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Thresholds for boreal biome transitions
Contributed by F. Stuart Chapin III, November 15, 2012 (sent for review June 15, 2012)

Abstract
Although the boreal region is warming twice as fast as the global average, the way in which the vast boreal forests and tundras may respond is poorly understood. Using satellite data, we reveal marked alternative modes in the frequency distributions of boreal tree cover. At the northern end and at the dry continental southern extremes, treeless tundra and steppe, respectively, are the only possible states. However, over a broad intermediate temperature range, these treeless states coexist with boreal forest (∼75% tree cover) and with two more open woodland states (∼20% and ∼45% tree cover). Intermediate tree covers (e.g., ∼10%, ∼30%, and ∼60% tree cover) between these distinct states are relatively rare, suggesting that they may represent unstable states where the system dwells only transiently. Mechanisms for such instabilities remain to be unraveled, but our results have important implications for the anticipated response of these ecosystems to climatic change. The data reveal that boreal forest shows no gradual decline in tree cover toward its limits. Instead, our analysis suggests that it becomes less resilient in the sense that it may more easily shift into a sparse woodland or treeless state. Similarly, the relative scarcity of the intermediate ∼10% tree cover suggests that tundra may shift relatively abruptly to a more abundant tree cover. If our inferences are correct, climate change may invoke massive nonlinear shifts in boreal biomes.
Footnotes
- ↵1To whom correspondence may be addressed. E-mail: marten.scheffer{at}wur.nl or terry.chapin{at}alaska.edu.
Author contributions: M.S. and M. Holmgren designed research; M.S., M. Hirota, E.H.V.N., and F.S.C. performed research; M.S., M. Hirota, and E.H.V.N. analyzed data; and M.S., M. Holmgren, and F.S.C. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1219844110/-/DCSupplemental.
Freely available online through the PNAS open access option.