Credit: ©iStockphoto.com/Nickolay Stanev

Plants living in mountainous regions are widely expected to move up-slope with climate change as they track changing temperatures. In some situations, however, water availability can dominate plants' response to climate change, facilitating down-slope movements even under warming conditions.

Shawn Crimmins from the University of Montana, USA and co-workers compared the altitudinal distributions of 64 plants over much of California since the 1930s and found that species' optimum elevations shifted down-slope by 88.3 m despite mean annual temperature increasing by about 0.6 °C1. Analysis of data from meteorological stations for the same period showed a net decrease in the climate water deficit — a measure of the difference between water lost through evaporation and transpiration and that added by precipitation — of 100 mm between the early and latter years of the twentieth century. Down-slope shifts in vegetation are likely to be a response to observed changes in water availability rather than changes in temperature, found the authors.

The study shows that considering changes in temperature alone may be inadequate for understanding how plant distribution is likely to shift in response to climate change.