Tree mortality, growth, and recruitment are essential components of forest dynamics and resiliency, for which there is great concern as climate change progresses at high latitudes. Tree mortality has been observed to increase over the past decades in many regions, but the causes of this increase are not well understood, and we know even less about long-term changes in growth and recruitment rates. Using a dataset of long-term (1958-2009) observations on 1,680 permanent sample plots from undisturbed natural forests in western Canada, we found that tree demographic rates have changed markedly over the last five decades. We observed a widespread, significant increase in tree mortality, a significant decrease in tree growth, and a similar but weaker trend of decreasing recruitment. However, these changes varied widely across tree size, forest age, ecozones, and species. We found that competition was the primary factor causing the long-term changes in tree mortality, growth, and recruitment. Regional climate had a weaker yet still significant effect on tree mortality, but little effect on tree growth and recruitment. This finding suggests that internal communitylevel processes-more so than external climatic factors-are driving forest dynamics.boreal forest | climate change | forest dynamics | tree demographic rates | tree competition F orests provide fundamental ecosystem services for sustaining the global environment, such as storing carbon and maintaining biodiversity. These services, however, are at risk for decline as evidence has increasingly shown that forests in many parts of the world are undergoing rapid changes (1-4). Climate at the regional or global scale is often presumed to be responsible for these changes (5-14), with surprisingly little attention being paid to the possible effects of endogenous processes despite the fact that competition is often an important force driving stand dynamics and succession (15-18). How climate change and competition interplay to affect the long-term change of demographic rates and what are their relative contributions to the change are unanswered questions (19,20).We addressed these questions by compiling data from 1,680 permanent sample plots (PSPs) that are located in undisturbed natural forests across western Canada (Fig. 1). The trees in these plots, which cover a wide geographic region spanning 32°of longitude and 10°of latitude primarily in the boreal zone, were censused over a period from 1958 to 2009 (Fig. 1). Within each plot, all standing trees with diameter at breast height (DBH) ≥ 9 cm were tagged, recorded, and remeasured at irregular time intervals (mean = 10 y) (SI Appendix, Fig. S1). Plot sizes ranged from 0.04 ha to 0.81 ha (mean = 0.14 ha). To reduce possible impact of plot sizes on our analyses, only the plots with at least 50 trees at their first census were selected. The plots have been censused three to eight times (mean of four times). In total, these plots contained 320,878 living trees over the study period (SI Appendix, Table S1).We analyzed the ...

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