Assessment of Post-Disturbance Regeneration in the Ubc Alex Fraser Research Forest’S (Afrf’S) Knife Creek Block

NATURAL REGENERATION DYNAMICS OF INTERIOR DOUGLAS-FIR: ASSESSMENT OF POST-DISTURBANCE REGENERATION IN THE UBC ALEX FRASER RESEARCH FOREST’S (AFRF’S) KNIFE CREEK BLOCK

Emmanuel Adoasi-Ahyiah B.Sc., Kwame Nkrumah University of Science and Technology, 2017

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Forestry)

THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)

September 2020

The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled:

Natural Regeneration Dynamics of Interior Douglas-fir: Assessment of Post-disturbance Regeneration in the UBC Alex Fraser Research Forest’s (AFRF’S) Knife Creek Block

submitted by Emmanuel Adoasi-Ahyiah in partial fulfillment of the requirements for the degree of Master of Science

In Forestry

Examining Committee:

Dr. Verena C. Griess, Associate Professor, Department of Forest Resources Management, UBC Supervisor

Dr. Adam Polinko, Post-doctoral Fellow, Department of Forest Resources Management, UBC Supervisory Committee Member

Dr. Allan Carroll, Professor, Department of Forest and Conservation Sciences, UBC

Supervisory Committee Member

Dr. Lori Daniels, Professor, Department of Forest and Conservation Sciences, UBC Additional Examiner

Abstract

Licensees are required to regenerate interior Douglas-fir (Pseudotsuga menzisii var glauca) forest gaps greater than 0.1ha to meet and maintain species distribution across age classes for stands, biodiversity (such as mule deer winter range (MDWR)), and recreational objectives. However, little is known about the dynamics of natural regeneration in these forests. To understand future stand structure and composition, I investigated the regeneration dynamics and growth of interior Douglas-fir post-disturbance, and the influence of growing space availability on the dynamics of interior Douglas-fir regeneration and growth. I identified 181 gaps greater than 0.1 ha in the Knife Creek block of the UBC-Alex Fraser Research Forest. Sanitation harvests to control Douglas-fir beetle (Dendroctonus pseudotsugae) created salvage gaps, while biotic disturbances (e.g. windthrow) created natural gaps. Regeneration density and growth attributes of interior Douglas- fir were assessed in 600 1,000thha randomly laid circular plots across identified gaps. Growing space variables, such as slope, leaf area index (LAI), topographic wetness index (TWI), elevation, and aspect were obtained using hemispherical photographs and digital elevation models for each plot. Additionally, the distance of each plot to the gap edge, ground cover, site series and site index growing space variables were determined. Based on the findings of the study, I do not accept the hypothesis that the establishment and growth of interior Douglas-fir will increase with increasing available growing space in the gaps of regenerating Douglas-fir stands, and that growing space availability will have an influence on the establishment and growth of naturally regenerating seedlings. . However, only light (LAI) was directly measured using hemispherical photographs whereas moisture (TWI) was modelled from DEM data that may not have reflected microsite conditions. The findings of the study revealed that approximately 70% of the gaps did not meet stocking requirements, out of which about 40% had no regeneration. Additional research is needed to fully understand the dynamics of interior Douglas-fir regeneration and growth in these gaps over temporal and spatial scales. This will enable management to make more informed decisions on post-disturbance silvicultural prescriptions within the Alex Fraser Research Forest.

Lay summary

Forest management in the central interior landscapes of British Columbia is challenged with conflicting timber production and mule deer winter range (MDWR) maintenance management goals. This is further exacerbated by the issues of disturbances (e.g. wildfires, bark beetle, defoliators and harvesting), and species succession following these disturbances. Interior Douglas- fir is instrumental in providing timber for harvest, and for mule deer habitat maintenance. Given its importance, the regeneration of interior Douglas-fir was assessed together with growing space variables that influence species establishment and growth in greater than 0.1ha gaps. The findings of the study indicated that natural regeneration was not sufficient to meet MDWR requirements without external management intervention. Growing space availability had a weak relationship with regeneration and growth. However, its potential impact is important and management prescriptions must assess and include the influence of growing space variables in seedling establishment and growth.

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Preface

This thesis was made possible with the collaboration of my supervisory committee members, Drs. Verena C. Griess, Adam Polinko and Allan Carroll, who provided me with utmost support and guidance in the development and execution of my research questions and objectives, research methodology, and provided detailed edits and comments in all chapters of the thesis write-up. Dr. Adam Polinko provided guidance in conducting data analysis for Chapter 3. Further, the Alex Fraser Research Forest provided some equipment and support during the data collection period for the research field work. Finally, I was responsible for developing the research questions and objectives, research methodology and sampling design, data collection and data analysis for chapter 3 coupled with the full write-up of the thesis. Chapter 3 is expected to be submitted for scientific publication.

Table of contents

Abstract ...... ii

Lay summary ...... iii

Preface...... iv

Table of contents ...... v

List of tables ...... vii

List of figures ...... viii

List of abbreviations ...... ix

Acknowledgements ...... x

Dedication ...... xii

Chapter 1: Introduction ...... 1

1.1 Background ...... 1

1.2 Thesis statement ...... 4

1.3 Thesis objective and implication for Knife Creek ...... 4

1.4 Thesis structure ...... 5

Chapter 2: Literature Review ...... 6

2.1 Physiological response of interior Douglas-fir...... 6

2.2 The dry interior Douglas-fir forest of British Columbia ...... 8

2.3 Biotic and abiotic disturbances in the dry interior Douglas-fir forest ...... 10

2.4 Fire disturbance in interior Douglas-fir forests ...... 12

2.5 Douglas-fir beetle disturbance in interior Douglas-fir forests ...... 13

2.6 Natural regeneration and growing space dynamics in interior Douglas-fir forests...... 15

2.7 Management of interior Douglas-fir forests ...... 18

Chapter 3: Douglas-fir natural regeneration dynamics in the Knife Creek block of the UBC AFRF post-disturbance ...... 22

3.1 Introduction ...... 22

3.2 Research objectives and questions ...... 26

3.3 Hypotheses and predictions ...... 27

3.4 Methodology ...... 27

3.4.1 Study area ...... 27

3.4.2 Gap selection ...... 28

3.4.3 Data collection ...... 29

3.4.4 Data Analysis ...... 31

3.5 Results ...... 32

3.5.1 Role of disturbance on interior Douglas-fir regeneration ...... 32

3.5.2 Drivers of interior Douglas-fir regeneration ...... 35

3.5.3 Drivers of interior Douglas-fir growth ...... 43

3.6 Discussion ...... 46

3.6.1 Role of disturbance on interior Douglas-fir regeneration ...... 46

3.6.2 Drivers of interior Douglas-fir regeneration ...... 47

3.6.3 Drivers of interior Douglas-fir growth ...... 51

Chapter 4: General conclusions ...... 54

4.1 Main findings ...... 54

4.2 Management implications ...... 55

4.3 Future research ...... 57

Bibliography ...... 60

Appendices ...... 74

List of tables

Table 3. 1: Summary of damage status of seedlings in natural and salvage gaps across sampling plots (0- No damage; 1- Low damage; some impact on growth; 2- Moderate damage; moderate impact on growth or stem quality; 3- Severe damage; serious impact on growth or stem quality)...... 42 Table 3. 2: Summary of vigour status of seedlings in natural and salvage gaps across sampling plots (1- Overall good health and good growth condition with greater than 75% foliage; 2- Reduced health and limited growth with greater than 50% foliage but less than 75% foliage; 3- Severe growth limitation and poor health condition with less than 50% foliage)...... 42 Table 3. 3: Summary of basal area contributed by each species surrounding natural and salvage gaps across sampling plots (Fd - Douglas-fir (Pseudotsuga menziesii var. glauca); Sx - Hybrid spruce (Picea hybrids); At- Trembling aspen (Populus tremuloides); Ep - Paper birch (Betula papyrifera); Pl - Lodgepole pine (Pinus contorta var. latifolia)...... 43

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List of figures

Figure 3.1: (A) Map of Canada showing administrative provinces. (B) Map of the province of British Columbia. (C) Map of UBC-AFRF KC block showing gaps and sampling plots...... 29 Figure 3.2: Douglas-fir regeneration density per hectare per gap in the Knife Creek block, AFRF...... 34 Figure 3.3: Douglas-fir regeneration density per hectare by distance to gap edge, and slope in natural and salvage gaps...... 36 Figure 3.4: Douglas-fir regeneration density per hectare versus leaf area index and topographic moisture index in natural and salvage gaps...... 37 Figure 3.5: Douglas-fir regeneration density levels per hectare versus elevation and aspect in natural and salvage gaps...... 38 Figure 3.6: Douglas-fir regeneration density per hectare versus site series and site index in natural and salvage gaps...... 40 Figure 3.7: Douglas-fir regeneration density per hectare versus ground grassy vegetation and soil...... 41 Figure 3. 8: Douglas-fir regeneration crown width growth versus topographic moisture index in natural and salvage gaps...... 44 Figure 3.9: Douglas-fir regeneration total height growth versus leaf area index in natural and salvage gaps...... 45

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List of abbreviations

AFRF – Alex Fraser Research Forest

BAF – Basal Area Factor

BC – British Columbia

DAP – Digital Aerial Photogrammetry

IDF - Interior Douglas-fir biogeoclimatic zone

KC – Knife Creek

LAI - Leaf Area Index

MDWR – Mule Deer Winter Range

RMSE – Root Mean Square Error

SIBEC- Site Index Estimates by BEC Site Series

TWI - Topographic Wetness Index

UBC - The University of British Columbia

Acknowledgements

My first thanks go to the Almighty God for His protection, love and strength to me for bringing this research work to completion. I would also like to extend many thanks and appreciation to my supervisory committee members, Drs. Verena C. Griess and Allan Carroll, for their constructive criticism and comments, support and ideas during my master’s study. A special warm and heartfelt thanks and gratitude goes to Dr. Adam Polinko for his utmost patience, expert advice and guidance, continual encouragement in the hardest times, constructive comments and feedback, and for being the best I could ever wish for during this journey. You were with me through thick and thin and was always cheerfully ever ready to help me whenever I called on you. It is only fair to say that I would not have gotten this far without you. Thank you once again for igniting my interest in science and research.

Many thanks also go to the staff of Alex Fraser Research Forest, Stephanie Ewen, Don Skea, Kylie Green and David Hamilton providing information, feedback, answering all my questions and making my stay in Williams Lake a memorable one full of laughs and lessons. My amazing field work team members through out the data collection period: Candy Lo, Sky Jarvis, Stefan Mertzger, Jose Santacruz and Nora Stampe are worthy of thanks and appreciation for their tireless and dedicated hard work irrespective of unfavourable conditions and for all the laughs and amusements we shared whiles working. Big thanks to Vivek Srivastava for his help in ArcGis and for being a good friend in and out of the lab. A big shout out to Galen Maennicke for reading and editing my thesis.

I will like to express my appreciation to the MasterCard Foundation staff, Jola Lekich, Rohene Buojam, Sarah Cameron, Natasha Philander, and Erin King-Brown, for all the support, mentorship and friendly environment that they provided me through the course of my study. To my MCF friends and family, Ransford Kofi Buah, Espoir Tuyisenge, M.fred Mutangana, Olivier Ntwali, Marian Orhierhor, Husna Simba and Diane Uwamariya for standing by me, encouraging me, laughing with me, and being the very best of themselves so I can push on to the finish line. I am forever grateful. Thanks to the Mastercard Foundation for funding my studies and the Moss Rock Foundation for funding my research.

To crown it all and with a grateful heart, a standing ovation, loves and warm hugs and kisses goes to my family, Kwesi Ahyiah (Nana), Monica Ntiriwaa (Honey Kuchikuchi), Rita Oforiwaa, Felicia

Otwuah Ansah, and Bismark Ampaabeng, for their spiritual and moral support in all fronts, love, encouragement, availability when I called on home and for being there for me. Finally, a very big thanks goes to my Canadian family, Dr. Sue Watts and Miss Tanka Watts, you have been more than I could ever ask for, and for that and many more I am forever grateful. For your love, care and support for me always, I say “Nyame nhyira wo”. I wish you many years that are more prosperous.

…still, SFM (StriveForMastery)!

Dedication

To God who gave me the strength in times of weary and unrest; and to my ever-loving parents, Kwasi Ahyiah (Nana) and Monica Ntiriwaa Ofori (Honey Kuchikuchi), and the rest of my family. I believe you have lived the words of Winston Churchill that reads, “We make a living by what we get, but we make a life by what we give.”

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Chapter 1: Introduction

1.1 Background

The change in climate evidenced by the continual rise in global temperatures, coupled with reduced precipitation over geologic time scales, has been predicted to increase the vulnerability of forests to disturbances (Pojar 2010; IPCC 2014). For example, empirical evidence exists to support the claim that variations in climate over time are associated with reported rises in forest disturbances driven by insect and pathogen outbreaks (Bentz et al. 2010; Anderegg et al. 2015; Ramsfield et al. 2016; Senf et al. 2016) and wildfires (Westerling et al. 2006). Climate forecasts for British Columbia (BC) predict that a large proportion of the central interior will become drier, including the Cariboo-Chilcotin Forest Region (Hamann and Wang 2006). The Regional Climate Model (RCM) for the Cariboo-Chilcotin Forest Region projects that summer and annual temperatures will increase across the region, but more winter warming is expected in the northern areas than in the south (Dawson et al. 2008). It is expected that this will also lead to changes in the geographic ranges of species. Species are more likely to shift towards higher elevations and northwards 100km per degree warming, which could have a significant impact on primary ecosystems (Davis 1989; Hamann and Wang 2006). Delays in forest succession may pose a serious problem for forest management in the face of a changing climate. A significant change in community type and composition occurs if species are lost due to natural, or anthropogenic disturbances, and are not replaced (Davis 1989). Climate studies by Hamann and Wang (2006) in BC suggest that niche distribution of important coniferous species (including interior Douglas-fir zones) change under warming scenarios and are projected to lose a substantial portion of their ecological habitat.

Forests undergo different stand dynamics processes including stand initiation and development cycles (Oliver and Larson 1996), growth and maturation of individual trees, competitive interactions among species (Lutz et al. 2014), and disturbances (Klenner et al. 2008; Frelich et al. 2018). Disturbances are major drivers in the modification of ecosystem functions, the arrangement of forest stands, and the composition of species (Oliver and Larson 1996; Ashton and Kelty 2018). A study by Scheuermann et al., (2018) on canopy structure and species diversity effects on primary productivity of disturbed forest landscapes demonstrated that the scale and severity of a disturbance has effects on the diversity and composition of species, the structural

complexity of the canopy, and net primary productivity (NPP). Disturbances may be classified based on their frequency, intensity, and severity. The removal of a single dominant canopy tree, a small group of trees or even a large branch from a canopy dominant tree species can cause gap- scale disturbances. On the other hand, catastrophic disturbances may lead to the complete removal of overstory tree species and vegetation on a large-scale. Between these two extreme endpoints of the classification range lies the intermediate severity disturbances, which are usually hard to determine and quantify (Hart and Kleinman 2018). Natural forests frequently experience disturbances, which result in the modification of the biophysical and microscale surroundings (Hart 2016). Since these disturbances have impacts on ecosystem functions, as well as the arrangement and composition of species (Oliver and Larson 1996), it is imperative to understand the responses of species to disturbances across temporal and spatial landscapes.

