Science: Driven by Warming, Plants in Western Europe are Climbing to Higher Altitudes

The background picture is a mountain stream located in the Ubaye valley (French South Alps). Patchwork of forest landscapes and plant species in western European mountains. The mosaic of larger pictures shows different mountain forest ecosystems spanning the whole altitudinal gradient from lowlands to the subalpine vegetation belt (from the bottom to the top) and encompassing the French latitudinal gradient from Mediterranean to temperate forests (from the right to the left). The mosaic of small pictures illustrates forest plant species diversity within western European mountain forests.

[Image © Science]

To escape warming temperatures, some forest plants are taking up residence in higher, cooler locales, but the upward climb of these plants could disrupt mountain ecosystems, says new research in the 27 June issue of Science.

Jonathan Lenoir and his research team surveyed 171 plant species in Western Europe and found an average upward shift of 29 meters per decade, give or take about 11 meters. The shift happened over the course of the 20th century with the steepest climb occurring between 1971 and 1993, when the average shift was nearly 65 meters per decade.

Lenoir, from AgroParisTech in Nancy, France, and his colleagues studied common forest plant species in six mountain ranges in Western Europe, including temperate and Mediterranean mountain forests. The forest communities varied from lowlands to the upper sub-alpine vegetation belt, or from sea level to 2600 meters in elevation.

The researchers restricted their study to areas beneath well-established forest canopies as opposed to open areas where plants are more susceptible to year-to-year climate variations or agricultural practices. Forest canopies act as buffer zones smoothing temperature fluctuations that might otherwise be interpreted as long-term warming trends. "Changes in species distribution under the forest canopy can therefore be considered as fingerprints of regional trends," Lenoir and his colleagues wrote. They ruled out nutrient variability, land-use changes and introduction of invasive plants as variables other than a warming climate that could have produced the upward shift.

Portions of the mountain ranges studied by the researchers are within France, where climatic change has been associated with substantial increases in average temperature compared to temperature increases in other areas of the world. Studies show at least a 0.6 degree Celsius increase in average temperature over the 20th century. Lenoir and his colleagues conclude that warming temperatures are "the main driving force" for their results.

The average size of the shift of forest plants—29 meters per decade—is similar to what has been observed in alpine plants above the tree line. What's more, the observed increases in altitude of certain forest plants add to previous findings that plants shift in latitude and longitude with a changing climate.

"In general, our results show that species displayed different rates of movement, behaving in a seemingly idiosyncratic way in response to climate change," Lenoir and his colleagues wrote. But the plants' movements might not be that haphazard. The researchers note that the plants occupying the same geographical area or sharing physical traits show the same patterns of altitude change.

They found that plants that climbed the most were species living in mountainous areas and species that have short life cycles, faster maturation and—once they reach maturity—are smaller in size. For instance, herbs, ferns and mosses tended to shift more than woodier plants like trees and shrubs. Greater dispersion of seeds by plants with short life cycles contributes to the greater uphill movement of smaller, grassier plants.

In contrast, trees did not climb as high or as fast as smaller plants with shorter life cycles. "At this point in time we believe that the grasses from lower altitudes will be on the same altitude with trees from a higher altitude," Lenoir said during a Science/AAAS teleconference. The change in the composition of plant communities could disrupt ecosystems, and the authors urge more studies on how climate change alters forests.

Brandon Bryn and Molly McElroy

26 June 2008