Science 323, 521–524 (2009)
Regional warming may be killing off trees in old forests across the western United States, finds new research. Until now, large-scale studies of tree mortality have been confined to the tropics, where new tree growth has counterbalanced a recent surge in tree mortality.
Phillip van Mantgem and Nathan Stephenson of the US Geological Survey and colleagues analysed mortality data from 76 undisturbed old-growth coniferous forest plots censused between 1955 and 2007 in the western United States. During this period, tree mortality increased in 87 per cent of the plots examined and doubled within 17 to 29 years, depending on region. Forest die-back was not confined to any one location or tree type. Rather, mortality increased at low and high elevations, in young and old trees, and in all major genera. The density of forests decreased over the study period, suggesting that competition for resources was not the main cause of increasing mortality. Rather, regional warming — which approached 0.5 °C per decade over the study period — and an associated rise in drought stress may have been responsible.
The researchers warn that the steep rise in tree mortality may be symptomatic of increasing stress on forests and could serve as a taste of things to come.