Do Plants in Mountainous Areas Always Migrate Uphill in Response to Warming?
Crimmins, S.M., Dobrowski, S.Z., Greenberg, J.A., Abatzoglou, J.T. and Mynsberge, A.R. 2011. Changes in climatic water balance drive downhill shifts in plant species' optimum elevations. Science 331: 324-327.
In a study they designed to explore this phenomenon, Crimmins et al. compared the altitudinal distributions of 64 plant species between the 1930s (when 13,746 plots were surveyed) and the first five years of the 21st century (when ~33,000 plots were surveyed) in the major mountain ranges of the state of California (USA) that are located above 35°N latitude, where mean annual temperatures rose by about 0.6°C over the interval of time between surveys, and where increases in precipitation resulted in a net decrease in the climatic water deficit, or the difference between potential evapotranspiration and precipitation.
Given such findings, the five researchers report that the majority of the plant species they studied "appear to be tracking their climatic niche by shifting their altitudinal distributions downhill in response to decreased climatic water deficit" in a "niche tracking of changes in water availability rather than changes in temperature." And as a result of that unusual expansion in the direction of warmer temperatures in a warming climate, they also found that "plant species in our study area are experiencing an increase in their optimum temperature (0.36°C) due to both climatic warming and downhill shifts" towards warmer temperatures.
The results of this enlightening study demonstrate that temperature is not the all-important factor when it comes to determining how plants will migrate in the face of a suite of multiple climatic factors in a state of flux. And it demonstrates that plants can actually alter what may have long been their standard optimum operating temperature when changing environmental conditions require such a change.
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