Background
An extremely common assumption, according to the authors, is that temperature is the principal factor defining species' distributions in mountainous areas. However, they indicate that "many species, including plants, are constrained by energy and water availability," citing the work of Stephenson (1990, 1998); and they say that the latter factor can sometimes override the direction of migration that would be expected in a warming climate, noting that "there is ample evidence of stasis and even downhill shifts in species' distributions despite climatic warming," in support of which statement they reference the studies of Archaux (2004), Lenoir et al. (2008) and Popy et al. (2010).

What was done
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.

What was learned
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.

What it means
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.

References
Archaux, F. 2004. Breeding upwards when climate is becoming warmer: no bird response in the French Alps. Ibis 146: 138-144.

Lenoir, J., Gegout, J.C., Marquet, P.A., de Ruffray, P. and Brisse, H. 2008. A 100-year survey shows that the optimal elevations for growth of plant species in European temperate forests have shifted upward by about 30 meters per decade. Science 320: 1768-1771.

Popy, S., Bordignon, L. and Prodon, R. 2010. A weak upward elevational shift in the distribution of breeding birds in the Italian Alps. Journal of Biogeography 37: 57-67.

Stephenson, N.L. 1990. Climatic control of vegetation distribution: The role of the water balance. The American Naturalist 135: 649-670.

Stephenson, N.L. 1998. Actual evapotranspiration and deficit: Biologically meaningful correlates of vegetation distribution across spatial scales. Journal of Biogeography 25: 855-870.