Neighbourhood effects in a temperate eucalypt forest

Maintaining the productivity of forest ecosystems requires a greater understanding of the factors controlling the growth and survival of individual trees. In the past, tree growth and survival has predominantly been associated with abiotic factors, such as local climatic conditions. However, it is now widely recognised that biotic features of the community itself, such as interactions among neighbouring plants, also play an important role. Neighbour interactions can range from competitive (negative) to facilitative (positive) with the net interaction effect of all neighbouring plants contributing to the growth and survival of individual trees. Changes in the environment, such as increased water stress due to drought, can amplify or reduce the strength of interactions among neighbouring plants. Whilst the effects of climate and neighbours are commonly studied in isolation to one another, their combined effect on tree growth and survival is not well understood.

In this thesis I explore the effects of neighbouring plants in an experimental temperate forest. I determine how the local neighbourhood influences tree growth (Chapters 2 & 3), patterns of canopy damage during drought (Chapter 4), and microclimatic conditions (Chapter 5). I relate the observed neighbourhood effects to plant physiology and local climatic conditions, and discuss their combined effects in determining tree growth and canopy damage.

In the first chapter, I provide an overview of the factors determining tree growth and survival. I outline current approaches to predict the impacts of altered climatic conditions on tree performance and discuss how interactions among neighbouring plants are an important but often under-represented component. I outline the importance of an integrated approach that considers species-specific neighbour interactions, plant physiology and direct climatic effects to better predict future forest productivity. I finish by detailing the overarching aims of this thesis, as well as the specific aims of each chapter.

In the second chapter, I use a literature search to determine how neighbourhood effects are commonly modelled in tree growth studies. Using the most common approach, I highlight potential biases in its ability to quantify the strength and direction of neighbour interactions. I show that ignoring or incorrectly describing the relationship between tree growth and size can lead to erroneous conclusions of the neighbourhood effect, namely overstating the strength of competition. I present a simple solution to minimise such biases and accurately estimate neighbour effects on plant growth.

In the third chapter, I use the solution outlined in Chapter 2 to investigate how neighbouring plants influence the annual growth of two eucalypts over a three-year period. I assess whether the strength of neighbour interaction varies temporally in the climatically distinct years. I show that tree growth was strongly influenced by the number, size and proximity of neighbours, but not the neighbour's identity. I demonstrate that competitive neighbour effects on eucalypt growth decreased with increasing growing season precipitation.

In the fourth chapter, I explore how individual plant physiology and neighbourhood density influence patterns of canopy drought damage in two eucalypts. I demonstrate that plant hydraulic traits correctly predicted a large proportion of the drought damage, but that damage was also related to the density and identity of neighbouring plants. I use several physiological and morphological traits to describe why species differed in their competitive effect on eucalypt drought damage. This chapter has been published in New Phytologist.

In the fifth chapter, I assess how neighbouring plants influence the above-ground microclimate over a year-long period. I investigate which neighbourhood characteristics have the strongest influence on diurnal and night-time temperature and relative humidity. I show that neighbours impact both the diurnal and night-time microclimatic conditions, however, diurnal trends were substantially clearer. In general, neighbouring plants had a moderating influence on temperature and relative humidity, particularly in extreme climate conditions. I relate these findings to growth and drought damage patterns in Chapters 3 and 4.

In the final chapter, I synthesise the findings of this thesis and discuss how integrating neighbour interactions with plant physiology and climatic conditions increases our ability to explain tree growth and canopy drought damage. I position these findings in the context of predicting future tree growth and survival, and outline potential future research avenues with implications for future forest productivity.