Insect outbreaks (such as the Douglas-fir beetle) are noted as one of the most common, and important, biotic agents that influence forest succession over time (Dale et al. 2001; Bentz et al. 2010). Dale et al., (2001), in their study on climate change and forest disturbances in the USA, reported that the highest economic costs were associated with insect and pathogen outbreaks compared to other disturbances, with economic losses of over 2 billion dollars and impacting approximately 20.4 million hectares. Insect outbreaks have been associated with many factors, such as the arrangement of an incipient population according to space, history of disturbance in the area, topography, availability and susceptibility of hosts, and fire occurrences (Aukema et al. 2006). However, climatic suitability for brood and development remains one of the most important factors influencing insect outbreaks (Carroll et al. 2004; Aukema et al. 2006). While the increasing number of insect outbreaks is associated to changes in climate (both locally and globally) (Allen et al. 2010), some studies have argued that observed competition between trees far outweighs the influence of climate on tree growth and mortality (Zhang et al. 2015). In a study based on a half- century of evidence using datasets of long-term forest observations (1958-2009) from western Canada, Zhang et al. (2015) showed that climate has a significantly weak effect on tree mortality and a negligible effect on the recruitment and growth of trees. This suggests that internally occurring processes at the species community-level are the dominant drivers of forest dynamics relative to external factors such as climate (Zhang et al. 2015).

Interior Douglas-fir (Pseudotsuga menziesii var. glauca) is one of the world’s most valuable and widely used timber species, which is native to the western part of North America (Hermann and Lavender 1990). It is a dominant species in the Interior Douglas-fir biogeoclimatic zone (IDF) in British Columbia and is extensively grown in even and uneven-aged stands (Hermann and Lavender 1990; Day 1996). Douglas-fir dominant stands cover approximately 4.5 million hectares (Hermann and Lavender 1999) of the landmass in British Columbia, and 92 million hectares (Mathys et al. 2014) within its natural range, from British Columbia (550N) in the western parts of North America to the central parts of Mexico (190N) (Hermann and Lavender 1990). The species range has however been expanded and has been planted on millions of hectares of land across Australia, South America, Europe and New Zealand (Hermann and Lavender 1999). The interior Douglas-fir forests are face mortality caused by alternating insect outbreaks over the last several decades (Erickson 1992; Day 1996; Schmitz and Gibson 1996). These forests have been associated with competition and density related stagnation arising from overstocking, which is a serious concern for management (Day 1998). Critical mule deer winter habitat is provided by mature interior Douglas-fir dominated forests, and conservation of these habitats remains a primary management objective in the IDF (Armleder et al. 1994). The low elevation of the interior Douglas-fir forests coupled with limited snowpack availability contributes to the importance of these forests as winter range habitat for mule deer (Armleder et al. 1986). The interior Douglas-fir forests are therefore of high economic and ecological interests in BC. Additionally, these forests are equally useful for ranching purposes as they may serve as grazing sites for cattle (Day 1998). Given the economic and ecological importance of interior Douglas–fir forests, there is the need to study the dynamics of interior Douglas-fir regeneration post-disturbance. The interior Douglas-fir was therefore chosen as the candidate species for this study based on its commercial and ecological importance and its vulnerability to insect disturbances.

In BC, interior Douglas-fir forests are managed to support an array of multiple management objectives, including the production of timber and wildlife habitat (Sullivan et al. 2011). As a result, uneven-aged management, such as group and single-tree selection, has been proposed to meet and maintain a distribution balance across age classes for stands, biodiversity, recreation, and range objectives (Day 1996; Sullivan et al. 2011). Interior Douglas-fir forests are often associated with tree clusters in central BC near grassland interfaces because of previous harvesting practices designed to improve mule deer habitat. However, management of interior Douglas-fir forests is

increasingly challenged by the impacts of the Douglas-fir beetle. It is therefore important that management decisions follow and satisfy proposed guidelines outlined for the management of the interior Douglas-fir forests to meet multiple objectives.

1.2 Thesis statement

The regional mule deer winter range strategy regulation for the Cariboo-Chilcotin Forest Region mandates that all licensees must regenerate all gaps greater than 0.1ha under the reforestation phase (CCLUP Mule Deer Strategy Committee 2014). This area provides an important ecosystem for mule deer and is designated to serve as thermal and forage habitat for mule deer wintering ranges. Critical mule deer wintering ranges are legally protected under the Cariboo-Chilcotin Land Use Plan (CCLUP) and more specifically, the Williams Lake Sustainable Resource Management Plan (SRMP) (Cariboo-Chilcotin Land-Use Plan 2005). The management of the UBC Alex Fraser Research Forest (AFRF) must therefore maintain or enhance Douglas-fir density in forested areas for mule deer winter range habitat maintenance (Dawson et al. 2006). The Douglas-fir beetle outbreak is thus of great concern to the management of the AFRF (Coggins and Jack 2008). Sanitation harvesting was consequently used by management to curtail the spread of the Douglas-fir beetle that was killing economically and ecologically important Douglas-fir trees in portions of the AFRF. This has subsequently led to the creation of gaps in the blocks post- disturbance which are currently naturally regenerating. However, it has been noticed by management that the rate of regeneration of interior Douglas-fir is below expected projections.

1.3 Thesis objective and implication for Knife Creek

This research investigated the establishment, growth, and dynamics of regeneration in the Knife Creek block of the Alex Fraser Research Forest (AFRF) after sanitation harvesting and natural disturbances, and the factors (biotic and abiotic) influencing the dynamics of regenerating stands. The results of this study will provide the management of AFRF with an understanding of the dynamics in establishment, growth, and composition of species regenerating in the gaps of the Douglas-fir forest in the Knife Creek block of the research forest. In addition, this study will help forest managers make better informed decisions regarding the regeneration of stands and improve

the management of the whole forest to meet its multiple objectives. Furthermore, this research will provide the management of AFRF with an understanding of the impacts of biotic and abiotic factors that influence the establishment, growth, and composition of species regenerating in the gaps of the Knife Creek block, which is important for stand development of the forest post- disturbance. This will help the forest managers make informed decisions when designing silvicultural systems and management practices in the Knife Creek block of the research forest to meet multiple management objectives. With ample understanding of biotic and abiotic factors influencing Douglas-fir regeneration in gaps and possible relationships, forest management activities in the AFRF could be designed to yield desired outputs for the mule deer winter range.

1.4 Thesis structure

This thesis is divided into four chapters. Chapter 1 encompasses the introduction of the thesis in the form of background, thesis statement and objective, and proposed implications of the study. Chapter 2 is a review of relevant literature to understand the context and discuss previous similar works and research carried out on the subject of the study. Chapter 3 provides the results and explores the dynamics of regeneration to understand the influence of growing space availability on the dynamics of the regeneration post-disturbance. The fourth and last chapter encompasses a synthesis to provide an overall analysis of the research, including conclusions and implications for sustainable forest management and the significance to current and future research. Chapter 4 also explains the overall importance and contribution of the research to the management of the Knife Creek block of AFRF and other Douglas-fir forests in the mule deer winter range with multiple management objectives.

Chapter 2: Literature Review

2.1 Physiological response of interior Douglas-fir

The Douglas-fir tree is categorised into the interior (Pseudotsuga menziesii var. glauca) and the coastal (Pseudotsuga menziesii var. menziesii) variety. Research on the interior variety of Douglas-fir, also known as the Rocky Mountain Douglas-fir, is limited. Most research conducted on Douglas fir has focused on the variety of menziesii (Lemay et al. 2009; Coleman et al. 2014). The two varieties of Douglas-fir can be distinguished and categorized using geographical, morphological and/or physiological attributes. One of the physiological attributes used is shade tolerance. The interior variety of the Douglas-fir is known to have a moderate tolerance to shade whilst the coastal variety is intolerant to shade (Hermann and Lavender 1990; Chen et al. 1996; Lemay et al. 2009). In central BC, interior Douglas-fir is considered as a late-successional species in the IDF zone that is moderately shade-tolerant and capable of regenerating under a canopy (Chen and Klinka 1997). Interior Douglas-fir is an important widespread species that occupies a very broad ecological range (Hermann and Lavender 1990; Day 1996) and grows in uneven-aged fire-dominated ecozones in the drier areas.

Several studies have shown relationships between the growth of naturally regenerated and planted interior Douglas-fir seedlings and light availability (Chen et al. 1996; Chen 1997; Chen and Klinka 1997; Vyse et al. 2006). The occurrence of sun leaves in whorled patterns around branches and shade leaves in nearly horizontal plane orientations (Chen and Klinka 1997) may account for the survival of seedlings in a gradient of light availability. In a study conducted on naturally regenerated interior Douglas-fir seedlings, there was an increase in specific leaf area and the ratio of total lateral to terminal increment with decreasing light availability (Chen et al. 1996). Chen and Klinka (1997) reported higher light availability under a forest canopy than previous reports for coniferous forests on low latitudes and tropical rain forests (Canham et al. 1990). Availability of light and intensity, and duration, of sunfleck was spatially variable among branches of different seedling species (Chen and Klinka 1997), which could be attributed to the variability of seedling distances from gaps (Canham et al. 1990) and the effects of clumping from surrounding overstory tree leaves (Chen and Black 1992). Under field conditions, Chen and Klinka (1997) also observed that seedlings of Douglas-fir photosynthetically acclimatized to the light available in the surrounding environment. They further demonstrated that understory-grown interior Douglas-fir

seedling branches had a faster photosynthetic induction in response to changes in photosynthetic photon flux density (PPFD) than branches of open-grown seedlings. Photosynthetic induction is dependent on the PPDF and length of a period, as well as other ecological indicators (e.g., water table) (Chen and Klinka 1997). At low PPFD, understory-grown seedling branches were more photosynthetically efficient relative to branches of open-grown seedlings (Chen and Klinka 1997).

In forest ecosystems, understory light availability may vary in two scales of time (i.e., diurnal and seasonal variations) and three scales in space mediated by the presence of gaps and distances from the canopy and neighbouring stems (Chen et al. 1996). In a study on Lodgepole (Pinus contorta var latifolia) pine and Douglas-fir (Pseudotsuga menziesii var glauca) in interior BC, specific leaf area of both species decreased with decreasing light availability, although Douglas-fir had a higher and faster marginal increase. Douglas-fir demonstrated higher plasticity to specific leaf area and crown architecture to a gradient of light availability (Chen et al. 1996). Lateral incremental growth was more reduced in Lodgepole pine than in Douglas-fir, however, terminal growth did not significantly differ. Douglas-fir also allocated more growth to laterals than to terminals in low light environments (Chen et al. 1996). Another similar study conducted by Vyse et al (2006) on Douglas-fir, Lodgepole pine, and Ponderosa pine under varying canopy cover and site conditions, showed that the level of light available strongly affected the survival and growth of all species. They also found that there was no threshold for canopy closure but that the most open canopy conditions had the best seedling survival and growth. However, the largest gap used within the study was 0.1ha. Linear growth was observed for all species with light availability, with the most growth occurring for Lodgepole pine, and the least growth occurring for Douglas- fir. Total light most significantly affected six-year seedling growth, while slope, aspect, and crown closure had less influence (Vyse et al. 2006). In wetter open canopies, all species exhibited a higher photosynthetic capacity, which positively correlated with light and moisture availability. Douglas- fir recorded a higher survival percentage (78%) relative to Lodgepole pine (76%) and Ponderosa pine (70%) (Vyse et al. 2006). Another study investigating the influence of, and interaction between, light availability and sapling size of interior Douglas-fir and Lodgepole pine demonstrated that sapling size had little influence on the morphological characteristics of species, although growth in height and lateral branches increased with increasing sapling size (Williams et al. 1999). Douglas-fir exhibited a more plastic crown morphology relative to Lodgepole pine. The shade-intolerant Lodgepole pine was able to survive together with Douglas-fir for 50 years with

available light as low as 5%PPFD. This study suggests that species have greater tolerance to shade on drier sites. In response to low light availability, interior Douglas-fir reduces its apical growth, maintains a live crown with limited healthy branches and allocates more resources to lateral growth, which serves as an adaptation to capture sun flecks in the understory. With increasing light availability, Douglas-fir allocates more resources for terminal growth than lateral growth leading to an increase in crown depth because of more light reaching the lower branches, while greater mortality of lower branches is expected with decreasing light availability due to self-shading (Williams et al. 1999).

2.2 The dry interior Douglas-fir forest of British Columbia

The Interior Douglas-fir (IDF) biogeoclimatic zone is the second warmest zone in the interior of BC and serves as home to diverse mixed-conifer species. Douglas-fir is the dominant tree species in the area but grows slowly because of the low amount of precipitation (Lloyd et al. 1990; Day 1996; Hermann and Lavender 1999; Swift and Ran 2012). However, the interior Douglas-fir occupies sites that are considered better than average in terms of productivity within the species’ range of occurrence (Hermann and Lavender 1999). These sites are characterized by luvisols and brunisols as the dominant soil types, with elevations ranging between 300 and 1450 m (Lloyd et al. 1990; Hope et al. 1991). The IDF forest species composition generally consists of primary dominant species like the interior Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco var. glauca], western larch [Larix occidentalis Nutt.] and ponderosa pine [Pinus ponderosa Douglas ex C. Lawson] (Hope et al. 1991). However, on wetter sites, other secondary species like paper birch [Betula papyrifera Marsh.], lodgepole pine [Pinus contorta Dougl. var. latifolia Engelm.], hybrid white spruce [Picea engelmannii x glauca] and trembling aspen [Populus tremuloides Michx.] are usually recognized (Hope et al. 1991; Klinka et al. 2004; Koot et al. 2015). The ground vegetation cover usually consists of shrubs, herbs and mosses as well as shade-tolerant regenerating trees species like hybrid white spruce and Douglas-fir (Hope et al. 1991; Klinka et al. 2004).

