The influence of demographic rates on stand structure in old-growth forests

Forests store significant quantities of carbon in their aboveground biomass, and represent the majority of the terrestrial carbon sink. Globally, increases in mortality rates are driving declines in this carbon sink; however projections of future changes are poorly constrained as we lack an understanding of the mechanisms of change. A better understanding of the links between demographic rates, forest stem size distributions, and aboveground biomass is needed to predict forest responses to changes in demographic rates. This thesis uses analysis of forest inventory data and a simulation modelling approach to extend our understanding of relationships between demographic rates and forest stand structure. In Chapter 2, I analyse structural variation in Amazonian forests, and show that relationships between demographic rates and stand structure vary with forest type, climate, and soils. Variation in mortality rates shapes stand structure in warm, wet forests with stable soils, but stand structure in dry and montane forests is more strongly shaped by variation in growth rates. In Chapter 3, I develop a novel individual-based simulation approach, using forest inventory and tree ring data from four bioclimatic domains in Quebec. This approach demonstrates the importance of demographic rates in determining forest structure, and provides a methodology which is applicable to many permanent sample plot networks. In Chapter 4, I apply this simulation approach to plot data from Amazonia, to analyse forest resilience to increases in mortality rates. For a given percentage increase in mortality rate, forests with lower baseline demographic rates show greater declines in basal area and longer recovery times than forests with higher baseline demographic rates. This thesis clarifies relationships between demographic rates and stand structure, and provides a novel simulation method with which to better understand how stand structure and forest biomass may respond to changes in demographic rates.