Switzerland, like most other temperate zone mountain regions, has seen a 1 K warming during the last 100 years, with milder winters and generally higher minimum temperatures. Summer temperatures and maximum daytime temperatures have been less affected, although June 2003 was the warmest on record in the Alps, inducing massive melting processes at high elevation. Will the current and potential further warming affect alpine plant life?

Here is a selection of warming effects ranked by likelihood:

1. The most likely effects will be faster tree growth at the treeline (Fig. 1). However, this does not imply a rapid advancement of the treeline. Treelines have been found to migrate very slowly with a several century delay. However, past climate-induced fragmentation of the treeline ecotone is likely to become reduced as warming continues (ongoing "infilling" of trees in gaps).

2. Alpine pioneer vegetation is likely to respond faster than late succesional vegetation. Several studies have documented an increase in the number of plant species in the summit regions of high mountains during the past 100 years (e.g. Grabherr and Pauli 1994, Bahn and Körner 2003). However, there is evidence that late successional alpine grassland will hardly be affected by warming. Clones of Carex curvula have been dated by DNA mapping (via clonal size) and were found to have been at the very same location for more than > 1500 years old. The climates over these long periods included medieval warm periods and the little ice age. This suggests great persistence, irrespective of climate, of such "old" vegetation types.

3. A change in snowbed vegetation. Earlier release from snow causes a longer season length. Note that an increase in snowpack by heavier late winter snowfall, as has been observed in recent years, can reverse the warming-only effect on snow duration.

1 - Treeline trees have shown increasing tree ring width in recent decades, which reflects the general increase in air temperatures. Tree ring data are from the central Alps (Valais). The chronology shown is composed of data for trees of equal age during respective periods (no age trend incurred). Data by Paulsen et al. 2000.

2 - Alpine pioneer plant species have been shown to migrate upslope in the Alps.

3 - Snow distribution and duration may override temperature-only effects. In the foreground, late successional alpine grassland.

29 August 2011
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