| Tree mortality influences forest structure, composition and ecosystem functions. To assess how recent climate changes affected tree mortality, observational studies conducted in old-growth forests have shown that tree mortality has increased with recent global warming, increasing atmospheric CO2, and decreasing water availability in tropical, temperate, and boreal forests. These studies could lead to biased estimation of climate effects on boreal forests. Boreal forests are a mosaic of stands at various developmental stages, with old forests accounting for only a small portion of the landscape. Additionally, uncertainty exists whether the observed temporal increases in tree mortality are attributable to climate changes or stand developmental processes. The overall objective of this thesis was to investigate how recent climate changes affected North American boreal forests, encompassing the variety of tree sizes, stand developmental stages and stand compositions which typify the boreal region. The aboveground biomass carbon pool had been examined and related to tree mortality.;In the first tree mortality study, I examined how endogenous factors, such as competition, species interaction and aging, affect tree mortality. I simultaneously tested, using Boosted Regression Trees (BRT) models, the effects of an individual's relative size, stand crowding, species interaction and ageing on mortality of Pinus banksiana Lamb., Populus tremuloides Michx., Betula papyrifera Marsh. and Picea mariana Mill. Data from 109 permanent sampling plots (PSPs) located in Ontario had been used for these analyses. I found that mortality increased significantly with decreasing relative size for all study species, and the size-dependent mortality was stronger for shade-intolerant than for shade-tolerant species. With increasing stand basal area, mortality increased for Pinus banksiana, Populus tremuloides and Picea mariana, but decreased for Betula papyrifera. Mortality was higher in stands with more conspecific neighbours for Populus tremuloides, Betula papyrifera and Picea mariana, but was lower for Pinus banksiana. Mortality also increased with stand age for all species. Furthermore, the size-dependent mortality was stronger in more crowded stands. These results suggest that tree mortality in boreal forest is driven by endogenous factors such as competition, aging, and species interaction.;The objective of the second tree mortality study was to disentangle the effects of climate change and endogenous processes on tree mortality. I conducted individual mortality probability analyses for five major boreal tree species Populus tremuloides, Populus balsamifera L., Pinus banksiana, Picea mariana, and Picea glauca (Moench) Voss, using Hierarchical Bayesian logistic regression model (HBLogit). The analyses were based on data from 887 PSPs that covered a wide range of stand developmental stages in the western boreal region, i.e., Alberta and Saskatchewan. I found that both climate change and forest development processes influenced temporal mortality increases. When endogenous factors were considered for all study species, the overall tree mortality increased during study period (1958-2007). Climate change-associated increases in tree mortality were significantly higher in young than old forests. I also found that, over the study period, annual temperature anomaly increased, and climate moisture index anomaly decreased, showing a global-change-type drought. Further analyses revealed that higher increases of tree mortality in younger forests were a result of their higher sensitivity to regional warming and drought.;Additionally, I examined climate change-induced tree mortality using data from 148 PSPs in Manitoba. I partitioned climate change effects from endogenous effects on tree mortality by developing individual tree mortality models using HBLogit. The analyses were conducted for five major boreal tree species Populus tremuloides, Populus balsamifera, Pinus banksiana, Picea mariana, and Picea glauca,. I found that tree mortality increased over the last three decades. Although there was significant warming in the Manitoba study area (i.e., annual temperature anomaly increased by 0.038 °C year-1 over the study period), there was also an increase in annual climate moisture index anomaly, suggesting that the study area did not experience global-change-type drought. Collectively, the mechanism that led to temporal increases of tree mortality in this area could be different from other areas of western North America where global-change-type drought may be the mechanism for observed increases in tree mortality. The neighborhood analyses provide the evidence that the temporal increases of temperature and water availability likely have increased tree-tree competition on tree mortality and led to a temporal increase of tree mortality.;Finally, I investigated temporal changes of biomass carbon pool and related it to recent increases of tree mortality in western boreal forest region. Using data from 871 permanent plots in Alberta and Saskatchewan, I found that aboveground biomass change (DeltaAGB) averaged at 1.11 (95% credible interval (CI), 1.02~1.21) Mg ha-1 yr-1 over study period (1958-2009), suggesting that the forests have been a strong carbon sink. After accounting for forest age-dependent decreases, I found that DeltaAGB has declined at -0.031 (CI, -0.037~-0.024) Mg ha-1 yr-1 yr-1 due to increased tree mortality and reduced growth of surviving trees with no increase in recruitment. The highest decline rate was found for late-successional coniferous forests that dominated by shallow-rooted Picea spp. at a rate of -0.074 (CI, -0.093~-0.053) Mg ha-1 yr-1 yr-1. Further analyses indicated that regional warming and drought were likely contributors to shrinkage of forest aboveground biomass carbon sink in this region. |
