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Increasing Precipitation Does Not Counteract Effects of Warming on Plant Species Distributions

Released: 2011
Citation:
WERC Publication Brief: Increasing Precipitation Does Not Counteract Effects of Warming on Plant Species Distributions. Updated October 2011.

THIS BRIEF REFERS TO:
Stephenson, N.L. and A.J. Das. 2011. Comment on “Changes in Climatic Water Balance Drive Downhill Shifts in Plant Species’ Optimum Elevations.”
Science. doi: 10.1126/science.1205740 (PDF: http://www.werc.usgs.gov/ProductDetails.aspx?ID=4546)

THIS BRIEF AND PUBLICATION IS IN RESPONSE TO:
Crimmins, S.M., S.Z. Dobrowski, J.A. Greenberg, J.T. Abatzoglou, A.R. Mynsberge. 2011. Changes in Climatic Water Balance Drive Downhill Shifts in Plant Species’ Optimum Elevations. Science. doi: 10.1126/science.1199040

A recent report in the journal Science (Crimmins et al. 2011) concluded that (1) over the last 70 years, plant species in northern California had migrated downward by an average of about 90 m, (2) the downward migration was caused by a precipitation-induced decline in climatic water deficit, and (3) similar downhill species shifts can be expected where future increases in precipitation outpace temperature-induced increases in evaporative demand.

In sum, the paper concluded that increasing precipitation can counteract the effects of increasing temperature. But in a technical response to Science, USGS scientists Nate Stephenson and Adrian Das demonstrate the conclusions of Crimmins et al. are incorrect.

First, Stephenson and Das show that the historical and contemporary vegetation plots compared by Crimmins et al. were geographically biased relative to one another. For example, the median latitude of their contemporary plots was about 140 km farther north than that of historical plots. Because a northward shift of 100 km is generally accompanied by a roughly 100 m decline in species’ elevations, this and other geographic biases likely account for the lower species’ elevations.

Second, Crimmins et al. miscalculated climatic water deficit. Corrected calculations show that the majority of extra precipitation in the contemporary period fell in winter and became biologically unusable surplus water.

Most importantly, increasing precipitation will not counteract the effects of increasing temperature. Increasing temperature and precipitation will lead to warmer, wetter environments, which are fundamentally different from cooler, drier environments. Other research also suggests that under a warming climate, most plant species will shift upward, not downward in elevation, regardless of changes in precipitation.



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Management Implications
Pacific dogwood, Yosemite National Park --Photographer: Ben Young Landis/USGS
Even if precipitation increases under climate warming, species such as Pacific dogwood will likely shift upward, not downward in elevation. Image Credit: Ben Young Landis/USGS.

  • An apparent climatically-induced downward elevational shift of California plant species over the last 70 years, reported in the journal Science, is likely an artifact of geographic data biases; further work is needed to determine which, if any, species have actually migrated.
  • Increasing precipitation should not be expected to counteract the effects of increasing temperature; other research suggests that most plant species will shift upward in elevation, regardless of changes in precipitation.


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