MISTLETOE AND THE BRUSHTAILED POSSUM IN SILVER BEECH FOREST, SOUTH WESTLAND, NEW ZEALAND. A thesis submitted in fulfilment of the requirements for the Degree of Master of Forestry Science in the University of Canterbury by Hamish John Owen -:;:: University of Canterbury 1993 Table of Contents Abstract Chapter I-General Introduction 1.1 Mistletoe 1 1.2 Possums in New Zealand indigenous forests 6 1.3 South Westland- a 'mistletoe stronghold' 8 1.4 General study aims 9 Chapter 2-Study Area .2.1 Location 10 2.2 Geology, landform and soils 10 2.3 Climate 12 2.4 Vegetation 12 2.5 Possum colonisation of the Haast catchment 14 2.6 Location of study areas 17 2.7 Description of study areas 17 Chapter 3-Distribution of Peraxilla colensoi in Silver Beech Forests 3.1 Introduction 20 3.1.1 Study aims 21 3.2 Study area and site selection 22 3.3 Methods 24 3.3.1 Field methodology 24 3.3.2 Data analysis 27 Page 3.4 Results 29 3.4.1 Host preferences 29 3.4.2 Classification of plots into forest types by 1WINSPAN 31 3.4.3 Differences in mistletoe density between forest types 35 3.4.4 Differences in mistletoe host size selection between forest types 37 3.4.5 Distribution of mistletoe 40 3.4.6 Physical variables and mistletoe abundance 43 3.4.7 Regression 45 3.5 Discussion 47 3.5.1 Mistletoe density as a function of time and host size 47 3.5.2 Mistletoe and beech forest structure 48 3.5.3 Influence of other environmental variables 53 3.6 Conclusions 55 Chapter 4 Mistletoe Browse Study 4.1 Introduction 56 4.4.1 Study aims 57 4.2 Study area and site selection 57 4.3 Review of methods 60 4.3.1 Methods of assessing possum damage 60 4.3.2 Relative subjectivity of techniques 61 Page 4..4 Methods 62 4.4.1 Field methodology 62 4.4.2 Data collection and analysis 67 4.5 Results 68 4.5.1 Overall patterns of leaf change at Pleasant Flat and Deer Stalkers flat 68 4.5.2 Intensities of possum browse on individual plants 71 4.5.3 Changes in leaf area: possum browsed and unbrowsed plants 71 4.5.4 Seasonal patterns in browse 75 4.5.5 Seasonal leaf loss and growth 75 4.5.6 Comparing intensity of browse to leaf area change 78 4.5.7 Phenology 78 4.6 Discussion 79 4.6.1 Possible sources of sampling bias and error 79 4.6.2 Possum impacts on browsed plants 80 4.6.3 Defolia tion and growth 82 4.7 Conclusions 85 Chapter 5 Diet Study 5.1 Introduction 86 5.1.1 Study aims 87 5.2 Study area and site selection 87 5.3 Review of methods 89 5.3.1 Diet assessment 89 Page 5.3.2 Estimating food availability 91 5.3.3 Plant preference ratings 93 5.4 Methods 94 5.4.1 Field and laboratory methods 94 5.4.2 Data analysis 96 5.5 Results 98 5.5.1 Important foods in possum diet 98 5.5.2 Seasonal variation in diet 102 5.5.3 Plant availability 112 5.5.4 Plant preferences 114 5.5.5 Kill rates 115 5.6 Discussion 117 5.6.1 Diet patterns 117 5.6.2 Competition with native birds 120 5.6.3 Possum densities and there impacts on preferred species 121 5.7 Conclusions 127. Chapter 6 Conclusions 128 Acknowledgements 131 References 132 Appendix A - % leaf area changes for individual plants (Pleasant Flat) 143 Appendix B % leaf area changes for individual plants (Deer Stalkers Flat) 143 List of figures Figure Page 2.1 Map showing location of Haast Valley 11 2.2 Distribution and density of South Westland 16 3.1 Map showing location of Thomas River transects 23 3.2 Mistletoe height classes 26 3.3a Silver beech stern and mistletoe frequency by dbh size class 30 b Silver beech basal area and mistletoe frequency by dbh size class 30 3.4a Size distribution of silver beech, kamahi and rata stems in Silver beech-broadleaved forest 33 b Size distribution of silver beech, kamahi and rata sterns in Simple silver beech forest 33 c Size distribution of silver beech, kamahi and rata stems in Kamahi-beech-rata forest 34 d Size distribution of silver beech, kamahi and rata stems in Rata-kamahi-beech forest 34 3.5 Varation in mistletoe density between forest types 36 3.6 Host size selection in different forest types a Silver beech-broadleaf 38 b Simple silver beech 38 c Kamahi-beech-rata 39 3.7 Vertical distribution of mistletoe within forest types 41 3.8 Size distribution of mistletoe 3.9 Altitudinal distribution of mistletoe 44 4.1 Map showing location of upper Haast study sites 59 4.2 Mistletoe leaf area index 66 Figure 4.3 Average % leaf area losses from mistletoe plants 69 4.4 Seasonal patterns of browse on mistletoe plants 76 4.5 Seasonal patterns of leaf fall and growth (browsed and unbrowsed plants) 77 5.1 Annual contribution to possum diet of