Why Plants Sometimes Migrate Downhill as Temperatures Rise
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.
Working in most of the major California (USA) mountain ranges north of 35°N latitude, the five researchers tested their theory, using two "temporally independent data sets," one comprised of 13,746 survey plots that were sampled in the 1930s and one comprised of about 33,000 plots that were sampled from 2000 to 2005, over which time period mean annual temperatures in the state rose by about 0.6°C. Concomitantly, they report there was an increase in precipitation across the northern portion of the state that resulted in a net decrease -- over this same time interval -- in the climatic water deficit, which they define as the difference between potential evapotranspiration and precipitation.
Results indicated "a higher proportion of species shifting their distributions downhill" between the 1930s and the early 2000s. They also say "there was moderate evidence of increases in optimum temperatures experienced by species, with a higher proportion of species shifting toward warmer conditions than cooler conditions," and as a result, they report that plants in their study area "are experiencing an increase in their optimum temperature (0.36°C) due to both climatic warming and downhill shifts."
To these findings, it should be noted that increases in the air's CO2 content also tend to increase most plants' optimal temperatures (i.e., the temperatures at which their photosynthetic rates are maximal), which latter observation also meshes well with Crimmins et al.'s final conclusion that "this combination of factors has likely increased net primary productivity in the region (Lenihan et al., 2003)."
Lenihan, J.M., Drapek, R., Bachelet, D. and Neilson, R.P. 2003. Climate change effects on vegetation distribution, carbon, and fire in California. Ecological Applications 13: 1667-1681.