Climate Warming Causes Change in Forest Dynamics [Letter]

If Forest Dynamics in Canada’s West are Driven Mainly by Competition, Why Did They Change? Half-century Evidence Says: Climate change
Proceedings of the National Academy of Sciences (August 11, 2015 v112 no32; doi: 10.1073/pnas.1508245112) / by David T. Price, et al.
http://www.pnas.org/content/112/32/E4340.short

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…

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