Contrary to growth performance, the heat spells negatively affected survival of the plants exposed to the extreme treatment. Interestingly, Marchand et al. (2005) found increased rates of mortality in spite of increased photosynthetic performance in three species exposed to experimentally-induced heat spells at a high Arctic tundra site. All in all, our results support the finding of several studies reporting detrimental effects of warming on alpine and arctic-alpine species when air temperature was increased by 4-8 °C above the site average, only when warming was associated with drought (Hernández-Fuentes et al. 2015, De Boeck et al. 2016, Berauer et al. 2019, Notarnicola et al. 2021). Notably, highest phenotypic plasticity coincided with the highest number of different haplotypes in the SWE population. There is much evidence that genetically diverse populations of plants inhabiting cold to temperate regions possess high capacity to respond to environmental changes. High phenotypic plasticity has been observed in North-American populations of the arctic-alpine cushion species Silene acaulis (Gussarova et al. 2015, DeMarche et al., 2018). Populations of Heterotheca brandegeei, a North American chasmophyte, presented high levels of phenotypic plasticity in the response of WUE to compound effects of heat and drought which may bufferH. brandegeei from the negative impacts of climate change (Winkler et al. 2019). Conversely, Wickander et al. (2021) did not observe any relationship between genetic variation and phenotypic plasticity in response to high temperatures in populations ofPersicaria vivipara, an arctic-alpine species mostly propagating clonally, with low levels of sexual reproduction. On the other hand, populations of Argyroxiphium sandwicense subsp.macrocephalum, a Hawaiian mountain plant, lack any genetic control of phenotypic plasticity although this species presented an overall plastic response, in terms of improved WUE, to a heating and drought treatment (Krushelnycky et al. 2020).