title: Half-century evidence from western Canada shows forest dynamics are primarily driven by competition followed by climate.
abstract: 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 community-level processes-more so than external climatic factors-are driving forest dynamics.
authors.affiliation: Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada T6G 2H1; and.
authors.forename: Jian
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Forest Management Branch, Alberta Department of Environment and Sustainable Resource Development, Edmonton, AB, Canada T5K 2M4.
authors.forename: Shongming
authors.initials: S
authors.lastname: Huang
authors.affiliation: Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada T6G 2H1; and fhe@ualberta.ca.
authors.forename: Fangliang
authors.initials: F
authors.lastname: He
mesh_terms: D002170:Canada; D002980:Climate; D057231:Climate Change; D003625:Data Collection; D016208:Databases, Factual; D065928:Forests; D005843:Geography; D015233:Models, Statistical; D013997:Time Factors; D014197:Trees / Q000254:growth & development*
keywords: boreal forest; climate change; forest dynamics; tree competition; tree demographic rates