After exposure for eight weeks to a daily heat spell of 30 °C the plants of all populations performed well in terms of growth performance. This supports the results of several studies reporting increased growth rates of arctic and alpine species exposed to high temperature (Arft et al. 1999, Abeli et al. 2012, Fazlioglu and Wan 2021). To our knowledge, heat tolerance has never been tested so far inV. alpina (but see White et al. 2023). However, previous studies reported critical leaf temperatures > 40 °C for several arctic and arctic-alpine species (Buchner et al. 2017, O’Sullivan et al. 2017). There is evidence that alpine plants can experience leaf temperatures up to 13 °C higher than air temperature (Neuner et al. 2000) when absorbing irradiance at PPFD > 1000 µmol m−2 s−1. The leaf temperature of rosette plants can rise up to 30 °C above air temperature at 2 m height above the ground in clear summer days (Larcher et al. 2010). However, our plants were grown at lower irradiance and net CO2exchange was also measured at light levels about half that of the aforementioned threshold, i.e. about 500 µmol m−2s−1. Even if we did not measure leaf temperatures in our experiment, it is extremely unlikely that temperature in the foliar tissues reached critical levels because the Fv/Fm values were always close to optimum (Goltsev et al. 2012). In fact, the Fv/Fm in plant tissues undergoing heat stress, especially at high irradiance, drops far below 0.7 (Streb et al. 2003, Braun and Neuner 2005, Karadar et al. 2018). Indeed, the Fv/Fm and the chl content both were higher in the plants of all populations experiencing the heat spells. This suggests that the heat spells did not induce damage in the photosynthetic machinery. On the contrary, the photosynthetic efficiency was even improved by the high-temperature treatment. Similarly, Marchand et al. (2005) observed better photosynthetic performance in four arctic plant species exposed to a temperature rise of approximately 9 °C for eight days. Our study provided indirect evidence that the extreme treatment reduced water content, because of evaporation-driven decline in available soil water when temperatures were raised without concurrent increase in water addition (Elmendorf et al. 2012). In fact, lower stomatal conductance under the extreme treatment revealed reduced water availability in the plants of all populations that experienced the heat spells.