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Growth and Physiology of Eucalyptus Nitens in Plantations Following Thinning

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Growth and Physiology of Eucalyptus Nitens in Plantations Following Thinning Growth and physiology of Eucalyptus nitens in plantations following thinning Jane Louise Medhurst, B. For. Sci. (Hons.) Submitted in fulfilment ofthe requirements for the degree of Doctor of Philosophy School of Plant Science & CRC for Sustainable Production Forestry, University of Tasmania December 2000 Declarations This thesis contains no material which has been accepted for a degree or diploma by the University of Tasmania or any other institution. To the best of my knowledge and belief this thesis contains no material previously published or written by another person except where due acknowledgment is made in the text of the thesis. Jane L. Medhurst This thesis may be made available for loan and limited copying in accordance with the Copyright Act 1968. JaneL. Medhurst Abstract In Australia, a new activity is the management of eucalypt plantations for solid wood products. Thinning is one option for improving log sizes without the need for long rotation periods. The timing and intensity of thinning operations has a large bearing on the final products. This thesis describes a series of experiments in which the growth and physiological responses to thinning in Eucalyptus nitens (Deane and Maiden) Maiden plantations were identified. The thinning treatments, in three plantations, included an unthinned control and a range of final stockings from 600 to 100 trees per hectare. Diameter growth was significantly improved by thinning. In general, the more trees removed by thinning, the higher the growth response by the retained trees. Dominant and co-dominant trees showed the greatest growth response to thinning. Plantations oflow to medium productivity will benefit from early-age thinning. Based on stand and individual tree responses, a final density of200 to 300 trees per hectare may be ideal. The hydraulic conductivity of sapwood increased with stem height and the ratio of leaf area to stem sapwood area increased with stem size. Thinning did not alter these relationships. A broader study across a wide range of E. nitens plantations found strong relationships between stem sapwood area and tree leaf area that were independent of site, age and silvicultural treatment. Crown structure and the development of stand leaf area index following thinning were investigated. Larger branches were found in the lower crown of thinned trees and the increase in leaf area as a result of thinning occurred on the northern aspect of the crown. For trees in unthinned stands, the vertical distribution of leaf area 11 was skewed towards the top of the crown and was correlated with live crown ratio. The vertical leaf area distribution of trees in a thinned stand was less skewed and was unrelated to tree size or dominance. Leaf area index, as estimated from light interception measurements, increased at a constant rate soon after thinning regardless of residual stand density. In the longer term, residual stand density had a strong influence on leaf area increase per tree and was correlated with changes in crown length. Water use of retained trees increased after thinning through changes in the radial variation in sap flow across the sapwood. Stand-level water use after thinning was reasonably predicted from the Penman-Monteith equation scaled by a simple canopy size factor. Increases in photosynthetic capacity and changes in foliar nitrogen distribution occurred after thinning. These responses were well correlated with measured changes in the light regime and were due primarily to changes in specific leaf area. Foliar nitrogen content was positively related to light levels throughout the crowns of trees in thinned and unthi1med stands. Thinning induced both short- and long-term physiological changes. A modelling analysis demonstrated that increases in crown light interception and light-use efficiency drive growth response. The relationship of these changes with environmental factors suggests that, in order to maximise individual tree and stand productivity, thinning regimes for E. nitens plantations should be designed to maintain adequate light levels for individual crowns between thinning and harvest. Ill Acknowledgments This study was conducted using funding provided by the Forests and Forest Industry Council of Tasmania. I would like to thank my supervisors, Chris Beadle and Neil Davidson, for their support and guidance throughout the study. Chris provided insight into the field of plant physiology with patience and good humour. Neil provided valuable input into the experimental design and ensured that the administrative side of the study ran smoothly. Bill Neilsen of Forestry Tasmani~ provided the growth data for the Goulds Country thinning