Leaf, tree and stand responses to nutrient supplementation in Eucalyptus nitens

Maximising the growth and productivity of commercial plantations is critical for meeting global demands for wood and paper products, without resorting to unsustainable harvesting of native forests or increasing the plantation estate at the expense of other primary production systems. In Australia, productivity of plantation eucalypts is often limited by low nutrient availability and maintaining high growth rates is dependent on external nitrogen (N) and phosphorus (P) inputs. Therefore, an important management tool for maximising productivity is optimising fertiliser use. To do this, an improved mechanistic understanding of the productivity response to nutrient supplementation is needed. One of the major factors determining the response of productivity to nutrient supplementation is the degree of leaf area stimulation. While an increase in leaf area index (LAI) in response to fertiliser application increases the photosynthetic area of the crown, extra leaves also incur costs to the tree through increased respiration and water loss. The vertical distribution of leaf area through the crown is also important, as lower, shaded leaves may become a net carbon cost to the tree if photosynthetic carbon gains outweigh respiratory losses. Further, the relative physiological costs and benefits of extra leaves are heavily dependent on site conditions such as temperature and water status. The balance between rates of photosynthesis, respiration and transpiration affects the carbon and water balance of the whole plant, and therefore is an important driver of growth rate. However, there are significant gaps in our understanding of how these processes are influenced by nutrition both directly and through the leaf area response. Further, the dependence of these mechanisms on environmental conditions is not well understood. Therefore, the core objective of this study was to provide new knowledge to support a detailed mechanistic understanding of the response of plantation productivity to nutrient supplementation. I consider the effects of N and P fertilisers on leaf area (Chapter 2), physiology (Chapter 3) and growth (Chapter 4) of Eucalyptus nitens trees. I present and discuss the results of three large-scale field fertiliser experiments in mid-rotation E. nitens plantations in Tasmania. In chapter 1, I provide an overview of the theoretical concepts underpinning the response of trees to nutrient supplementation. I put these concepts into the applied context of plantation productivity and discuss the importance of large-scale field experiments in advancing research in this area. Finally, I outline the general aims of this thesis and the specific aims of each chapter. In chapter 2, I investigate the effects of the fertiliser treatments on both total leaf area and its vertical distribution through the crown. I demonstrate that N fertiliser application stimulates LAI development but not linearly, and that much of the extra leaf area is located lower in the crown. I discuss the potential implications of this for physiology and growth. In chapter 3, I compare rates of photosynthetic and water-use parameters between fertiliser treatments measured on leaves in the lower, middle and upper crown. Drawing on these results alongside high-resolution diurnal trunk shrinkage data, I present evidence for a water-loss cost associated with high N which I hypothesise can be attributed to high LAI. In chapter 4, I quantify tree- and stand-level growth in response to the fertiliser treatments, and present evidence of seasonal variation in the observed response. I then relate these findings to the observed patterns in leaf area production and physiology described in the previous chapters. In chapter 5, I synthesise the results of this thesis by presenting a conceptual framework explaining the interplay between nutrition, leaf area, physiology and growth. I discuss how site conditions and environmental variables fit into this framework, and consider the overarching implications for forest management, particularly in a changing climate.