In BC, the IDF is found to dominate the low- to mid-elevations of the south-central regions of the interior and stretches into Alberta, Idaho, Washington, Montana, and Oregon (Lloyd et al. 1990; Hope et al. 1991; Daigle 1996). The IDF covers over approximately 4.5 million hectares

(5%) of forestland in BC and contains over 90 percent of interior Doulas-fir in Canada (Ministry of Forests 1995; Hermann and Lavender 1999; Klenner and Walton 2009; Heyerdahl et al. 2012). This zone is described as the “dry-belt” with a continental climate consisting of long and cold winters, and hot and dry summers. The mean temperature for the zone ranges from 1.6 to 9.5°C, with 2 to 5 months of the year recording 0°C. Mean precipitation ranges from 300mm to 750mm yearly (Lloyd et al. 1990; Hope et al. 1991; Pojar and Meidinger 1991; Li et al. 1994; Hermann and Lavender 1999). In BC, southerly latitudes and lower elevations experiencing summer drought limit interior Douglas-fir establishment. On the other hand, the characterized moist, warm climatic conditions in the more easterly climatic region permit the establishment of a higher productive forest, however, seedling establishment is limited by competition for light (Simard 2012). Interior Douglas-fir is projected to expand in territory towards the north as a result of global warming and to shrink in the south of BC due to the predicted increase in summer drought. In future years, the range of interior Douglas-fir may become limited to mountainous regions with a moist climate where there will be increased competition from the resident or invading tree species for survival and establishment (Pojar 2010).

The combined effects of topographic and edaphic conditions have contributed towards shaping the stand structure and species composition of the interior Douglas-fir forests (Hope et al. 1991). These conditions influence species occurrence and competition for resources with one species being successful over another based on it’s abilities to allocate photosynthate to growing parts and endure low levels of some growth factors such as light and moisture (Oliver and Larson 1996). Interior Douglas-fir of all sizes and ages occurs throughout the IDF zone, ponderosa pine occurs mostly on drier sites, hybrid spruce grows on wetter and cooler sites, and lodgepole pine exists on higher elevations previously disturbed by fire (Swift and Ran 2012). Stand development in the IDF is such that following a disturbance (such as a fire), the area is first colonized by ponderosa or lodgepole pine contingent on-site elevation. Lodgepole pine is the symbolic successional species at higher elevations, but over time this is substituted by Douglas-fir (Swift and Ran 2012). Hybrid spruce has a growing advantage, along with Douglas-fir, in the cooler wetter areas of the IDF where fires may be less intense and frequent (Swift and Ran 2012).

The interior variety of Douglas-fir differs in its growth on a broad range of site conditions, ranging from high elevations with cool and moist sites, to low elevations characterized by warm

and dry sites. Interior Douglas-fir is intolerable to droughty conditions and poorly drained soil. Growth occurs on soils with a diverse range of parent materials some of which are attributed to its growth limitations (e.g., low nutrient availability) (Coleman et al. 2014). Besides nitrogen (N), numerous other factors, such as moisture availability, may also limit the growth of interior Douglas-fir (Coleman et al. 2014). An imbalance in nutrient availability at growing sites of interior Douglas-fir forests may reduce the defensive capabilities of the trees leading to increased mortality (Mandzak and Moore 1994). Past research efforts have been directed towards interior Douglas-fir nutrition deficiencies. There are well known N limitation in the IDF and the species has been shown to be responsive to N additions. However, the application of a single nutrient could result in nutrient imbalances and lead to reduced growth response and/or mortality (Coleman et al. 2014). The foliar nutrient ratios of Douglas-fir on 131 study locations revealed that a majority of the proportions of Douglas-fir stands were below optimum nutrient contents (Coleman et al. 2014).

2.3 Biotic and abiotic disturbances in the dry interior Douglas-fir forest

Growing space is the sum of all factors necessary for the growth of tree species. Growing space may be grouped into to the tangible (e.g. space for root and shoot growth) and intangible (e.g. light, water, and nutrient) capacities available for tree growth (Oliver and Larson 1996). Every site is limited by growing space. Growing space may be created in an area by a disturbance through the removal of plants, and through the alteration of the total available growing space available to trees by altering nutrients, oxygen and water availability (Oliver and Larson 1996). A forest disturbance has been defined as any discrete event or occurrence that causes a change in the availability of resources and substrate in space and/or time (White and Pickett 1985). Abiotic agents (such as a fire) and/or biotic agents (such as herbivorous insects or parasites) may cause a forest disturbance. The occurrence of a disturbance event influences cycling of soil nutrients through its effect on organic matter distribution and the rate of release of nutrients (Harmon et al. 1986; Brown et al. 2003; Marañón-Jiménez and Castro 2013). They may affect an ecosystem's biodiversity and forest composition significantly by altering the allocation, distribution, and use of resources that lead to a significant alteration in the structural and functional composition of a forest, and/or destroy pre-existing biotic assemblages in an ecosystem (e.g. Haggstrom and Kelleyhouse 1996; Sullivan and Sullivan 2001; Lyons et al. 2008; Millington et al. 2011; Latif et al. 2016).

Disturbances may leave behind legacies upon which many other ecosystem disturbances may act on due to the alterations in ecosystem structure and function, such as a significant change in an ecosystems resource use and availability (Buma 2015). These legacies may serve as the underlying precursors to an ecosystem’s structural and functional changes from the microsite to landscape level (McCullough et al. 1998; Rhemtulla and Mladenoff 2007; Fraterrigo and Rusak 2008).

Remnant biological legacies from preceding disturbances in an ecosystem vary from each other depending on the time difference between different disturbance occurrences, intensity, type, and extent (Paine et al. 1998; Romme et al. 1998; Turner et al. 1998). Ecosystem alterations and legacies from disturbances occur at multiple scales (temporal and spatial); hence, it is important to determine the impact of interactions between disturbances, such as bark beetles and fire (Bebi et al. 2003; Simard et al. 2012). The interaction between two disturbances may be affected by the period between these disturbances, which may lead to an increase or reduction in the severity of one disturbance following another in a variable temporal gradient scale in short and/or long term (e.g. Fleming et al. 2002; Sturtevant et al. 2012). The influence of one disturbance on another is not easily quantified as one disturbance can potentially last longer and/or influence the occurrence of another disturbance or the diversity of species present post-disturbance (Dupouey et al. 2002; Hart et al. 2014b). The characteristics of a disturbance, including intensity, spatial extent, frequency, and severity, is shaped by a suite of complex factors such as the interaction between disturbance interactions, land cover, and climate (Harvey et al. 2018).

Disturbances and their interactions have helped shape the composition and structure of the mixed-conifer dry forests of interior BC (Pojar and Meidinger 1991). Forests in this area are open and are structurally heterogeneous (Allen et al. 2002), with trees occurring alone or in clumps (Maclauchlan and Brooks 2009). These dry forest ecosystems typically experience mixed-severity disturbance regimes throughout their range that includes fire (low, medium and high severity), bark beetles, and defoliators (Klenner et al. 2008; Simard 2012; Swift and Ran 2012). However, the key drivers that have created the current conditions in the interior Douglas-fir forests have been the result of an interaction of several factors including climate, extensive fires, European settlement, harvesting, fire suppression, and insect outbreaks (Klenner et al. 2008). The primary dominant natural disturbance agents in this ecosystem that have a significant influence on stand dynamics, forest composition and structure are wildfires, bark beetles, root rots, and defoliators

such as the Douglas-fir tussock moth (Orgyia pseudotsugata), and more importantly, the western spruce budworm (Choristoneura freemani) (Swift and Ran 2012). Among these disturbances, wildfires have driven the historic range of variability in these forests. Drier sites within this area are characterized predominantly by low severity fire regime disturbances that result in variable stand structure that is open, multi-aged, with grassy and herbaceous understory (Wong 1999; Heyerdahl et al. 2001; Baker et al. 2007; Swift and Ran 2012). On the other hand, moist sites of these forests are characterized by dense even-aged stands that arise from less frequent, and more severe, fire regimes (Vyse et al. 2006; Klenner et al. 2008). In the drier areas of the IDF, frequent fires usually occur at an interval of 10-20 years and burn less than 50 ha, helping to maintain current forest structure and composition (Swift and Ran 2012). Historically, occurrence of fires reduced young Douglas-fir abundance partly because of the low hanging branches, which increases their vulnerability to crown scorch. However, older Douglas-fir trees are considered “fire-adapted” and tend to survive fires due to the formation of a thick bark (Daigle 1996; Swift and Ran 2012).

2.4 Fire disturbance in interior Douglas-fir forests

A broad scale of disturbances including wildfire, insect infestation and harvesting beginning at the time of European settlement (1860), which is relatively more recent (Klenner et al 2008), continuously affects dry forests of interior BC. Fires are one of the primary disturbance and natural ecological process in the dry mixed conifer forests in BC that influence stand structure and development (Day 1996; Daigle 1996; Stocks et al. 2003; Harvey et al. 2017). Common and extensive fires give these forest types a mixed severity fire regime (Daigle 1996; Heyerdahl et al. 2012; Harvey et al. 2017). Studies have reported varying fire frequency and a range of fire return intervals for the IDF. One study found that these dry forests have been reported to have a historical average fire frequency and return interval of about 7-20 years (Daigle 1996). However, another study reported a varied mean fire frequency of 150-200 years for large (50 ha or greater) high severity fires and 10-20 years for low severity (Swift and Ran 2012). Both surface and crown fires have been historically common in these forests (Hope et al. 1991). More frequent low severity ground fires gave room for the creation of small gap openings that allowed the germination of trees, forbs, shrubs, and grasses providing interspersed shrubby and grassy openings in uneven- aged stands (Daigle 1996). Occasional crown fires are instrumental in creating large open patches

within the landscape interspersed with multi-aged forested stands (Daigle 1996). This mixture of low to high severity fires leave behind patches of areas with mildly scorched trees and intact forest floor litter as well as patches with complete destruction of all trees and forest floor litter (Wong 1999). The frequency and intensity of fires in the IDF differ with changes in topography, which affects the distribution of soil moisture. This influences the frequency, spread, and intensity of the fire, which in turn leads to the formation of a complex forest structure of poorly defined boundaries of multi-aged stands with patches (Swift and Ran 2012). Stands in these forests have experienced both widespread stand altering and frequent stand maintaining fires (Daigle 1996; Harvey et al. 2017).

To safeguard timber resources, the area of dry forest burned by fire was kept below 1% of historic occurrences through increased fire suppression, an extensive road network system, and aerial fire suppression technology (Klenner et al. 2008). The suppression of fire, coupled with other land management practices, has led to the alteration of the ecological structure, composition, and function of these forest stands and grasslands that are radically different from the historical conditions. This includes an increase in woody plant encroachment into grasslands and an increase in forest in-growth (Hope et al. 1991; Daigle 1996; Bai et al. 2004; Klenner et al. 2008; Swift and Ran 2012; Harvey et al. 2017). This has led to increased forest stand densities, changes in species composition, slowed organic matter decomposition, increased mortality from forest pests, and tree root competition, and increased fire size and intensity (Daigle 1996). The nature of disturbance in these forests has transformed from one primarily dominated by fire and insect infestations, to one that is dominated by harvesting and insect outbreaks (Klenner et al. 2008). To restore these forest stands to historic conditions, prescribed burning and thinning treatments have been proposed as a way to mimic the low severity fire disturbance that commonly characterizes these stands (Daigle 1996; Klenner et al. 2008; Feller and Klenner 2011; Heyerdahl et al. 2012; Harvey et al. 2017).

2.5 Douglas-fir beetle disturbance in interior Douglas-fir forests

Bark beetle disturbances play an important role in shaping the structure and composition of dry forests in British Columbia. The Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins) is one of the major biotic disturbance agents which causes mass mortality of Douglas-fir in the IDF (Armleder and Dawson 1992; Erickson 1992; Powers et al. 1999; Humphreys 2000; Day

2007). The Douglas-fir beetle, like most beetles, spends most of its life beneath the bark of attacked host species laying eggs that develop into larvae that feed solely on phloem tissue of the host plants (Wood 1982; Christiansen et al. 1987; Humphreys 2000). Generally, the Douglas-fir beetle produces two broods per year and completes its yearly life-cycle by the laying of eggs. After hatching, the beetles go through a four-instar larvae stage, a pupa stage and finally mature into adults. Adult Douglas-fir beetles construct holes through the bark of candidate species after emergence and lay strands of eggs in egg galleries under the bark of host trees parallel to the grain of the tree. Eggs hatch into the larva, which feeds on the phloem in feeding channels positioned at right angles to the galleries. Larvae pupate after four instars, develop into an adult Douglas-fir beetle under the bark of the host tree, and await ambient temperature above 16° C to initiate flight. Usually, flight occurs between May and August (Province of British Columbia 2018). Like the Mountain Pine beetle (Dendroctonus ponderosae), climatic factors suitable for brood and development, and the availability of susceptible host species (veteran trees) coupled with suppression of fire in forest landscapes favours the establishment of the Douglas-fir beetles (Carroll et al. 2004; Aukema et al. 2006; Allen et al. 2010). The Douglas-fir beetle preferably attacks Douglas-fir that are of low vigour (Rudinsky 1961), damaged by fire (Furniss 1965; Hood and Bentz 2007), or newly killed (Humphreys 2000; Aukema et al. 2016). Although this species of beetle mostly exists in endemic conditions (Christiansen et al. 1987), at high population densities driven by an abundance of host species and suitable climate conditions, the Douglas-fir beetle has the potential to attack healthy well-defended populations of Douglas-fir trees leading to widespread outbreaks (Rudinsky 1962, Rudinsky 1966).

During epidemic outbreaks, large portions of healthy, mature trees are attacked and killed, and the spread to other green trees is rapid, leading to devastating losses (Erickson 1992; Schmitz and Gibson 1996). The infestation of healthy well-defended trees by the Douglas-fir beetle leads to the formation of patches of dead trees within forest stands at epidemic states (Rudinsky 1961; Wood 1982; Powers et al. 1999). This is influenced by susceptible host species abundance, weakened and freshly downed trees allowing Douglas-fir beetle build-up, and the existence of high population beetle densities (Rudinsky 1961; Negron 1998; Aukema et al. 2016). During droughty conditions, the susceptibility of host species to Douglas-fir beetle infestation is heightened (Rudinsky 1961; Hart et al. 2014a). Damage prediction of Douglas-fir beetle may be achieved using relative proportions of stand density and basal area. In general, stands with high

densities comprised mostly of Douglas-fir are more likely to be susceptible to infestation (Negron 1998; Negron et al. 1999; Dodds et al. 2006). Susceptibility of trees to bark beetle infestation or attack can be reduced by practices such as treatment and/or removal of slash, and thinning of young stands (Ross and Daterman 1997). In the case of the Douglas-fir beetle, fire prevention plays an important and beneficial role through the reduction of potential beetle habitat by decreasing the number of burned trees and hence lowering susceptible and injured host trees available for infestation (Furniss 1965; Ross and Daterman 1997). Implementation of sanitation harvests in the IDF forests could control and reduce Douglas-fir beetle habitat, and lead to the creation of forest gaps that will either regenerate naturally or require planting.

