Commentary: Is climate change making plants go up mountains?

Abstract

ISSN 1948-6596 news and update commentary Is climate change making plants go up mountains? Paleontological evidence indicates that about 2.5 million years ago the world climate entered a pe- riod of instability. Climatic fluctuations, probably forced by cyclical changes in the earth’s orbit around the sun, resulted in a series of ice ages and today we are in a period between glacial ad- vances. Alterations in solar radiation are also thought to cause climatic variation such as the Medieval Warm Period (~950-1200 AD) and Little Ice Age (~1650-1850 AD); and major volcanic eruptions lead to short-term changes by changing atmospheric reflectance. In addition to these natural factors, climate is also increasingly thought to be influenced by human activities, an issue that is the topic of intense international dis- cussion under the United Nations Framework Con- vention on Climate Change (UNFCCC) and where the scientific indicators of climate change have been under a media spotlight. During the last century almost all environ- mentally important atmospheric trace gases have increased dramatically, and the consensus opinion of the Intergovernmental Panel on Climate Change (IPCC) is that there is correlation between changes in atmospheric composition and ob- served changes in the global climate. Major rea- sons are increasing use of fossil fuel energy, to- gether with land use changes such as for food and energy production. These human activities lead to the mobilisation and volatilisation of soil and plant carbon and nitrogen stocks, increasing the flux of CO 2 , CH 4 and N 2 O into the atmosphere (IPCC 2007). During the last century, global mean sur- face temperature has increased with unprece- dented speed since records began and CO 2 levels range beyond any of those of the last 650,000 years (IPCC 2007). In a study of the altitudinal distribution of 171 western European mountain forest plant spe- cies based on an impressive database, Lenior et al. (2008) have provided some evidence that ob- served increases in regional temperature are be- ing translated into upward shifts of plant species. They compared the altitudinal distribution of their sample between two time periods: 1905-1985 and 1986-2005, over an elevational range of 0-2600 m, and calculated an aggregate upwards shift of 64 meters from 1971 (mean year of the first interval) and 1993 (mean year of the second interval) in the core ranges of the plants investigated. As might be expected with such rapid changes in climate, trees showed less response than herbaceous species. In an additional study to investigate whether or not woody plants were also upwardly mobile, Lenoir et al. (2009) tried to demonstrate that, even if adult trees did not show a climate change signal, their seedlings were indeed responding. Both studies made attempts to eliminate confounding variables, but in doing so included some bias. They investigated the spatial core of the species’ ranges, rather than the edges of distributions, where the effects of climate change are likely to be most apparent, but where there is also likely to be greater natural fluctuation. In addition they only included forest communities, which are buff- ered by the micro-climate created by the forest canopy and so are less likely to show short-term responses. Moreover they ruled out the effects of increasing nitrogen deposition, land-use change, invasive species and CO 2 fertilisation. On the other hand they excluded trees with a high risk of hu- man introduction such as spruce, and in so doing lost one of the most important trees of the upper forest line. How important is this evidence in the con- text of policy making? It shows that, in western European mountains at least, regional tempera- ture increases are partly being translated into up- ward shifts of plant communities. However, as one would expect from an individualistic response to environmental change, the species did not be- have uniformly. Mountain plants showed greater shifts than elevation generalists, as did plants with faster life history traits. Of the 171 species in the first paper (Lenoir et al. 2008) two-thirds went up but a third went down. Similarly of the tree spe- cies analysed in detail (Lenoir et al. 2009), the seedlings of 10 species showed an upward trend frontiers of biogeography 2.1, 2010 — © 2010 the authors; journal compilation © 2010 The International Biogeography Society