Divergent vegetation growth responses to the 2003 heat wave in the Swiss Alps

In 2003, Europe experienced its hottest summer in >500 years. Satellite-derived photosynthetic activity estimates across the Alps revealed a pattern of high elevation growth enhancement and low elevation growth suppression in response to these extreme summer temperatures. Surface weather-derived effective growing season lengths were shorter in 2003 by an average of 9% and 5% for colline and montane areas respectively and were 2%, 12% and 64% longer for subalpine, alpine and nival areas respectively. In situ forest growth measurements of 244 trees at 15 sites across Switzerland verified this pattern and revealed that this divergent response was consistent between species. We suggest that warmer summer temperatures lengthened the snow-free growing season at high elevations while they increased summertime evaporative demand at lower elevations. Our investigation demonstrates that climatic changes are affecting plants beyond simply shifting the elevation belts upwards.