Canadian Forest Service Publications
If forest dynamics in Canada's west are driven mainly by competition, why did they change? Half-century evidence says: climate change. 2015. Price, D.T.; Cooke, B.J.; Metsaranta, J.M.; Kurz, W. Letter in Proceedings of the National Academy of Sciences of the United States of America 112(32):E4341.
Issued by: Northern Forestry Centre
Catalog ID: 36196
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In a recent paper (1), Zhang et al. present analyses of “forest dynamics” inferred from measurements collected during 1958–2009 at permanent sample plots (PSP) distributed across Canada’s western forests. Their results are almost unanimous in showing widespread increases in mortality, and declines in relative growth and recruitment (figure 2 in ref. 1). Zhang et al. conclude these trends are explained primarily by changes in stand-scale competition, and that recent changes in climate are of secondary importance. Surprisingly, Zhang et al. do not explain the temporal changes in competition they detected. We accept that stand dynamics depend upon competition for light, nutrients, and water, but argue that climate affects the supply of these resources. We find some major problems with the report by Zhang et al., including misinterpretation of results and a critical lack of clarity on key model assumptions, which cast serious doubt on their conclusions.
Plain Language Summary
Forests worldwide have been changing rapidly in recent years, but it has been difficult to figure out why these changes are occurring. In this letter to the Proceedings of the National Academy of Sciences (PNAS) we draw attention to deficiencies in a recent article published in PNAS, the authors of which argued (incorrectly, in our opinion) that the underlying biological and physical forces that shape forests (“forest dynamics”) are driven mainly by competition within forest stands (communities of trees) and that climate is less important. We point out some major problems with this article, including misinterpretation of results and a critical lack of clarity on some key assumptions of the analysis they used. Together, these concerns cast serious doubt on the authors’ findings. There is an urgent need to account for climatic effects in models that predict forest yield, to ensure that forest management remains sustainable in the coming decades.