2.6 Natural regeneration and growing space dynamics in interior Douglas-fir forests

The regeneration potential of trees following a disturbance is spatially and temporally variable (Weetman and Vyse 1990; Oliver and Larson 1996). Natural regeneration of the desired trees species is dependent on the abundance of seed sources within, or at the edge of the disturbed area, a suitable seedbed, and favourable environmental conditions for seedling germination and early establishment (Weetman and Vyse 1990; Ministry of Forests 1995). Generally, all disturbed forest stands will naturally restock with enough time. However, the period for this to be achieved is variable and dependent on tree species, natural productivity, and the geographic location of the forest stand in question. The time for natural restocking may range from several years to many decades for the establishment of commercially valuable tree species. A delay in regeneration and/or a reduced growth rate increases the length of rotations and drastically reduces the yield of timber available for harvesting (Weetman and Vyse 1990; Ministry of Forests 1995).

Natural regeneration in the dry interior Douglas-fir forests of BC has been practised in the past following stand-level partial cutting prescriptions (Li et al. 1994; Vyse et al. 1998; Waterhouse 1998; Huggard and Arsenault 2009). Dry interior Douglas-fir forests in the IDF zone have been associated with highly variable natural regeneration in terms of the amount and timing of seedling establishment (Huggard et al. 2005). This variability in regeneration or species recruitment is influenced by the supply of seeds, seedbed conditions, and the surrounding/environmental conditions (Ministry of Forests 1995; Huggard et al. 2005). Historically, poor or scattered conifer regeneration in dense patches has been documented in these

forests that arises from the combined effects of climatic variables (hot, dry conditions; characterizing the area), unsuitable or inadequate prepared sites, competition with other vegetation (e.g., grass), and herbivory (e.g., livestock, small mammals) (Vyse et al. 2006; Vyse 1991 as cited in Huggard and Arsenault 2009). In addition, tree-crop production, ecosystem natural variability, and species succession challenges post-disturbances are exacerbated by conifer seed predation (Huggard and Arsenault 2009).

Microsites play crucial roles in the success of natural regeneration. Unavailable suitable microsites are suspected to be the main factor in the low germination rate and recovery of seedlings (Weetman and Vyse 1990; Ministry of Forests 1995; Huggard et al. 2005; Vyse et al. 2006). In a summary of studies in the Opax Mountain IDF forests silvicultural systems, Huggard et al. (2005) found that there was no natural regeneration in larger openings after harvesting. Individual tree selection (ITS) treatments had slow regeneration attributed to erratic seedfall, high seed consumption (predation), and hot dry summers leading to poor germination. Conversely, germination was enhanced on exposed mineral soils, but seedlings did not survive droughty conditions (Huggard et al. 2005). Like the Opax Mountain IDF forests, insufficient regeneration in gaps of stands at the Knife Creek block may stem from microsite conditions (environmental factors) and the availability of seed sources at the time of, and after, salvage logging and the competition for available growing space by species. Contrary to these observations, Waterhouse (1998) found that natural regeneration, irrespective of partial cutting harvesting density in three IDF subzones (IDFdk3, IDFxm and IDFxw) stands, was sufficient at 2,000 seedlings per hectare after a decade, with the exception of stands with steep southerly slopes and low overstory crown closure. This study summarized two unpublished data sets from variable partial cuttings that occurred between 1980 and 1987 in the IDF forest. The disparity observed in this study from other studies, like the Opax Mountain IDF forests, may stem from the differences in disturbance type and severity, seed productivity and source, seedbed conditions, and growing space availability from created gaps. This supports the notion that for the same ecotone, different prevailing conditions and growing space availability leads to different responses by trees and vegetation.

The spatial and temporal availability of growing space may influence tree growth and development (Oliver and Larson 1996). Any one growing space factor has the potential to limit growth as a result of its absence, its use by another tree, or its variability in quantity available

(Oliver and Larson 1996). Growing space may include light, water, nutrients, oxygen, temperature, and carbon dioxide. Growing space availability is one way by which spatial pattern changes in forest stands are determined over time, and is a critical and limiting factor for forest growth (Gower et al. 1992; Lemay et al. 2009). Growing space that favours regeneration of shade-intolerant species is released through the mortality of overstory species leading to the creation of large gaps (Lemay et al. 2009), which in the IDF are common following high severity fires (Daigle 1996; Heyerdahl et al. 2001). Moisture, light, and temperature are prerequisites for seed germination, but also growth and development. For example, in a study of 14 conifer species, including Douglas- fir (var. glauca and menziesii), Li et al. (1994) found that light was beneficial for the germination all of species. For both varieties of Douglas-fir, germination had a positive response to light in the absence of seed stratification, but was unaffected by light when stratified. In addition, moisture availability and elevation are instrumental in determining the site index, which is the potential of a species to grow on a specific site, for interior Douglas-fir (Coleman et al. 2014).

Regeneration and recruitment of trees in gaps of IDF forests may undergo variable interactions that may be in the form of competition and/or facilitation with large trees in and/or around gaps based on limiting factors present at these sites. While first-year seedlings can thrive and grow well under light shade, older seedlings of Douglas-fir would normally require full sunlight for growth (Hermann and Lavender 1990). Competition from vegetation has the potential to limit the regeneration of Douglas fir by creating intolerable shade levels and moisture stresses that kill young seedlings (Hermann and Lavender, 1990). On wetter sites in the multi-aged stands of IDF forests, light is the limiting factor for the regeneration of tree species in gaps leading to clusters of small trees away from large trees that shows a pattern of repulsion. Conversely, moisture is the limiting factor in drier sites of the IDF forests which leads to the formation of small clusters of tree regeneration closer to large trees where soils are moister depicting a pattern of attraction (Lemay et al. 2009). In the dry IDF forest, Douglas-fir seedlings regenerate closer to large trees than away from them in large gaps due to the moisture microsite provided (Lemay et al. 2009). Moisture limitations on dry IDF sites result in clumped spatial patterns of regeneration and growth of trees rather than an even spatial distribution (Lemay et al. 2009). Ectomycorrhizae (EM) fungi colonization post-fire has been hypothesized to be important for interior Douglas-fir seedling establishment on dry and moist sites by increasing access to scarce resources for seedlings (Simard and Durall 2004). Interior Douglas-fir seedlings may benefit from EM fungal colonization

through mammal, soil, or wind dispersed inoculum, and through mycorrhiza networks, however, benefits diminish with an increasing loss of residual trees and the severity of disturbance (Teste et al. 2009). In an experiment to determine the carbon dynamics of Rocky Mountain Douglas-fir, researchers found that nutrient and water availability had a strong influence on stand canopy dynamics and affected biomass distribution at the stand level by altering water and nutrient availability using irrigation, fertilization, and carbon (wood chips) (Gower et al. 1992).

Understanding the influence of natural regeneration and growing space availability on interior Douglas-fir is important to ensure continuous survival, growth, and development of the species, which is also instrumental in the maintenance of range habitat for mule deer. Different management objectives may require that forest managers apply silvicultural treatments that mimic natural disturbance regimes (Radeloff et al. 2000), promote specific wildlife habitats (e.g. Mule deer) (Koot et al. 2015), or convert forest lands into other land uses such as agriculture and grazing (Hessburg et al. 2005). However, there is the tendency for forest management practices geared towards achieving and/or addressing a specific objective, at the expense of others, to exacerbate or create problems (Hessburg et al. 1994, 2000).

2.7 Management of interior Douglas-fir forests

Management of the dry IDF forests is complex due to conflicting management objectives, coupled with the occurrence of mixed-severity disturbances regimes. In these forests, multiple- entry partial cuttings and reliance on natural regeneration of trees post-harvest persists as the dominant silvicultural system (Huggard and Arsenault 2009). Conifer regeneration in disturbed dry forests is continuously a main operational concern for management in the southern interior of BC (Huggard et al. 2005; Vyse et al. 2006). Natural regeneration of tree species, alone or joined with active planting, can be used to restock disturbed and/or harvested forest stands (Weetman and Vyse 1990; Ministry of Forests 1995). With natural regeneration, the cost of planting is eliminated, which is generally advantageous from a financial perspective, but additional costs can arise in later years as a result of the need to correct irregular spacing and timing of regeneration throughout the silvicultural cycle (Ministry of Forests 1995). Natural regeneration in the IDF arises from seed sources in and/or adjacent to gaps created from disturbances. Direct seeding of interior Douglas- fir can be challenging because of seed predation (Huggard and Arsenault 2009). First year interior

Douglas-fir seedlings have relatively slow, indeterminate, and moisture limited growth. On moisture limiting sites, ground vegetation competes aggressively with Douglas-fir seedlings for moisture available in the site (Simard et al. 1998). Fire has the ability to destroy the seed bank of potential competitor species, and thereby promote regeneration of tree species (Daigle 1996) while suitable areas can be made available for non-tree vegetation by forest harvesting (Simard et al. 1998). Douglas-fir regeneration and understory vegetation may undergo a dynamic interaction. Understory vegetation may provide young germinated seedlings with shade protection but can concurrently limit water and nutrient availability through competition (Oliver and Larson 1996). An example of competitive understory vegetation in the IDF is the pinegrass (Calamagostris rubescens Buckl) (Lloyd et al. 1990). Simard et al. (1998) found that there was a significant increase in interior Douglas fir regeneration when large patches of pinegrass was removed from the understory. Based on the silvics of interior Douglas-fir, the species is well suited to an uneven- aged management regime (Day 1996). The Douglas-fir forests in the IDF are highly desirable for timber harvesting, because of the high-quality timber produced, and their proximity to highways, for transportation, and manufacturing plants for processing (Day 1996). These forests are important for mule deer winter habitat due to their low elevations and limited snowpack during the winter (Armleder et al. 1986), and also serve as grazing lands providing forage for cattle (Day 1996; Klenner and Walton 2009).

The history of uneven-aged harvesting in the dry-belt interior Douglas-fir stands spans over 100 years in BC (Waterhouse 1998; Huggard and Arsenault 2009). In these forests, harvesting occurs at varying densities, ranging from natural shelterwood (cutting of trees over a fixed diameter) to single-tree selection silvicultural systems, aimed at improving residual stand quality, reducing logging damage, and improving regeneration establishment on dry sites post-harvesting (Waterhouse 1998). Considering pre-harvest stand structure, the interior Douglas-fir stands are highly variable and require carefully applied site-specific prescriptions to meet management objectives (Waterhouse 1998). Complex management objectives such as timber harvesting, mule deer habitat maintenance, and public recreation direct prescriptions for management of these dry forests (Armleder et al. 1986; Day 1996; Klenner and Walton 2009).

Wildlife management and timber production are the main management concerns that characterize the mixed-conifer dry forests in the interior of BC. The Mule deer (Odocoileus

hemionus Rafinesque) is an important ungulate species in BC whose survival during winter depends on the provision of critical winter habitat by Douglas-fir stands (Armleder and Dawson 1992; Armleder et al. 1994). Douglas-fir old-growth (>140 years) forests with multilayered and moderate to high crown cover provide suitable winter ranges that are utilized by mule deer (Armleder and Dawson 1992; Armleder et al. 1994). Mule deer feed on fallen arboreal lichen and Douglas-fir foliage that drops as a result of the influence of snow breakage and wind (Waterhouse et al. 1991, 1994; Waterhouse 2009). Equally importantly, Douglas-fir is a very valuable timber species that is desired for harvest in BC (Armleder et al. 1998; Dawson et al. 2007). This opposing, and yet equally important, objectives make the management of these forests complex. The ability of the IDF to provide high-quality food, snow interception, and thermal cover to mule deer makes it an important factor and determinant of mule deer survival in BC (Armleder and Dawson 1992). This highlights the importance of maintaining the stand density and composition of Douglas-fir forests to ensure the continuous provision of winter range habitats.

In BC, mule deer conservation has been designated as a high priority due to the animal’s importance as a game species (Blood 2000). A reduction in winter range habitat, as a result of past forest management such as clearcutting, may be the cause of declining mule deer populations in BC (Armleder et al. 1989). As such, silvicultural treatments that create suitable conditions (i.e. multilayered moderate to high crown cover) were developed to ensure mule deer population survival and increase (Armleder and Dawson 1992; Dawson et al. 2007). These treatments have been comprised of the proportionate removal of tree species, based on their abundance and diameter, to obtain a multilayered uneven-aged stand with a clumpy spatial tree species distribution in the long-term, achieved by focusing on small-diameter and weakened tree removal (Armleder et al. 1986; Armleder and Dawson 1992; Dawson et al. 2007). In light of achieving the competing management objective of timber production and wildlife habitat conservation in the dry forests in BC, these silvicultural treatments continuously hold enormous potential for meeting management objectives through the selective harvest of valuable Douglas fir species.

The term "sustainable forest management" encompasses management that ensures the balance between the extraction, use, and conservation of forest resources and other forest ecosystem services (Wilkie et al. 2003). This also includes the objective of meeting specified goals of management without jeopardizing the environment, now, and for future generations. In the dry

forest, an ideal scenario is a balance between the increasing demand for timber production and the maintenance of biodiversity (e.g. habitat for Mule deer and other small mammals) (Sullivan and Sullivan 2001; Sullivan et al. 2011). This dilemma usually leads to a trade-off between opposing management objectives whereby prioritizing one objective over the other results in negative effects on the other (Rees 2003). For example, management objectives that maintain MDWR requirements or enhance species habitat may impose some restrictions timber harvesting (eg. Armleder et al. 1989; DellaSala et al. 2015; Koot et al. 2015; Sutherland et al. 2016). Similarly, management geared towards maximizing timber production may create conditions with forest structure, composition, and/or function that are undesirable for wildlife habitat (Haggstrom and Kelleyhouse 1996; Sullivan and Sullivan 2001; Thompson et al. 2003; Millington et al. 2011). The effects of forest management may be latent and not realized immediately. However, the long-term impacts could affect stands resilience to successive disturbances (Hessburg et al. 2000, 2005; Allen et al. 2002; Carroll et al. 2004), and influence other ecological processes (Franklin et al. 2002; Palik et al. 2002).

Chapter 3: Douglas-fir natural regeneration dynamics in the Knife Creek block of the UBC AFRF post-disturbance

3.1 Introduction

Interior Douglas-fir (Pseudotsuga menziesii var. glauca Franco) is a dominant species in the Interior Douglas-fir biogeoclimatic zone (IDF) of BC (Hermann and Lavender 1990, 1999; Lloyd et al. 1990; Day 1996; Lemay et al. 2009; Swift and Ran 2012). It is the main tree species in the “dry-belt” region with a continental climate characterized by long and cold winters, and hot and dry summers in most western parts of North America (Hermann and Lavender 1990, 1999; Hope et al. 1991; Pojar and Meidinger 1991). Interior Douglas-fir possesses slower growth relative to the coastal variety, with less stem tapering and a high wood strength, and produces highly valuable timber (Hermann and Lavender 1990, 1999; Day 1996). Additionally, many of these Douglas-fir forests are in close proximity to manufacturing plants for processing, and highways for transportation (Day 1996). Consequently, these attributes make the interior Douglas-fir one of the most important species managed for timber production in BC. More so, the interior Douglas- fir is ecologically important in the maintenance of mule deer winter range (MDWR) because of its ability to intercept snow, and provide food (e.g. arboreal lichens and foliage) and thermal cover (Armleder and Dawson 1992; Waterhouse et al. 1994; Waterhouse 2009). The IDF in the southern interior of BC is characterized by an “open” canopy Douglas-fir forest with understorey patches of shrubland and grassland, and scattered lodgepole pine (Pinus contorta) (Lloyd et al. 1990; Sullivan et al. 2011).

