Global environmental change effects on forest dynamics: lessons from permanent sample plot networks

Abstract

Global environmental change caused by anthropogenic emissions of greenhouse gasses is a major threat to forest ecosystems globally. Before we can begin to adequately mitigate these threats, we first must understand how these systems are being affected. The purpose of this dissertation is to provide an understanding of how best to estimate these effects, investigate possible mechanisms of mitigation, and provide the first ground-based global estimate of changes in forest productivity in response to global environmental change. In my first study, I examined how the use of size-thresholds can bias estimates of forest biomass gain, loss, and net change. Permanent sample plot networks are intensively sampled, spatially extensive, and as a result often quite costly. In an effort to reduce costs of these networks, tree size-thresholds are often used which leads to measurement of solely large trees. However, it is unclear whether these size-thresholds bias estimates of the effects of global environmental change on stand biomass gain, loss, and net change. Using a network of 141 permanent sample plots from Manitoba, Canada, with all trees of >1.3 m in height repeatedly measured, I constructed three distinct data sets: using a 10 cm, 5 cm, and no diameter at breast height threshold. These three data sets were then used to demonstrate that stand biomass gain and loss were increasingly underestimated as thresholds increased. This underestimation was particularly noticeable in the relationship between biomass gain and age: the peak productivity was estimated to be 20 years later when using a 10 cm threshold in comparison to no threshold. Despite bias in estimates of stand biomass gain and loss as stands aged, there was little evidence for any bias in estimates of global environmental change effects on forest biomass gain, loss, or net change. These results suggest that, if not properly controlled for, the use of tree size thresholds can significantly bias estimates of forest biomass gain, loss, and net change. In my second study, I examined how tree longevity has responded to global environmental change. In particular, acceleration in tree life cycles (i.e., reduced longevity in trees with faster lifetime growth rates) has been hypothesized as a cause of increased tree mortality in response to global environmental change. However, this link has never been explicitly tested. In light of this, I defined two testable hypotheses: (i) the probability of ageing driven tree mortality increases with global change and (ii) the mortality probability associated with global change is higher for faster-growing trees. To test these hypotheses, I examined the temporal changes of tree mortality probability in 539 permanent sample plots monitored from 1960 to 2009, with ages greater than 100 years at initial censuses, across the boreal region of Alberta, Canada. I demonstrated that tree longevity has been reduced as a result of global environmental change. Further, I demonstrated that this reduction in longevity was strongest for those trees with the fastest lifetime growth rates. These reduced longevities were linked to increasing atmospheric carbon dioxide and reduced water availability, indicating that the reductions in longevity will likely continue with future global environmental change. In my third study, I examined the potential of enhancing niche complementarity as a way to mitigate the negative effects, or enhance the positive effects, of global change on individual tree productivity. Specifically, I hypothesized that individual trees with functionally and phylogenetically dissimilar neighbours would grow more quickly and experience less competition under global environmental change than those with functionally and phylogenetically similar neighbours.