Climatic extremes improve predictions of spatial patterns of tree species

doi:10.1073/pnas.0901643106

Climatic extremes improve predictions of spatial patterns of tree species

^aLand Use Dynamics, Swiss Federal Research Institute WSL, 8903 Birmensdorf, Switzerland;
^bDepartment of Biology, Faculty of Science, University of Tromsø, N-9037 Tromsø, Norway;
^cU.S. Geological Survey, Utah Cooperative Fish and Wildlife Research Unit, Utah State University, Logan, UT 84322-5290;
^dLaboratoire d'Ecologie Alpine, Unité Mixte de Recherche–Centre National de la Recherche Scientifique 5553, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France; and
^eDepartment of Ecology and Evolution, University of Lausanne, CH-1015 Lausanne, Switzerland

Edited by David D. Ackerly, University of California, Berkeley, CA, and accepted by the Editorial Board September 26, 2009 (received for review March 15, 2009)

Abstract

Understanding niche evolution, dynamics, and the response of species to climate change requires knowledge of the determinants of the environmental niche and species range limits. Mean values of climatic variables are often used in such analyses. In contrast, the increasing frequency of climate extremes suggests the importance of understanding their additional influence on range limits. Here, we assess how measures representing climate extremes (i.e., interannual variability in climate parameters) explain and predict spatial patterns of 11 tree species in Switzerland. We find clear, although comparably small, improvement (+20% in adjusted D², +8% and +3% in cross-validated True Skill Statistic and area under the receiver operating characteristics curve values) in models that use measures of extremes in addition to means. The primary effect of including information on climate extremes is a correction of local overprediction and underprediction. Our results demonstrate that measures of climate extremes are important for understanding the climatic limits of tree species and assessing species niche characteristics. The inclusion of climate variability likely will improve models of species range limits under future conditions, where changes in mean climate and increased variability are expected.

Footnotes

¹To whom correspondence should be addressed. E-mail: niklaus.zimmermann{at}wsl.ch
Author contributions: N.E.Z., N.G.Y., T.C.E., E.S.M., W.T., A.G., and P.B.P. designed research; N.E.Z., E.S.M., D.R.S., and P.B.P. performed research; N.E.Z., N.G.Y., E.S.M., D.R.S., and P.B.P. analyzed data; and N.E.Z., N.G.Y., T.C.E., E.S.M., W.T., A.G., D.R.S., and P.B.P. wrote the paper.
This paper results from the Arthur M. Sackler Colloquium of the National Academy of Sciences, “Biogeography, Changing Climates and Niche Evolution,” held December 12–13, 2008, at the Arnold and Mabel Beckman Center of the National Academies of Sciences and Engineering in Irvine, CA. The complete program and audio files of most presentations are available on the NAS web site at www.nasonline.org/Sackler_Biogeography.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. D.D.A. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/cgi/content/full/0901643106/DCSupplemental.