The interior Douglas-fir forests typically experience a mixed severity disturbance regime that shapes their structure and composition (Klenner et al. 2008; Simard 2012; Swift and Ran 2012). Following disturbance and/or mortality, interior Douglas-fir forests are formed from single species or multicohort stands resulting from the regeneration of gaps (Oliver and Larson 1996; Huggard et al. 2005; Lemay et al. 2009). Wildfires have historically been a major disturbance across this landscape, the primary natural disturbances that drive stand regeneration and development are bark beetle infestations, root rots, and attacks by defoliators such as the Douglas- fir tussock moth (Orgyia pseudotsugata), and western spruce budworm (Choristoneura freemani) (Klenner et al. 2008; Simard 2012; Swift and Ran 2012).

The Douglas-fir beetle is an important herbivorous bark beetle native to BC and located within the interior dry forest zone (Erickson 1992). Feeding, reproduction, and the majority of the life cycle (usually one year) of the Douglas-fir beetle is spent under the bark (Rudinsky 1961; Wood 1982; Aukema et al. 2016). Douglas-fir beetle populations are characterized by a low- density endemic phase that consists of infestation of freshly downed trees, tree stumps, large branches, and defensively compromised host trees (Schmitz and Gibson 1996; Humphreys 2000). The occurrence of suitable climatic conditions for Douglas-fir beetle survival, such as drought (Hart et al. 2014a), and the abundance of defensively compromised host tree species or other suitable hosts favours the proliferation of Douglas-fir beetle populations that attack large, vigorous host trees (Rudinsky 1961; Christiansen et al. 1987). The density and basal area of Douglas-fir in a stand (Negron 1998), coupled with the abundance and distribution of defensively compromised host Douglas-fir trees (Dodds et al. 2006) are two useful metrics in determining the overall susceptibility of a stand to an outbreak. Outbreaks of Douglas-fir beetle may leave behind biological legacies related to changes in stand composition, structure, or health upon which other disturbance agents may act on in the future (Buma 2015) such as wildfires and wind.

Regeneration of interior Douglas-fir stands are limited by seasonal deficits in available water, while the occurrence of drought increases the susceptibility of stands to dieback, pathogens, stand-replacing wildfires, and insect outbreaks (Klenner et al. 2008; Littell et al. 2008; Beiler et al. 2015). These disturbances open up growing space (the sum of all factors for tree growth), including physical space for regeneration. Stand development processes in disturbed forests promote forest succession by enabling the establishment of cohorts of tree and enhancing their growth in the understory, as well as creating room for the development of larger size trees in overstory (Oliver and Larson 1996; Ashton and Kelty 2018). Furthermore, forest gaps serve as areas for new plant establishment, enhance the growth of current vegetation (Phillips and Shure 1990), and increase specialized animal habitat (Vitt et al. 1998). Creation of canopy gaps in mature forests tends to affect the population dynamics of species, the speed and trajectory of plant growth, and the heterogeneity of the whole forest in space and time post-disturbance (Oliver and Larson 1996; Ashton and Kelty 2018).

In Douglas-fir forests, growing space availability and limitations determine the structural nature of stands. Thus, they are considered important variables related to the self-organization of

the ecosystem (Simard 2009). A gradient exists in the IDF where areas with lower elevation and lower precipitation tend to be grasslands, while the higher elevation areas that experience increased levels of precipitation tend to favor mixed-species forests (Hope et al. 1991; Huggard et al. 2005). More so, variability in growing space leads to the formation of a mosaic of aboveground and belowground available resources and growing conditions in space, which may be related to tree position around and in gaps, and the sizes of gaps themselves (Gray and Spies 1996; Gray et al. 2012; Ashton and Kelty 2018). Tree regeneration levels and site productivity are two important factors affecting the rate of forest gap canopy closure. Based on the rate of tree regeneration, which is primarily influenced by site productivity, environmental conditions and seed sources, slower and/or low-density tree regeneration in forest gaps may result in increasing the time before canopy closure occurs (Weetman and Vyse 1990; Ministry of Forests 1995).

Forest regeneration is influenced by growing space availability. Biotic factors influencing conifer forest regeneration may be related to facilitation (Simard and Durall 2004; Teste and Simard 2008; Teste et al. 2009; Barker et al. 2014), species traits, stand structure (Dobrowski et al. 2015), seed predation (Vander Wall 2008; Huggard and Arsenault 2009), competition (Dodson and Root 2013), and insect outbreaks (Bentz et al. 2010; Anderegg et al. 2015; Senf et al. 2016). On the other hand, broad-scale abiotic factors, such as climate and topography as well as variations in fine-scale abiotic factors, such as microclimates are critical for conifer regeneration and establishment (Ashton and Kelty 2018). For broad-scale abiotic drivers, discrepancies may exist between climate parameters and regeneration niches of occupied tree species (Dobrowski et al. 2015). Synchrony between the growing season of Douglas-fir and low levels of precipitation in Douglas-fir forests makes soil water availability a limiting factor and an important determinant in tree growth and development, as well as plant community composition (Littell et al. 2008). Responses to growing space availability in canopy gaps have significant effects on the reproduction and growth of vegetation (Oliver and Larson 1996; Ashton and Kelty 2018).

The importance of forest disturbances in the succession of forests is well recognized (Oliver and Larson 1996; Ashton and Kelty 2018). However, an understanding of the effects of forest gaps on spatial heterogeneity of forest processes, such as growth and mortality, and forest succession remains limited. While it is widely known that growing space availability varies within and/or around gaps (Oliver and Larson 1996); observed patterns are not constant across different

forest types and studies (Gray et al. 2002, 2012). Thus, the study of dynamics in species establishment, growth, and composition in gaps remains important for many forest types. A better understanding of the relationships that exist between regeneration processes and patterns of severity of disturbances may be essential to forestall probable shifts in vegetation structure and types, and mitigate the effects of these possible shifts on ecosystem services and ecological processes in landscapes (Turner et al. 2013). Studies on post-fire regeneration have indicated that shifts in the spatial patterns of disturbance severity pose substantial challenges for forest regeneration, especially in cases where there is a mismatch between disturbance scale and severity, and the regeneration traits of the leading climax species (Collins et al. 2017; Stevens et al. 2017). Management of forest stands to maintain biodiversity and a range of essential forest services must be the ultimate goal of forest managers. However, to achieve this goal, it is important for forest managers to have a comprehensive, and in-depth understanding of the dynamics of developmental processes and structures that occur in natural forests.

Knowledge and understanding of the agents (biotic and abiotic) influencing the establishment and growth of interior Douglas-fir is of utmost importance because of the species’ commercial value (Day 1996; Cruickshank 2017) and role in providing forage and habitat for mule deer (Dawson et al. 2006). Limited information exists on the changes in the abiotic environment and their variability over forest succession at different temporal and spatial scales. In interior Douglas-fir forests little research has focused on light availability influences on Douglas-fir regeneration (naturally or planted) and growth, with or without other species (e.g. Chen et al. 1996a, 1996b; Chen 1997; Chen and Klinka 1997). Available growing space is an important consideration for understanding species distribution and other forest ecosystem functions, such as nutrient cycling and photosynthesis (Oliver and Larson 1996). Growing space has the potential to significantly influence the establishment, growth, and development of interior Douglas-fir, when considering physiographic attributes (such as aspect, slope and elevation), drought, precipitation/moisture, light intensity or radiation, and nutrient availability within gaps (Hope et al. 1991; Oliver and Larson 1996; Huggard et al. 2005; Littell et al. 2008; Ashton and Kelty 2018). For example, high temperatures and low precipitation have been reported to affect the growth of interior Douglas-fir (Chen et al. 2010).

In this chapter, I investigated the establishment and growth of interior Douglas-fir post sanitation harvest. I assessed gaps greater than 0.1 ha, in accordance with the regional mule deer winter range strategy regulation that mandates all licensees to regenerate all gaps greater than 0.1 ha under the reforestation phase (CCLUP Mule Deer Strategy Committee 2014). In addition, I assessed the impacts of growing space availability on the establishment and growth of the regenerating interior Douglas-fir seedlings in gaps greater than 0.1 ha in size within forested stands. I sought to understand the dynamics of interior Douglas-fir regeneration and the influence of growing space availability on growth and development to gain insight into the establishment and growth responses of interior Douglas-fir. To achieve this, I assessed the regeneration and growth of interior Douglas-fir after sanitation harvesting implemented because of Douglas-fir beetle attacks, and natural disturbance in gaps greater than 0.1 ha.

3.2 Research objectives and questions

The objectives of this chapter are to:

1. Determine the levels of establishment and growth of interior Douglas-fir regeneration in gaps greater than 0.1 ha following sanitation harvest and natural disturbance. 2. Assess the influence of growing space availability on the establishment and growth of interior Douglas-fir regeneration in the gaps of the interior Douglas-fir stands to understand future species composition and development.

To address the above-stated research objectives, this study will answer the following research questions:

a) What is the role of disturbance on the regeneration of interior Douglas-fir? b) What growing space factors are driving the dynamics of interior Douglas-fir regeneration in the gaps of the research forest?

3.3 Hypotheses and predictions

The establishment and growth of interior Douglas-fir species will increase with increasing available growing space in the gaps of regenerating interior Douglas-fir stands. In addition, growing space availability will have an influence on the establishment and growth of naturally regenerating seedlings in the gaps of the interior Douglas-fir stands.

Based on these hypotheses, I predicted that:

Gaps with an optimum available growing space would have higher establishment and growth of interior Douglas-fir regenerating in gaps. In addition, available growing space will influence Douglas-fir regeneration and seedling growth, and may cause changes in species composition in the short-term. This may lead to an increased stand recovery time of species in the longer term and change the overall stand structure and composition with time.

3.4 Methodology

3.4.1 Study area

The study was conducted in the Knife Creek (KC) block of the University of British Columbia’s Alex Fraser Research Forest (AFRF) situated within the Cariboo Forest District near 150-mile house and adjacent San Jose Valley. The Knife Creek block (52°03’N, 122°49’W,) is approximately 3,500 ha and is approximately located 16 km southeast of Williams Lake, British Columbia. The block has a dry climate with gentle terrain and topography interspersed with steep slopes, short ridges, and gullies. The mean annual temperature of the area is 4.7o C with mean summer night and daytime temperatures ranging from 7.3o C and 22.1oC, respectively. Mean annual precipitation is 569 mm. The area receives 277 mm of precipitation during the 104 days frost-free period from May to September each year (Wang et al. 2012).

The Knife Creek (KC) block is predominantly located in the Interior Douglas-fir biogeoclimatic zone (IDF). However, a small percentage of the block transitions into the Sub- boreal Pine Spruce zone (Day 2007). The KC block covers three biogeoclimatic ecosystem classification (BEC) sub-zones that comprise four ecosystem areas from the east to the west. These are the IDFxm (Very Dry Mild Interior Douglas-fir Subzone), the IDFxm/IDFdk3 transition, the

IDFdk3 (Fraser Dry Cool Interior Douglas-fir Variant), and the IDFdk3/SBPSmk (Sub-Boreal Pinespruce Moist Cool Subzone) transition (Coggins and Jack 2008). The forestland area is managed as a winter range to provide forage and habitat for mule deer (Armleder et al. 1986; Day 1998). Historically, the composition and structure of the forest has been influenced and shaped by several different silvicultural practices, periodic fires, management by First Nations for food-plants and animals, fire suppression, and cattle ranching (Klinka et al. 2004; Day 2007). The leading tree species in the area are Douglas-fir (Pseudotsuga menziesii var. glauca), hybrid spruce (Picea hybrids), and lodgepole pine (Pinus contorta var. latifolia) with Douglas-fir being the dominant tree species and forming about 90% of the total forested area (Jang et al. 2018). Other secondary species present include paper birch (Betula papyrifera) and trembling aspen (Populus tremuloides). Well-drained Luvisols and Bunisols are the dominant soils mainly occurring on the Knife Creek block (Hope et al. 1991). Minimal logging activities were carried out in the management of the Knife Creek block of the Alex Fraser Research Forest subsequent to its establishment in 1987. Pre-commercial thinning and brushing, together with salvage-logging activities were carried out periodically to curb the spread of the Douglas-fir beetle (Day 2007). The major natural disturbances recorded in the Knife Creek block are the periodic occurrences of forest fires historically, but has not burned in a century, and Douglas-fir beetle outbreaks (Day 2007).

3.4.2 Gap selection

A total of 181 individual gaps greater than 0.1 ha were identified in the Knife Creek block of AFRF. Gaps greater than 0.1 ha were identified from digital aerial photogrammetry (DAP) data obtained from drone flight over the Knife Creek block in 2018 (Hamilton 2019). The DAP data from the drone identified all gaps greater than 0.1 ha irrespective of the cause of gap formation. Identification of gaps greater than 0.1 ha in size was carried out before the beginning of fieldwork. I cleaned and verified the DAP data prior to fieldwork using ArcGis by removing gaps created by the construction of pipelines and/or landings (areas cleared for storing and transportation of harvested/salvaged timber), and gaps that were less than 0.1 ha in size. I designated gaps as natural gaps (e.g. arising from windthrow) and salvage gaps (e.g. arising from sanitation from Douglas- fir beetles) based on the agent responsible for disturbance.

Figure 3.1: (A) Map of Canada showing administrative provinces. (B) Map of the province of British Columbia. (C) Map of UBC-AFRF KC block showing gaps and sampling plots.