important foods 100 5.2 Seasonal diet patterns of 6 most important food items 106 5.3 Duncans Multiple Range Testshowing seasonal changes in consumption of 10 major food items 108 5.4 Duncans Multiple Range Test showing seasonal changes in number of food items consumed 111 4.1 Possum and insect browse on mistletoe leaves 65 List of Tables Table Page 2.1 Research topics and study areas 19 2.2 Physical descriptions of study areas 19 3.1 Proportion of host stems, forest area and mistletoe in each forest type 36 3.2 Forest aspect and mistletoe density 44 3.3 Physiography and mistletoe density 44 3.4 Regression equations showing relationship between stand parameters and mistletoe infestation 46 4.1 Average % mistletoe leaf area losses 70 4.2 % mistletoe leaf area eaten by possums and insects 73 4.3 Mean leaf area changes: unbrowsed vs browsed plants 74 5.1 Vegetation plot height tier intervals and weights 97 5.2 Foods contributing >1 % to diet in anyone month 101 5.3 ANOV A showing seasonal changes in quantities consumed of 10 food items 105 5.4 ANOVA showing seasonal changes in number of food i terns consumed 105 5.5 Proportions of leaf biomass 113 5.6 Preference Index Scores 116 Abstract The distribution of mistletoe in South Westland silver beech forest was investigated. Stand structure and densities of Peraxilla colensoi (pirita) were assessed along six altitudinal transects. Host size and forest structure were both found to be important factors influencing density and distribution of mistletoe. Generally large beech trees were favoured as mistletoe hosts. There was also significant variation in mistletoe density between forest types In a different study the leaf area removed by possums and insects from a population of mistletoe plants was quantified over a nine month period. Insects browsed all plants, and on average consumed around 4% of each mistletoe's leaf area. The overall consumption of mistletoe leaf area by possums was around 2%, but possum browse was not evenly spread over the population. Of the forty plants sampled, seven were attacked by possums with one plant being heavily browsed. By the end of the monitoring period the leaf area retained by possum browsed plants was significantly lower than that of unbrowsed plants. If individual mistletoes continue to be selectively browsed then defoliation and death of some mistletoe plants seems a likely outcome. Possum diet was assessed by means of gut sample analysis and compared to estimates of food availability within silver beech forest. Possums utilised a wide range of food types but two or three species were dominant at any particular time of the year. Seral species such as wineberry, pohuehue, fuchsia, and lawyer were generally the most important foods. Mistletoe was not an important food item, contributing less than 1 % of annual possum diet. 1 CHAPTER ONE General Introduction Chapter outline: In this chapter the New Zealand mistletoe flora and some basic information on mistletoe biology is outlined. Evidence that New Zealands mistletoes have declined and that this contraction is linked to the spread of the possum is reviewed. Objectives of the study are presented. 1.1 Mistletoe Mistletoes are a polyphyletic group of shrubby angiosperms that largely parasitise the aerial sterns of perennial plants although some root parasites also occur. They are distributed throughout the world but are most diverse in the tropics. There are 1400 species of mistletoe most of which belong to the Loranthaceae (950 species) and the Viscaceae (400 species), a few species OCCU.r in the Eremolepidaceae, Myzodendraceae and Santalaceae . The New Zealand mistletoe flora comprises three Viscaeae and six Loranthaceae species including Peraxilla colensoi (pirita) which forms the focus of this study. Mistletoe biology Little is known of the biology of Peraxilla or other New Zealand mistletoes. It is probable that the biological characteristics of Peraxilla are similar to those of other hemi-parasitic Loranthaceae. These plants have green leaves and thus produce organic food by photosynthesis, but are dependant on the host for water and nutrients. In order to achieve this, stern parasitic mistletoes have acquired specialized features. They have forsaken a conventional root system for one or more haustoria (woody intrusive organs that anchor the mistletoe 2 into the hosts vascular elements). Upon germination of a mistletoe seed the radicle (embryonic root) grows towards the host branch and forms, upon contact, a haustorial disk.