Unraveling the role of light and biotic interactions on seedling performance of four Pyrenean species along environmental gradients
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The predicted upward displacement of forest species due to climate warming is expected to be modulated by a medley of abiotic and biotic factors acting at microsite level. Species-specific differences in plant responses to this set of environmental factors can thus have strong implications in the future
dynamics of forest ecosystems. To gain a better understanding of the main fine-scale factors and processes driving present and future species performance in the montane and subalpine belt of the Eastern Pyrenees (NE Spain), we established a set of experimental mixed plantations along elevational and environmental gradients using the four tree species dominating these areas (Pinus sylvestris, Pinus uncinata, Abies alba and Betula pendula). Once the plantations had been established, the performance and growth of 72 seedlings of each species was monitored and linear and non-linear models were fitted to identify the main factors controlling their survival and growth. We found most of the mortality to occur during the third growing season, following a harsh winter and a drought period during summer. Mortality patterns were highly species- and site-specific. At the subalpine belt, shrubs were found to have a facilitative effect on winter survival of P. sylvestris (mortality < 10%) but not on the other species. At the montane belt, A. alba mortality during the summer increased in areas with high light exposure and herbaceous cover (mortality > 30%). All species except P. uncinata showed lower height growth at high elevation, with differences between sites matching differences in growing season duration (20%). Our results underline the strong impact that short periods of extreme climate can have in the performance of plants developing in mountainous areas far from their optimal elevational range. However, they also underline a potentially critical role played by biotic and abiotic microsite factors in mediating species responses to these climatic events.