3.4.3 Data collection

A total of 600 1,000th ha circular plots were randomly installed in the gaps identified in the Knife Creek block (Figure 3.1). Out of these 600 plots, 477 plots were located in salvage gaps and 123 plots were located in natural gaps. Both live and dead seedlings were tallied. The total height of each seedling per plot was measured to the nearest centimetre. The "height" of a seedling was defined as the length of the stem from ground level (uphill side) to the base of the apical terminal bud or the tip of the highest green branch. A measuring tape was used for the height measurement. Green height of seedlings was measured for seedlings with broken or dead tops. The height growth from the previous year was measured for each seedling per plot by measuring the distance between the bud scale scar of the previous year and the base of the apical terminal bud. The diameter of

each seedling was measured using a digital calliper at the collar. The collar of the seedling is the transitional point where the stem and root intersect (forming a junction or little swelling). In instances where the collar was absent, as may be the case for Douglas-fir, measurements were taken at the lowest branch or needle or from 2 mm on the uphill or high side of the seedling parallel to the bole or stem (Menes and Gina 1995). The uphill or high side refers to the highest point of mineral soil or humus layer around the base of the seedling, but not lower than the germination point. The width of the longest branch of each seedling per plot was measured to the nearest centimetre. The "crown width" was defined as the length of the longest branch from the main stem of the seedling to the tip of the branch. A measuring tape was used for the crown width measurement. For tree seedlings with broken or dead branch tips, the green crown was recorded. Data were also obtained on the basal area of surrounding overstory trees by determining the total number of the cross-section of tree trunks and stems at the base per species that occupied each plot using a glass prism of BAF 3. From the centre of each plot, the total number of species was obtained and recorded.

Damage was classified into four categories to assess evidence of browsing/herbivory, stress, and/or mechanical damage. These were: 0 = no damage; 1 = low damage, some impact on growth; 2 = moderate damage, moderate impact on growth or stem quality; and 3 = severe damage, serious impact on growth or stem quality. Vigour of the Douglas-fir seedlings was then assessed. Vigour status ratings of the regenerating seedlings were subjectively assigned to each seedling per plot. Three vigour classes were employed: 1 = Overall good health, good growth condition (single leader, dense and green foliage, damage absent), leader growth (current and previous years exhibiting very good performance), greater than 75% foliage; 2 = Presence of competing neighbours or reduced health and limited growth (lighter and/or less dense foliage, damage present), greater than 50% but less than 75% foliage; and 3 = Severe growth limitation and poor health condition (poor foliage, multiple leaders, browsing, etc.), less than 50% foliage.

The next part of the data was collected on growing space variables including solar radiation and light availability, and physiographic attributes. To measure the amount of solar radiation or light reaching within the forest gaps, hemispherical photographs were taken with the aid of a fisher eye digital camera mounted on a tripod stand levelled horizontally using a bubble level (Hardy et al. 2004). One hemispherical photograph was taken per plot. Photographs were obtained during

uniform sky-conditions (i.e. either early in the morning or late in the afternoon) or uniform overcast days. Images were saved in JPEG and RAW format. Images were processed to obtain the leaf area index (LAI) of the plots. The LAI is a dimensionless quantity defined as the total area leaves cover per unit ground, which directly affects light interception. Data on physiographic attributes (aspect, slope, and elevation) and the topographic wetness index (TWI) were obtained from a digital elevation model (DEM). The TWI modelled from DEM indicates the horizon depth and potential water content of the soil, and is used in hydrological processes to quantify topographic control (Schillaci et al. 2015). Generally, TWI is a derived from fine-scale landform and up-gradient contributing land surface area interactions expressed as TWI = ln [CA/Slope], where; “CA” is the local upslope catchment area that drains through a grid cell and “Slope” is the steepest outward slope for each grid cell measured as drop/distance, i.e., tan of the slope angle (Schillaci et al. 2015). Aspect and slope per plot were also measured using a compass and clinometer in the field for validation of DEM data. Data on site series was extrapolated from the BC Ministry of Forests shape files, whereas site index was obtained from the Site Index Estimates by BEC Site Series (SIBEC).

3.4.4 Data Analysis

The statistical program R (version 3.6.3) was used to analyze research data. Non- parametric methods such as Krustal-Wallis Rank Sum Test, Non-metric Multi-Dimensional Scaling (NMDS) and Generalized Additive Model (GAM), were used to analyze factors associated with interior Douglas-fir regeneration density and growth. Although data was collected on first year seedlings (germinant), they were not considered in the regeneration density and growth analysis of interior Douglas-fir. First year seedlings were not included in the analysis because they were not established seedlings therefore including them in regeneration analysis would lead to increased bias. Regeneration data assessed per plot was extrapolated to per hectare basis to obtain regeneration per hectare for each plot. Following this, the average of the total regeneration per hectare for plots located in each gap was assessed to obtain the regeneration density per hectare per gap. The regeneration density per hectare per gap was used to assess stocking density for each gap. Differences in regeneration density (total trees per hectare per gap) were tested using the Krustal-Wallis Rank Sum Test. The influence of growing space availability and the interactions

between growing space availability on interior Douglas-fir regeneration and growth was modelled using NMDS and GAM to determine relationships. The NMDS was modelled in PC-ORD 6 software with a distance measure set to tie handling (default with no penalty) and autopilot set to “slow and thorough”. The GAM was modelled in R software using the case study number 3 from Fisher et al. (2018) on simple functions for full-subset multiple regression as a guide. Non-linear exponential, negative exponential, and polynomial functions were used to fit observational results to determine relationships. ArcGIS (version 10.7) was used in extracting information on elevation, aspect, slope, and TWI from DEM data. Douglas-fir regeneration status was extrapolated and converted to a per hectare basis for the plots and gaps. Aspect data was transformed using the formula: Transformed aspect = cos (rad (0 - measured aspect)) +1). I modelled TWI from DEM data in ArcGIS. The image processing software, Gap Light Analyzer (GLA; Frazer, Canham, & Lertzman, 1999) was used to analyze and process the digital hemispherical photographs that were obtained per plot in the gaps. The software allowed for the extraction of data on the canopy structure (such as canopy openness and effective leaf area index (LAI), and gap fraction) and gap light transmission indices. Ggplot2 was used to generate all graphs.

3.5 Results

3.5.1 Role of disturbance on interior Douglas-fir regeneration

Interior Douglas fir regeneration density ranged from 0 to 57,000 seedlings per hectare per gap in both natural and salvage gaps, with a mean value of 2,915 seedlings per hectare (Figure 3.2). Interior Douglas-fir germinants (first year seedlings) ranged from 1,000 to 789,000 seedlings per hectare across the gaps, with a mean value of 47,500 seedlings per hectare. Out of the 181 gaps sampled, 49.2% (89 gaps) had a regeneration density between 0-600 seedlings per hectare per gap, out of which 39.8% (72 gaps) had no regeneration (recorded as a zero value). Only four gaps recorded greater than 20,668 seedlings per hectare. Interior Douglas-fir regeneration density was significantly (p<0.05) higher in natural gaps than salvage gaps, although the highest regeneration density was recorded in a salvage gap (Appendix 1). The highest density of interior Douglas-fir regeneration recorded in natural gaps was 48,000 seedlings per hectare, whereas the highest density of regeneration within salvages gaps was 57,000 seedlings per hectare. There was no uniformity in the interior Douglas-fir regeneration density recorded in plots across both natural

and salvage gaps. Rather, regeneration density appeared to occur in clumps or clusters across both gap types. 29.3% (53) of the total gaps assessed were fully stocked, while the remaining 70.7% (128 gaps) were not fully stocked at the required stocking density of 2,500 seedlings per hectare.

Figure 3.2: Douglas-fir regeneration density per hectare per gap in the Knife Creek block, AFRF.

3.5.2 Drivers of interior Douglas-fir regeneration

I modelled the influence of growing space availability on interior Douglas-fir regeneration and growth attributes using NMDS and GAM to determine the effects of growing space availability and the interactions between growing space variables on regeneration and growth. Results showed that the models generated were not a good fit to predict growing space availability nor the effect of interactions on interior Douglas-fir regeneration and growth. Because of this, I report only observational figures on these topics.

Regeneration versus distance to gap edge and slope.

Interior Douglas-fir regeneration density had no relationship with distance to gap edge and slope. Non-linear negative exponential functions were not good fits with high RMSE values. For distance to gap edge, RMSE values were 7,708 and 6,092 for natural, and salvage, gaps respectively. RMSE values of 7,485 and 6,095 for natural and salvage gaps were obtained for the fit for slope. Although a good fit was not obtained, observational results showed that interior Douglas-fir appeared to decline further away from the gap edge and as slope of plots became steeper for both natural and salvage gaps (Figure 3.3). Higher regeneration density was recorded between 0 m and 5 m from the distance to gap edge, and between 0% and 50% slope in both natural and salvage gaps.

Figure 3.3: Douglas-fir regeneration density per hectare by distance to gap edge, and slope in natural and salvage gaps.

Regeneration versus LAI and TWI

The LAI and TWI variables had no evident influence on the regeneration density of interior Douglas-fir. Non-linear polynomial functions did not provide a good fit with high RMSE values. LAI RMSE values were 7,598 and 6,066, and TWI RMSE values were 7,712 and 6,102 for natural and salvage gaps, respectively. However, observational results showed that interior Douglas-fir regeneration somewhat appeared to increase up to a peak point and then appeared to decline as the LAI and TWI values became greater (Figure 3.4). The highest interior Douglas-fir regeneration density recorded for LAI was slightly diffirent and occurred at 1.23 within a natural gap and 0.88 within a salavge logging created gap. Similarly, values for TWI were 3.66 within a natural gap and

2.84 inside a salvage gap. The highest interior Douglas-fir regeneration density in both natutral and salvage gaps occurred on a relatively dry site (3.66 and 2.84 out of 12.28) with a mid-range LAI value (1.23 and 0.88 out of 2.67). A comparison between interior Douglas-fir regeneration density and the LAI and TWI variables recorded, showed a fairly similar observation in both natural and salvage gaps. Statistically, LAI values recorded were significantly higher in natural gaps compared to salvage gaps (Appendix 5) while TWI values did not differ between gap types (Appendix 6).

Figure 3.4: Douglas-fir regeneration density per hectare versus leaf area index and topographic moisture index in natural and salvage gaps.

Regeneration versus elevation and aspect

Elevation and aspect had no significant influence on interior Douglas-fir regeneration density across natural and salvage gaps. Non-linear exponential functions were not a good fit for aspect with high RMSE values of 7,707 and 5,826 for natural and salvage gaps, respectively. For elevetion, the calculated non-linear exponential functions did not fit. Though good fits were not obtained, observational results showed that interior Douglas-fir regeneration density appeared to be greater at higher elevation (Figure 3.6). Aspect was transformed from a circular radiance (0- 3600) into a linear form (0-2) at 00, where 0 represents true south and 2 represents true north. For the aspect, most regeneration occurred between 1.5 to 2 (towards the true north) in both natural and salvage gaps (Figure 3.6). While the study site has mostly south and west-facing slope aspects and elevation ranging between 700 m to 1,000 m, higher densities of interior Douglas-fir regeneration occurred at an elevation greater than 900 m in both natural and salvage gaps.

Figure 3.5: Douglas-fir regeneration density levels per hectare versus elevation and aspect in natural and salvage gaps.

Regeneration versus site series and site index.

Interior Douglas-fir regeneration density had no relationship with the site series and site index values of the plots. Non-linear negative exponential functions for site series and non-linear polynomial functions for site index did not fit well with very high RMSE values. RMSE values for site series were 7,330 natural gaps and 6,046 for gaps. For site index, RMSE values were 7,499 and 5,895 for natural and salvage gaps, respectively. Results however showed that interior regeneration density appeared to reduce with an increase in site series number for natural gaps, while salvage gaps displayed a somewhat “V-shaped” observational relationship with regeneration density (Figure 3.6). Based on an edatopic grid, most of the interior Douglas-fir regeneration occurred on mesic (moderate moisture) sites with a range of poor to rich soil nutrients (site series “01”- FdPl- Pinegrass-Feathermoss site). On the other hand, interior Douglas-fir regeneration density appeared to occur the most on areas with a site index of 17.1 for both natural and salvage gaps (Figure 3.6). A similar pattern for regeneration density compared to site index was observed for both natural and salvage gaps. Site series and site index values were significantly higher (p<0.001) in natural gaps than in salvage gaps (Appendix 10 and 11).

Figure 3.6: Douglas-fir regeneration density per hectare versus site series and site index in natural and salvage gaps.

Regeneration versus ground grass and soil

Ground vegetation cover had no obvious relationship with interior Douglas-fir regeneration. Non-linear polynomial functions for grass cover and non-linear negative exponential functions for soil cover did not fit well with high RMSE values. Grass cover RMSE values were 7,518 and 6,084, and soil cover RMSE values were 7,669 and 6,087 for natural and salvage gaps, respectively. Observational results however showed that for most of the plots sampled there was greater amount of ground grassy vegetation than available bare mineral soil (Figure 3.7).

Figure 3.7: Douglas-fir regeneration density per hectare versus ground grassy vegetation and soil.

Mortality of Douglas-fir seedlings was low across sampled plots in both gap types. Overall, the total percentage of dead seedlings was 1.6% in natural gaps and 3.8 % in salvage gaps. Correspondingly, the number of alive seedlings was 98.4% in natural gaps and 96.2% in salvage gaps.

A substantial percentage of undamaged interior Douglas-fir seedlings was recorded for natural gaps (52.7%) and salvage gaps (51.4%) (Table 3.1). For damage, there was a slightly lower record of low and severe damage (1 and 3) for interior Douglas-fir seedlings in sampled plots within natural gaps compared to salvage gaps but slightly higher moderate damage (damage 2) of interior Douglas-fir seedlings in natural compared to salvage gaps.

Status Natural (%) Salvage (%) 0 52.7 51.4 1 21.8 22.3 2 19.1 18.2 3 6.4 8.1

The result on the vigour of interior Douglas-fir seedlings revealed that the majority of interior Douglas-fir regeneration in both the natural and salvage gaps are in good health (Table 3.2).

Table 3. 2: Summary of vigour status of seedlings in natural and salvage gaps across sampling plots (1- Overall good health and good growth condition with greater than 75% foliage; 2- Reduced health and limited growth with greater than 50% foliage but less than 75% foliage; 3- Severe growth limitation and poor health condition with less than 50% foliage).

Status Natural (%) Salvage (%) 1 57.6 61.5 2 37.4 28.9 3 5.0 9.6

Basal area of the surrounding overstory trees was dominated by interior Douglas-fir trees in natural gaps (40.3%) and salvage gaps (36.9%) (Table 3.3). However, salvage gaps contained a higher percentage of Hybrid spruce trees than natural gaps (Table 3.3).

Table 3. 3: Summary of basal area contributed by each species surrounding natural and salvage gaps across sampling plots (Fd - Douglas-fir (Pseudotsuga menziesii var. glauca); Sx - Hybrid spruce (Picea hybrids); At- Trembling aspen (Populus tremuloides); Ep - Paper birch (Betula papyrifera); Pl - Lodgepole pine (Pinus contorta var. latifolia).

