The shape of plant community to come: linking species functional response and climate-induced shifts in alpine tundra dwarf shrub community

Shrub expansion is a prominent effect of climate warming in both arctic and alpine regions, which is leading to a vast arctic and alpine greening (Myers-Smith et al., 2020). For this reason, understanding the response of dwarf shrubs to main climate drivers is of outmost importance. Climate changes induce plants to acclimate by both morphological and physiological traits due their phenotypic plasticity. A functional trait approach is hence crucial to foresee the response of key species of the ecosystems, which changes could also affect the entire community (Boscutti et al., 2018). Moreover, species with similar functional traits are assumed to respond to environmental modifications in comparable ways. Hence, the response of key species sharing similar traits might reverberate on the entire ecosystem proprieties, promoting an effective upscaling perspective (Fig. 1). Temperature is a well-known driver of shrub expansion, while its interplay with other climate variables (e.g. rainfall regime) has been often overlooked. This study aims at parsing the relationships between the climateinduced functional response of the key species Vaccinium myrtillus L. and the entire plant community. We here present the results of an observational study along an elevation gradient and a rain exclusion experiment conducted in alpine tundra in the north-eastern Alps. In the observational study, we found alpine greening of dwarf shrub community to be associated to temperature, but only when precipitation sustained the V. myrtillus growth (i.e. high temperature and high precipitations). In the rain manipulative experiment, instead, we found precipitation to induce a trait-mediated plant community response, favouring graminoid species in contrast to forbs, especially in stands where shrub expansion was not completed (half cover of dwarf shrubs), suggesting that species acclimation to drought relies not only on species response, but also on the whole community traits. This might argue that the ongoing increase of summer heat wave frequency in the alpine range could partially contrast the shrub expansion and, in turn, favor alpine grassland species, more adapted to water scarcity. These results might reverberate on the whole plant community, affecting the overall diversity, by changes in species richness and species composition. We suggest that future scenarios depicting the fate of alpine plant community should not neglect for the interplay of temperature and precipitation regimes and that a plant trait approach would be crucial in the understanding of ecological mechanisms underlying the change of plant community, especially in high elevation ecosystems.