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 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.