trial prior to 1996. The cooperation of the Bass and Huon District staff ofFores try Tasmania in establishing the Lisle and Creekton thinning trials is gratefully acknowledged. The Lisle and Creekton trials were each thinned by skilled and obliging contractors. I thank Lance Pearce of Mechanised Logging and Frank Frankcombe of Aprin for their efforts. I thank Michael Battaglia for his modelling and statistical assistance and advice. Maria Cherry, Mark Hunt and Don White are thanked for generously providing their data on leaf area and sapwood area for the analysis of allometric relationships in E. nitens. Mark Hunt provided invaluable assistance in the method for measuring sap flow and analysis of the sap flow data. Ross Edwards is thanked for his advice regarding the measurement ofhydraulic conductivity of sapwood. Throughout the project, field and laboratory assistance was provided by a range of people including Ray McLeod, Maria Cherry, Dale Worledge, Rick Hand, IV Ann Wilkinson, Martin Pi esse, Keith Churchill, Ann LaSala, Sandra Paffe, Martin Tyson, Anne Medhurst, Dan Raymond and Geoff Morris. I thank them all for their time and expertise. I received support and constructive criticism throughout the project from a range of people including Libby Pinkard, Grant Westphalen, Bill Neilsen, Peter Sands, Don White, David Fife, and Peter Naughton. I thank all my colleagues at the Cooperative Research Centre for Sustainable Production Forestry for providing such a pleasant working environment. Lastly, but most importantly, I thank Peter McLoughlin. His love, support and encouragement made this thesis possible. -I I :\ ! v Table of contents DECLARATIONS ABSTRACT II ACKNOWLEDGMENTS IV TABLE OF CONTENTS VI CHAPTER 1. INTRODUCTION 10 1.1 THEISSUE .......... ..................... .. .................................. .. .................................................... IO 1.2 BACKGROUND .. ... ... .... .. .................... ... ............... .......... ............................. ......... .............. l2 1.3 THE SPECIES ....................................................... ... .................................. ......................... 12 1.4 THE CURRENT THINNING PRESCRIPTION ........................................................................... 13 1.5 OBJECTIVES ..................................................................................................................... 14 1.6 SUMMARY OF EACH CHAPTER .......................................................................................... 14 CHAPTER 2. DESCRIPTION OF THINNING TRIALS 18 2.1 THE PLANTATIONS ...................................................... ..................................................... 18 2.2 TRIAL DESIGN AND TREATMENTS ... .................................................................................. 24 CHAPTER 3. GROWTH, DOMINANCE AND INTRA-SPECIFIC COMPETITION AFTER EARLY-AND LATER-AGE THINNING 26 3 .I INTRODUCTION ... ... .......................................................... .. ............................................... 26 3 .2 MATERIALS AND METHODS ............. .................. ............................... ............ ................... 28 3.2.1 Sites and treatments....... .................. .. ............ .. ...... .. .. .... ........... .. .................. 28 3.2.2 Measurements ....................................... ..... .............. .................... ... ... ..................... 28 3.2.3 Data analysis ....... .. ....... .. ..... ..................... .. ...... ... .................................... ............... 30 3.3 RESULTS ......... .......................................................................... .............. ... ....................... 31 3.3.1 Early-age thinning ......... ........ .................. .. ........ .. ........ .. ............... .. ............... ...... 31 3.3.2 Later-age thinning .. .......... .. .. .. .. ................ .. ... .. .. ........ 37 3.4 DISCUSSION ... ............................................ ...................................................................... .42 3.4.1 Changes in basal area ..... .... .... .. .................................................... .... ....................... 42 3.4.2 Dominance class ..................... .. ........................... .. ...................... .............. ............. 46 3.4.3 Height growth ......................... .. .......................... .. .................................................... 47 3.4.4 Thinning intensity ....................... ........ ................................ .. ........ .................. .......... 48 3.5 CONCLUSIONS ... .................... ... .......................................................................... .............. 49 CHAPTER 4. LEAF AREA- SAPWOOD AREA RELATIONSHIPS AND SAPWOOD HYDRAULIC CONDUCTIVITY 51 4.1 INTRODUCTION
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