Where has all the flora gone?

By Rachel Berkowitz

One potential effect of climate warming could be the shift to higher altitudes of mountain vascular plant species due to a reduction of alpine habitat. Empirical evidence for the shifts is provided largely by historical data. Complementing the historical records is a new worldwide observation network, initiated in 2000, that recorded the numbers of plants on 66 European mountaintops in two different years, 2001 and 2008. Harald Pauli of Austria’s Institute of Mountain Research and his large team of international collaborators compared the data and published their results last month in Science.

By comparing the number of species from each summit (down to the 10-meter contour line) in both years, Pauli’s team showed that the average number of species per summit increased from 34.9 to 37.7—a statistically significant 8% increase. An average altitude, weighted by species’ frequencies on the respective summits in either 2001 or 2008, of the distribution in each region indicated that species had indeed moved up slope by an average of 2.7 m.

However, the type of alpine habitat was found to play an important role: Whereas the number of vascular plant species on summits in temperate and boreal regions increased, the number in Mediterranean regions decreased. The species reduction in Mediterranean regions is thought to be due to a decrease in the availability of water, also associated with global warming.

Because Mediterranean mountains are particularly rich in endemic species, meaning species unique to a defined geographic area, a continuation of these trends could shrink overall diversity in the future, even though the average species count increased across the overall region from 2001 to 2008.

Of the species recorded in 2001 that had disappeared by 2008, 31% were endemics. But of the species recorded for the first time in 2008, only 13% were endemics. Those findings reflect the loss of species in areas rich in endemics, such as the Mediterranean, and the increase in non-endemic species in boreal and temperate areas, which have fewer endemics. The decrease in overall proportion of endemic to non-endemic species, from 24.5% to 23.4%, supports the idea that the composition of alpine species is becoming more homogeneous.

Pauli and his collaborators propose that range expansion of boreal and temperate alpine species is the result of global-scale warming, whereas range retraction of Mediterranean species is the result of both rising summer temperatures and reduced precipitation.

The implications of this work are disconcerting. If Mediterranean summits become warmer and drier, as is forecast by climate models, the trends in the fate and distribution of species are likely to continue. Because of the large number of endemic species in southern Europe, the number of species making up continental mountain flora might be reduced even if the species diversity on the majority of boreal and temperate mountaintops increases.

Although in general the observations reported in Science support the expectation that species will shift to higher altitudes because of climate warming, the result will not necessarily be an increase in species diversity on continental mountaintops.