Species Natural (%) Salvage (%) Fd 40.3 36.9 Sx 20.2 23.7 At 17.4 16.6 Ep 12.4 12.9 Pl 9.7 9.9

3.5.3 Drivers of interior Douglas-fir growth

Crown width growth compared to LAI and TWI

There was no relationship between the influences of growing space variables on interior Douglas-fir growth. Nonlinear polynomial functions were not good fits for the influence of LAI and TWI on crown width of interior Douglas-fir. RMSE values for LAI and TWI were 14 and 8 for natural and salvage gaps, respectively. It was observed that the crown width growth of regenerating interior Douglas-fir was sporadic in natural and salvage gaps when compared to the LAI variable. On the other hand, crown width growth appeared to lessen with increasing TWI values for seedlings of interior Douglas-fir, which follows the trend for regeneration density (Figure 3.7). The total height and crown width recorded in the natural gaps was significantly (p<0.05) higher than the total height and crown width recorded in the salvage gaps. Interior Douglas-fir crown width growth showed similar patterns for LAI and TWI variables in natural and salvage gaps.

Figure 3. 8: Douglas-fir regeneration crown width growth versus topographic moisture index in natural and salvage gaps.

Total height growth versus LAI and TWI

Growth attributes of regenerating interior Douglas-fir, specifically total height growth, likewise had no relationship with LAI and TWI variables. Similarly, nonlinear polynomial functions did not fit well for LAI and TWI influences on total height of interior Douglas-fir seedlings. RMSE values for LAI were 28 and 24, and 28 and 26 for TWI within natural and salvage gaps, respectively. However, total height growth appeared to show an observational sporadic relationship with the LAI variables recorded in both natural and salvage gaps. On the other hand,

interior Douglas-fir height growth observationally appeared to decline with higher TWI values in both natural and salvage gaps (Figure 3.8) which follows the trend of regeneration density. Interior Douglas-fir total height growth showed similar patterns for LAI and TWI variables in natural and salvage gaps.

Figure 3.9: Douglas-fir regeneration total height growth versus leaf area index in natural and salvage gaps.

3.6 Discussion

3.6.1 Role of disturbance on interior Douglas-fir regeneration

The goal of the research was to assess and quantify the status of interior Douglas-fir regeneration in gaps greater than 0.1 ha in the AFRF. Regeneration of interior Douglas-fir was highly variable across both gap types, and usually appeared to occur in clusters that were dispersed within gaps. The results revealed that at a required stocking of 2,500 seedlings per hectare, natural regeneration was not sufficient with 70% of the gaps assessed being understocked. Out of this, approximately 40% had no interior Douglas-fir regeneration. Possible reasons for the low and variable interior Douglas-fir regeneration in these forests could be related to the supply of seeds/seedfall densities, as well as conditions associted with the seedbed and the environment in general (Huggard et al. 2005). In addition, regeneration of interior Douglas-fir is known to be enhanced when mineral soils are available. As such, an unavailability of a suitable seedbed and suitable microsite conditions may affect the regeneration rate of interior Douglas-fir (Huggard et al., 2005; Coleman et al., 2014).

My findings are consistent with the study of Huggard et al. (2005) that indicated that dry interior Douglas-fir forests in the IDF zone are known for expressing highly variable natural regeneration in the amount, and timing, of seedlings. In addition, similar to my finding of interior Douglas-fir regeneration density as high as 57,000 seedlings per hectare across the gaps, Burton et al. (2000) reported a dense interior Douglas-fir regeneration density ranging between 1,533 - 30,433 stems per hectare across different treatments of rotting wood, mineral soil, intact forest floor, and live moss mats. Vyse et al. (2006) documented that poor regeneration in the dry interior Douglas-fir forests results from the combined effects of climatic variables (hot, dry conditions; characterizing the area), unsuitable or inadequately prepared sites, herbivory (e.g., livestock, small mammals), and competition with other vegetation (e.g., grass). My findings suggest that most of the gaps sampled had a greater percentage of grass compared to bare mineral soil, which may contribute to the low and variable interior Douglas-fir regeneration across the gaps. This observation is in agreement with the study of Simard et al. (1998) who found that there was a significant increase in interior Douglas fir regeneration when large patches of pinegrass in the stand was removed from the understory. Furthermore, Huggard et al., (2005) reported that slow regeneration was observed in individual tree selection (ITS) treatments attributed to erratic

seedfall, high seed predation, and hot dry summers leading to poor germination. This could be said to be the case for AFRF since sanitation harvests to curtail beetle populations may mimic ITS prescriptions. A study of natural regeneration in a uniform shelterwood dominated by interior Douglas-fir indicated that there was considerable variation in seed rain density and seed predation every year. However, seed predation and losses were significantly higher in partially cut stands compared to uncut stands and ranged between 70.8 - 99.6% over autumn and winter (Burton et al 2000). All of these discussed factors may have contributed to the low and variable regeneration found within the gaps I sampled of the AFRF. In contrast to my observations, Waterhouse (1998) reported a sufficient natural regeneration of 2,000 seedlings per hectare, with the exception of stands with steep southerly slopes and low overstory crown closure, irrespective of partial cutting harvesting density a decade after disturbance. However, this study was a summary of two unpublished data sets from variable partial cuttings that occurred between 1980 and 1987 in the IDF forest. The disparity observed may therefore stem from the difference in the years before the post-disturbance study, disturbance type (partial cutting harvesting densities) and severity (low, medium and/or high), seed productivity and source, seedbed conditions, and growing space availability from created gaps. The time-span post disturbance influences stand development processes such as regeneration, establishment and growth of tree species (Oliver and Larson 1996). This study is limited in the ability to report on regeneration density of interior Douglas-fir following Douglas-fir beetle disturbance at a specific time (year). Temporal data on the years of salvage of interior Douglas-fir beetle infected trees leading to gap formation in the KC block of AFRF could not be assessed and processesd. This study is therefore not able to predict the influence of time post-disturbance on the regeneration of interior Douglas-fir.

3.6.2 Drivers of interior Douglas-fir regeneration

My findings on the effect of growing space availability on interior Douglas-fir regeneration density and growth were observational. Models to predict growing space availability on interior Douglas-fir regeneration and growth were weak and had very small R2 values (<1) for both the NMDS and GAM used. The inability of the models to predict the effect of growing space availability on regeneration and growth of interior Douglas-fir could stem from the high number of zero counts (no regeneration) within roughly 40% of recorded plots. Alternatively, Zero-Inflated

Poisson regression (ZIP) and Zero-Inflated Negative Binomial regression (ZINB) models could be used to attempt to predict the effects of growing space availability on interior Douglas-fir regeneration and growth. However, the predictability of these models may be limited by over- dispersed data in which variance is much larger than the mean. Although observational, my findings suggest that there may be a possible effect of growing space on interior Douglas-fir regeneration and growth, however, additional studies need to be conducted.

I sought to determine the effects of available growing space factors driving the regeneration of interior Douglas-fir in natural and man-made gaps. My findings demonstrated that there was no statistically significant difference on the influence of growing space availability on interior Douglas-fir regeneration. However, observational findings indicated that with increasing distance to gap edge and slope, there appeared to be a decrease in interior Douglas-fir regeneration density for both gap types. In addition, there appeared to be a greater density of interior Douglas-fir regeneration with increasing elevation gained, which may be caused by the lower temperatures and greater amounts of precipitation associated with the higher elevations within the study site. These observations could be attributed to the attraction and/or repulsion of interior Douglas-fir seedlings in response to variability in available growing space in the gaps. In addition, a weak irregular relationship was observed for the effects of LAI, TWI and aspect on interior Douglas-fir regeneration across the gaps. However, a greater proportion of interior Douglas-fir regeneration occurred slopes with a northerly aspect. This observation could be associated with cooler and wetter aspects towrds the north and hence the regeneration thereof. Because LAI values, which are associated with light, did not show any particular relationship with interior Douglas-fir regeneration, I cannot specifically conclude that light had a role to play in this observation. Similarly, modelled TWI was comparable to the LAI relationship with interiror Douglas-fir regeneration. The TWI is a function of moisture for the site.

My study revealed that the mortality rate for interior Douglas-fir regeneration in both the natural and salvage gaps was low. The vigour and damage assessment of interior Douglas-fir regeneration in the sampled gaps indicating that a greater proportion of the seedlings were in good health, displayed adequate growth, and were undamaged. These observations demonstrate that the low and variable interior Douglas-fir regeneration rate recorded in the gaps is not a result of seedling mortality. Additionally, my findings revealed that interior Douglas-fir remained the most

abundant tree species in the surrounding overstory canopy, despite the removal of interior Douglas-fir trees as a result of salvaging logging for beetle infected trees and natural disturbances from beetle infestation. However, there is a possibility for the stand structure and composition to change in the future if the regeneration of interior Douglas-fir post-disturbance is low and variable.

Interior Douglas-fir regeneration may respond differently to any one factor or a combination of factors of available growing space in a gap (Weetman and Vyse 1990; Oliver and Larson 1996). Regeneration and recruitment of species in gaps of IDF forests may undergo a pattern of attraction or repulsion to large trees based on limiting factors present at these sites (Lemay et al. 2009). In the dry IDF forest, interior Douglas-fir seedlings regenerated closer to large trees than away from them in large gaps due to the moisture microsites provided by large trees (Lemay et al. 2009). In addition, a possible explanation to the observed attraction of interior Douglas-fir regeneration to gap edge with large trees may be attributed to ectomycorrhizal (EM) fungal networks that may be promoting seedling establishment (Teste and Simard 2008; Bingham and Simard 2012; Simard 2012; Barker et al. 2013). Furthermore, growing space availability provides a mosaic of available resources and growing conditions in space and time that is variable above and belowground which may be associated with tree position around and/or in gaps and gap sizes in Douglas-fir forests (Oliver and Larson 1996; Gray et al. 2012). In agreement with my findings of declining interior Douglas-fir regeneration with increasing gap sizes, Huggard et al. (2005) found that there was no natural regeneration in larger openings after harvesting. Also, the interior Douglas-fir differs in its growth on a broad range of site conditions, ranging from high elevations with cool and moist sites, to low elevations characterized by warm and dry conditions. Growth occurs on soils with a diverse range of parent materials, some of which are associated with the species’ growth limitations, such as low nutrient availability (Coleman et al. 2014). The dynamics in precipitation and elevation have been reported to influence the distribution of interior Douglas-fir forests initiating transitions from Douglas-fir to grasslands at lower precipitation threshold and lower elevation, to mixed species of Douglas-fir forest at higher precipitation threshold and higher elevations (Hope et al. 1991; Huggard et al. 2005). This findings correspond to those of Hope et al. (1991) and Huggard et al. (2005) in that greater interior Douglas-fir regeneration density was observed at higher elevations.

Moisture, nutrient availability, and light availability are three factors that effect spatial pattern changes in forest stands over time and can limit forest growth (Lemay et al. 2009; Gower et al. 1992). In a study on 14 conifer species, including Douglas-fir (var. glauca and v. menziesii), Li et al., (1994) found that Douglas-fir germination had a positive response to light in the absence of seed stratification, but was unaffected by light when stratified. IDF forests are characterized by light and moisture limiting factors. On wetter sites of the IDF where light is the limiting factor in Douglas-fir regeneration it leads to a clustering of small trees away from large trees showing a pattern of repulsion (Lemay et al. 2009). However, my findings suggest that light was not a limiting factor for interior Douglas-fir regeneration within my study site. Equally, moisture is the limiting factor in drier sites of the IDF forests leading to the formation of a small cluster of tree regeneration closer to large trees where soils are moister depicting a pattern of attraction (Lemay et al. 2009). My results partially agree with this observation in that regeneration was higher closer to gap edges where large trees were present and vice versa. However, the TWI had a weak relationship with interior Douglas-fir regeneration, which may be caused by the modelling of TWI from DEM data that was not at a high enough resolution to capture the differences in microsites of the sampled plots. Tree regeneration and site productivity are two important factors that affect the rate of forest gap canopy closure, which is important for MDWR. Slower seedling growth and reduced densities in regeneration in forest gaps may result in prolonging the number of years before stand canopy closure occurs, sometimes by as much as several decades (Weetman and Vyse 1990; Ministry of Forests 1995; Franklin et al. 2002). The variable and low rate of regeneration recorded indicates that the MDWR will be strongly affected and could become inadequate if natural regeneration is not supplemented with active planting.

Furthermore, given that a high percentage of the gaps did not meet regeneration requirements (understocked), and a low seedling mortality rate was recorded, it can be deduced that most of the seedlings (germinant) established at one time, but that this initial establishment was not enough to meet current regeneration guidelines (Burton et al 2000). Severe damage of interior Douglas-fir seedlings was low. However, seedling vigour and damage were both higher in salvage gaps compared to natural gaps. This could be attributed to a higher exposure of seedlings to unfavourable environmental conditions, such as summer frost and winter damages post- disturbance but also more light which favours growth. More so, compaction of soil from machine use, and debris from logged trees and fallen branches could lead to a reduction in a suitable seedbed

for Douglas-fir regeneration and cause reduced growth. The optimal conditions for species regeneration, growth, and development are continuously variable at the microsite level occurring in the form of microsite topography, climate, shading effects from other plants, and seedbed conditions (Li et al. 1994; Huggard et al. 2005). An imbalance in resource and nutrient availability at growing sites of interior Douglas-fir forests may reduce the defensive capabilities of the trees leading to increased mortality (Mandzak and Moore 1994). The foliar nutrient ratios of Douglas- fir on 131 study locations revealed that the majority of Douglas-fir stands were below optimum nutrient contents (Coleman et al. 2014). This suggests that IDF within the study site are likely nutrient deficient. Since my study did not consider the nutrient availability of the sites, I am unable to conclude that the variable regeneration rate may be attributed to nutrient dynamics, but it is a factor worth considering. In addition, competition from vegetation has the potential to limit interior Douglas-fir regeneration and growth by creating intolerable shade levels and competing for soil moisture (Hermann and Lavender 1990; Simard et al. 1998). All these factors can affect the future stand structure and composition of the forest by limiting regeneration. If external efforts are not implemented (such as planting), it is possible that the MDWR habitat may be reduced in quality or size (Sullivan & Sullivan, 2001). The effects of current management may not realized immediately, even though long-term forest structure, function, and composition may be altered to affect overall stand resiliency (Hessburg et al. 2000, 2005; Allen et al. 2002; Carroll et al. 2004) and other ecological processes (Franklin et al. 2002; Palik et al. 2002).

3.6.3 Drivers of interior Douglas-fir growth

Post-regeneration, interior Douglas-fir seedlings utilize available growing space for establishment and future growth. I assessed the effects of growing space factors on the growth of regenerating interior Douglas-fir seedlings in salvage and natural gaps. There was no statistically significant relationship between modelled growing space variables and interior Douglas-fir growth. However, observational findings suggest that interior Douglas-fir total height and crown width growth increased in growth proportionally. Proportional vertical and lateral growth is a characteristic of interior Douglas-fir. Total height and crown width growth was relatively higher (p<0.05) in natural gaps than in salvage gaps. Total height and crown width growth were both

sporadic when compared to LAI across gaps. Total height and crown growth appeared to be less when TWI values were higher.

