University of Washington - Department of Statistics
Advisor: Peter Guttorp
Local competition between trees affects growth and mortality, from which emerges spatial patterns of surviving trees. Often, the patterns resulting from this unspecified process are treated as instances of spatial patterns and analyzed with point process methods. Alternatively, forest simulation models assume mechanistic processes and parameters to examine the effects of these assumptions on tree patterns over time, and assess sensitivity to changing conditions, such as climate. Inference for growth and mortality mechanisms from actual data are usually separate analysis exercises; further these are often based on cross-sectional data due to difficulty in using longitudinal data. We propose a joint treatment of longitudinal size and mortality data for 8 plots of Abies amabilis collected from 1978 to 2009 in western Washington state, with a deterministic zone-of-influence competition mechanism for growth and a growth-dependent stress model for mortality. This model considers irregular observation for size and/or mortality, a common problem given the labor involved in taking observations. We find evidence of strong asymmetry in competition based on relative size of competitors, as well as confirmation of the direction of the stress model. The method gives a framework for statistical treatment of non-destructively sampled observational tree stand data; the results suggest future directions for improving and expanding the methods.