Phenotypic plasticity
We aimed to assess phenotypic plasticity by determining the response of V. alpina populations to short-term heat spells mimicking heat-wave conditions. Despite the overall good tolerance to the heat spells exhibited by V. alpina plants, we did observe differences among populations in the level of phenotypic plasticity towards the simulated heat wave. Such differences among populations were principally due to different water acquisition capabilities (Chaves et al. 2002) which results in different WUEs at a given level of stomatal opening (Medrano et al. 2009). The significant increase of net photosynthesis and WUE under heat spells showed by SWE (Fig 4b) resulted in a simultaneous increase of biomass (Fig. 4a), consistent with the overall effects of climate warming on plants (Wang et al., 2018; Song et al., 2019). Phenotypic plasticity can evolve and can be selected to cope with variable environments (Ackerly et al 2000; Richards et al 2006), promoting population persistence through the development of more adapted traits (Nicotra et al. 2010; Richardson et al. 2017). However, we also found that such plastic response to warmer growing conditions did not occur in the other populations of V. alpina, whose biomass was lower and did not change between treatments. This apparent lack of responses may occur in extreme environments, which select so strongly for stress adaptations that populations evolve decreased plasticity, maintaining traits better adapted to an extreme environmental disturbance (Matesanz et al. 2010). Furthermore, low above ground biomass is common under limited water availability, allowing plant to better conserve water and reduces evapotranspiration (Basu et al., 2016; White et al. 2023). This may explain why the populations showing the lowest biomass (PRA), also showed the lowest mortality in response to heat spells (Fig. 4g).