Growing space variability and dynamics also affect individual tree growth after seedling establishment has already occurred. For example, in response to low light availability, interior Douglas-fir reduces its apical growth and allocates more resources towards lateral growth to help maintain a live crown (Williams et al. 1999). This serves as an adaptation to capture sun flecks reaching the understory. With increasing light availability, crown depth increases because of more light reaching the lower branches whereas more mortality of lower branches is expected with decreasing light availability due to self-shading (Williams et al. 1999). In a similar study to determine the effects of light availability on the growth of seedlings, it was observed that with decreasing light availability naturally regenerated interior Douglas-fir seedlings decreased in the incremental terminal shoot growth to incremental lateral shoot growth but increased in specific leaf area (Chen et al. 1996). At a low photosynthetic photon flux density, Chen and Klinka (1997) observed that understory-grown seedling branches were more photosynthetically efficient relative to branches of open-grown seedlings. These observations are consistent with the findings of my study, in that I found crown width growth to be greater in natural gaps with a higher LAI and lower light availability.

The spatial variability in the intensity, duration, and availability of light among the branches of different species of seedlings (Chen and Klinka 1997) could be attributed to the distances (and position) of these seedlings from (or in) gaps (Canham et al. 1990) and the resultant effects of clumping from surrounding overstory tree canopies (Chen and Black 1992). In a study on Lodgepole pine and Douglas-fir in interior BC, specific leaf area of both species decreased with decreasing light availability, however, Douglas-fir demonstrated higher plasticity to specific leaf area and crown architecture to a gradient of light availability (Chen et al. 1996). Another similar study conducted by Vyse et al. (2006) on interior Douglas-fir, Lodgepole pine, and Ponderosa pine under varying canopy covers and conditions, ranging from open to closed, and all aspects and slopes, in a hot dry interior Douglas-fir forest, showed that the level of light available strongly affected the survival and growing conditions of all species. Vyse et al. (2006) found that in species response to growth, there was no threshold for canopy closure, and that the most open canopy conditions facilitated the best seedling survival and growth. However, the largest gap size used

was 0.1 ha. I did not find a strong correlation or relationship between Douglas-fir regeneration and growth and light availability indices. However, I assessed gaps greater than 0.1 ha in size, which may be one of the reasons for the disparity in my observations. In addition, this could also mean that smaller gaps are the most preferred for the regeneration and growth of Douglas-fir fir in terms of light availability. Vyse et al. (2006) also observed that total light available significantly affected seedlings growth, while slope, aspect, and crown closure had minimal impacts. This observation is consistent with our findings of a weak relationship between interior Douglas-fir growth and growing space variables.

Finally, regeneration and growth of interior Douglas-fir may be influenced by many factors, biotic and abiotic, over different temporal and spatial scales. However, the intensity of influence of these factors may be weak or strong depending on the variability of prevailing factors.

Chapter 4: General conclusions

My research provides forest managers at the AFRF with first-hand comprehensive information regarding the natural regeneration dynamics, and influence, of growing space dynamics on interior Douglas-fir establishment and growth in the gaps of the research forest post- disturbance. My study will help managers make informed decisions during the design of silvicultural systems for the Knife Creek block for interior Douglas-fir stand improvement, as well as improve management decisions related to Douglas-fir planting. Furthermore, this research provides useful information on the drivers that potentially affect the regeneration of interior Douglas-fir and will help in the formulation of policy and implementation of best practices aimed at improving the management within the AFRF, and other interior Douglas-fir forests in the MDWR.

4.1 Main findings

This research established that roughly 70% of gaps greater than 0.1 ha in size within the AFRF Knife Creek block are understocked and do not meet the requirements of the reforestation phase plan established for MDWR management and the Cariboo-Chilcotin sustainable land use plan. However, I did find gaps with very dense interior Douglas-fir regeneration, up to 57,000 seedlings per hectare. The mortality rate was low for interior Douglas-fir seedlings and approximately 50% of the regenerating seedlings were undamaged, in good health, and displayed high vigour. The good health and vigour of the majority of interior Douglas-fir seedlings, coupled with the high proportion of seedlings showing no signs of damage, illustrate that low regeneration may be attributed to other factors other than the death of established seedlings. The stand composition, based on the basal area of surrounding overstory trees, demonstrates that interior Douglas-fir remains the leading tree species in the area. However, maintaining current practices may alter the future composition of this forest and affect MDWR management prescriptions.

I do not accept the hypothesis that the establishment and growth of interior Douglas-fir will increase with increasing available growing space in the gaps of regenerating Douglas-fir stands, and that growing space availability will have an influence on the establishment and growth of naturally regenerating seedlings in the gaps of interior Douglas-fir stands. However, only light

(LAI) was directly measured using hemispherical photographs whereas moisture (TWI) was modelled from DEM data that may not have reflected microsite conditions. The influence of growing space availability variables on interior Douglas-fir regeneration that was modelled using NMDS, GAM and non-linear negative exponential, exponential and polynomial functions did not produce good fits. The relationship of interior Douglas-fir regeneration to growing space variables were mostly sporadic, with some growing space variables suggesting a sort of false negative exponential, exponential and/or polynomial relationship with interior Douglas-fir regeneration. Similar trends and relationships were observed in both natural and salvage gaps for interior Douglas-fir regeneration. The elevation of plots showed a somewhat false positive exponential relationship with Douglas-fir regeneration, which could be related to the site index and productivity, as well as precipitation and temperature. The growing space variables that showed a somewhat false negative exponential relationship with interior Douglas-fir regeneration were distance of plots to gap edge and slope of plots. On the other hand, LAI, TWI, and aspect variables did not reveal a definite trend or relationship when compared with Douglas-fir regeneration.

Likewise, growth attributes of interior Douglas-fir regeneration were variable when compared to growing space variables. Growing space variables influence on interior Douglas-fir regeneration growth attributes that was modelled using NMDS, GAM and non-linear negative exponential, exponential and polynomial functions likewise did not produce good fits. Most of the observational relationships for growing space variables influence on interior Douglas-fir regeneration growth attributes were sporadic for both natural and salvage gaps or showed a form of false negative exponential relationship.

4.2 Management implications

Management of the interior Douglas-fir dry forests is complicated by competing objectives related to the optimization of timber production, and maintaining wildlife habitat. Unrestricted timber harvesting may lead to the alteration of the structure, composition, and function of the forest (Haggstrom and Kelleyhouse 1996; Sullivan and Sullivan 2001; Thompson et al. 2003; Millington et al. 2011). However, management objectives are in place for maintaining landscapes suitable for wintering range of mule deer, which may put restrictions on the extraction of timber resources (e.g. Armleder et al. 1989; DellaSala et al. 2015; Koot et al. 2015; Sutherland et al. 2016).

Natural regeneration of interior Douglas-fir has been practiced following stand-level partial cutting prescriptions in dry Douglas-fir forests in BC for the past several decades (Li et al. 1994; Vyse et al. 1998; Waterhouse 1998). Interior Douglas-fir regeneration after disturbances in the dry interior Douglas-fir forest (including the KC block of AFRF) of BC is an important prerequisite for a successful and sustainable silvicultural system to balance the competing objectives of timber production and MDWR management. Regeneration of trees following disturbances can be achieved naturally, or together with active planting (Weetman and Vyse 1990; Ministry of Forests 1995). For the KC block of AFRF, interior Douglas-fir natural regeneration must be supplemented with planting to help meet the current stocking requirements that falls within historic range within the shortest possible time following a disturbance. This is an important management activity to ensure the continuous provision of mule deer wintering habitat and to maintain current stand compositions and structure (Dawson et al 2006; Sullivan et al 2011). However, if only natural regeneration is used to restock stands, it may be several decades before the MDWR is actually impacted (Chapter 3; Weetman and Vyse 1990).

In the past, declining mule deer populations in BC have been attributed to a reduction in winter range habitat as a result of past forest management activities, such as clearcutting (Armleder et al. 1989). However, this eventually led to development of silvicultural treatments that created multilayered moderate, to high crown cover to ensure mule deer population survival (Armleder and Dawson 1992; Dawson et al. 2007). Unfortunately, using natural regeneration in the IDF may not maintain this multilayered stand structure in the future (Chapter 3; Huggard et al. 2005) and mule deer populations may decline due to the unavailability of suitable habitat, lack of snow interception, and reduced forage (Waterhouse et al. 1991; Armleder and Dawson 1992; Armleder et al. 1994).

Current conditions within the IDF could be considered outside the historic range of variability. Disturbance regimes within these forests have transformed from one primarily dominated by fire, to one that is dominated by harvesting and insect infestation (Klenner et al. 2008). However, fires have played an instrumental role historically in natural regeneration in this area. Natural regeneration is promoted by fire in the IDF due to its ability to destroy the seed bank of potential competitor species (Daigle 1996). In addition, bare mineral soils that are exposed post fire are the most favorable for natural regeneration of interior Douglas-fir. Therefore, besides the

use of active planting to supplement natural regeneration, prescribed burning can be used to remove competing forms of vegetation and provide bare mineral soil to increase the potential of successful interior Douglas-fir regeneration. It is however important to note that some fire types may provide suitable areas for the establishment of competitor species (Simard et al. 1998), and destroy all ground vegetation. Daigle (1996) reported that more frequent low severity surface fires gave room for the creation of small gap openings that allowed for the germination of trees, forbs, shrubs, and grasses providing interspersed shrubby and grassy openings in an uneven-aged stand. More so, mechanical site preparation practices, such as scalping, ripping, plowing, and trenching, could be used to provide suitable seedbed conditions, preferably bare mineral soil, to improve natural regeneration of interior Douglas-fir.

In summary, no one technique or prescription will suffice to address all the challenges and concerns of forest management within the IDF, including the conflicting management objectives of timber production optimization and maintenance of MDWR. The future stand structure, function, and composition of a forest may be impacted in the long term by silvicultural prescriptions for forest management, although they may not be observable immediately (Hessburg et al. 2000, 2005; Allen et al. 2002; Carroll et al. 2004). Interior Douglas-fir stands are highly variable and will require carefully applied site-specific prescriptions to meet management objectives (Waterhouse 1998). Moving forward, a suite of management prescriptions are needed to address the complex management objectives of the IDF forests that are centered around timber production, maintaining the MDWR, and providing opportunities for public recreation (Armleder et al. 1986; Day 1996; Klenner and Walton 2009).

4.3 Future research

My research addressed the knowledge gap that exists regarding the patterns of natural regeneration dynamics and the drivers of these patterns in the dry interior forest zone of British Columbia, using the Knife Creek block of the Alex Fraser Research Forest as a study site. There is additional need to determine the effects of these observed patterns of natural regeneration on future stand structure and composition, and for maintaining MDWR habitat amidst continuous Douglas-fir beetle outbreaks. This is particularly important for the interior Douglas-fir forests where MDWR management and Douglas-fir beetle management interact. In addition, my data

collection took place during only one-season, following 2 years with extremely dry summers and increased wildfire activity. Fluctuations in seasonal precipitation have the ability to influence soil moisture. Since, TWI showed a weak relationship with regeneration and growth, which could be attributed to the resolution of the DEM data used in modelling TWI, it is important to install a network of soil moisture sensors across the research forest to obtain a true reflection of soil moisture levels across the sites. In addition, a similar experiment could be conducted in the same study area using fixed permanent plots to ascertain the effects of all growing space variables on regeneration, establishment, and growth of species in these dry forests. This would allow the continuous recording of seedling establishment and growth attributes as influenced by growing space factors in a time series over a period of years. This will further allow comparisons to be made on the significance of the influence of growing space factors on establishment and growth of species with time, and the quantification of the role of these factors in natural regeneration dynamics. In addition, re-running the analysis of the data in a more advanced model (if possible) to help predict current trends in regeneration and growth of species post-disturbance as influenced by growing space factors could provide managers with a better idea of the effects of growing space factors on the dynamics of regeneration of species in dry interior Douglas-fir forests post- disturbance.

Finally, while the importance of maintaining late-successional species for MDWR management in the dry interior Douglas-fir forests has been demonstrated (Sullivan et al. 2011), the dynamics of natural species regeneration and the drivers of these regeneration dynamics have received less attention in the past decades. By far, most regeneration concerns have been directed towards the planting of seedlings to meet stand density requirements. It is extremely important that these forests are regenerated. However, it is also important that we address the question of how many seedlings must be replanted to meet stand density requirement if the dynamics of natural regeneration are unknown and not quantified. The answer to this question will have significant implications on management decisions and help managers in making more informed decisions regarding replanting programmes post-disturbance. Additionally, this may have economic implications, as planting seedlings can be cost prohibitive and must be carried out in the most financially feasible way possible. My findings may serve as a baseline for future studies looking at the same growing space availability variables. This could help identify patterns and trends in the observed variable natural regeneration dynamics of species in the dry interior Douglas-fir

forests, and enable the prioritisation of areas for replanting programmes. Analogous to the work of Huggard et al. (2005), the establishment of silvicultural trials across all biogeoclimatic variants of the dry IDF forest zone could identify patterns of natural regeneration dynamics within landscapes managed for timber production, mule deer habitat, and recreation.

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Appendices Appendix 1: Boxplot of Douglas-fir regeneration in natural and salvage gaps.

p=0.044

Appendix 2: Boxplot of distance to gap edge in natural and salvage gaps.

p =0.048

Appendix 3: Boxplot of slope in natural and salvage gaps.

p <0.001

Appendix 4: Boxplot of elevation in natural and salvage gaps.

p = 0.003

Appendix 5: Boxplot of LAI in natural and salvage gaps.

p <0.001

Appendix 6: Boxplot of TWI in natural and salvage gaps.

p =0.868

Appendix 7: Boxplot of aspect in natural and salvage gaps.

p = 0.007

Appendix 8: Boxplot of site series in natural and salvage gaps.

p <0.001

Appendix 9: Boxplot of site index in natural and salvage gaps.

p <0.001

Appendix 10: Boxplot of height growth in natural and salvage gaps.

p =0.149

Appendix 11: Boxplot of diameter in natural and salvage gaps.

p =0.043

Appendix 12: Boxplot of total height in natural and salvage gaps.

p =0.008

Appendix 13: Boxplot of crown width in natural and salvage gaps.

p =0.010

Appendix 14: Douglas-fir regeneration growth attributes versus distance to gap in natural and salvage gaps.

Appendix 15: Douglas-fir regeneration growth attributes versus elevation in natural and salvage gaps.

Appendix 16: Douglas-fir regeneration growth attributes versus aspect in natural and salvage gaps.

Appendix 17: Douglas-fir regeneration growth attributes versus slope in natural and salvage gaps.

Appendix 18: Canopy openness versus distance to gap edge

Appendix 19: LAI versus distance to gap edge

Appendix 20: LAI versus canopy openness