Climate sensitivity of bilberry (Vaccinium myrtillus L.) dwarf shrub communities in Alps: interactions between plant growth, temperature and precipitation.

Shrub expansion is a prominent effect of climate change in the alpine and arctic ecosystems. One of the major drivers of the enhanced shrub growth is the increase in temperatures and many studies have investigated on this. Instead, the effects of the interplay between temperature and other climate variables (e.g. rainfall regime) on the shrub growth has been often neglected. This study aims at parsing the relationships between annual growth of Vaccinium myrtillus L., key species in the alpine tundra, temperature, precipitation and their interaction in the north-eastern Alps. We considered a 500 meters elevation gradient analyzing xylem rings of 100 cross sections of underground ramets of V. myrtillus referring to the period 1995-2015. We found altitude to affected both ramet age (i.e. number of rings) and the mean ring width. The mean temperature and annual precipitation showed significant interaction effects on mean ring width and xylem mean lumen area. At low temperatures an increase of precipitations led to a reduction in the mean ring width, while high temperatures and precipitation increased annual ring width. Mean lumen area was affected by precipitation only in cold years. Our results suggest that future scenarios should not overlook for rainfall regime effect by virtue of its possible role in snowpack permanence and summer drought