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Thresholds for warming-induced growth decline at elevational tree line in the Yukon Territory, Canada
Tree-Ring Laboratory, Lamont-Doherty Earth Observatory, Palisades, New York, USA
Center for Energy and Environmental Studies, Boston University, Boston, Massachusetts, USA
Tree-Ring Laboratory, Lamont-Doherty Earth Observatory, Palisades, New York, USA
Tree-Ring Laboratory, Lamont-Doherty Earth Observatory, Palisades, New York, USA
Center for Energy and Environmental Studies, Boston University, Boston, Massachusetts, USA
Department of Geography, Boston University, Boston, Massachusetts, USA
Forest Ecosystems and Ecological Risks, Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland
A few tree ring studies indicate recent growth declines at northern latitudes. The precise causes are not well understood. Here we identify a temperature threshold for decline in a tree ring record from a well-established temperature-sensitive site at elevational tree line in northwestern Canada. The positive ring width/temperature relationship has weakened such that a pre-1965 linear model systematically overpredicts tree ring widths from 1965 to 1999. A nonlinear model shows an inverted U-shaped relationship between this chronology and summer temperatures, with an optimal July–August average temperature of 11.3°C based on a nearby station. This optimal value has been consistently exceeded since the 1960s, and the concurrent decline demonstrates that even at tree line, trees can be negatively affected when temperatures warm beyond a physiological threshold. If warming continues without significant gains in effective precipitation, the large-scale greening of recent decades could be replaced by large-scale browning. Such browning could slow or reverse carbon uptake by northern forests.
Received 1 March 2004; accepted 13 July 2004; published 24 September 2004.
Citation: (2004), Thresholds for warming-induced growth decline at elevational tree line in the Yukon Territory, Canada, Global Biogeochem. Cycles, 18, GB3021, doi:10.1029/2004GB002249.
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