Water availability determines heat stress response in temperate broadleaved tree species

Abstract

Heat waves and co-occurring droughts are increasing in frequency and magnitude in Central Europe with major impacts on tree water and carbon dynamics. So far, knowledge on how common temperate broadleaved tree species respond to combined heat and drought remains limited. To understand if tree species might tolerate future climate conditions, knowledge on their ability to regulate canopy temperature, sustain gas exchange and prevent leaf damage is essential. In our study we explored the impacts of increasing temperature and vapour pressure deficit (VPD) combined with a mild soil drought on three broadleaved tree species Fagus sylvatica, Acer platanoides and Quercus robur. Seedlings were subjected to a gradual temperature increase from 25°C to 45°C in a greenhouse, experiencing either well-watered control or mild drought conditions. Using single tree chambers, we assessed changes in leaf temperature, gas exchange and leaf senescence. We found that with higher temperatures and VPD transpirational cooling of the leaves increased in all species, even though it was lowest for A. platanoides. Under mild drought conditions leaf cooling was strongly limited and leaf temperatures mostly exceeded air temperatures. Drought limitations were also reflected in the gas exchange with overall lower net assimilation, stomatal conductance and transpiration rates.  Still, drought and control trees both exhibited increased transpiration rates with higher temperature and VPD. At the same time stomatal conductance and net assimilation decreased with increased heat. Consequently, water-use efficiency strongly declined under well-watered and drought conditions for all species, emphasizing the crucial role of water during heat stress. For Q. robur these declines in gas exchange only happened at temperatures beyond 38°C under well-watered conditions, indicating higher thermal thresholds. This was also reflected in the leaf senescence as Q. robur avoided visible leaf damage entirely. In contrast, F. sylvatica and A. platanoides demonstrated high leaf senescence particularly in combination with drought. In summary, our study highlights the importance of water availability for thermal regulation and sustaining positive net carbon uptake. Even though all species showed similar trends in their heat response, sustained net assimilation and avoidance of leaf damage point towards a potentially better heat resistance of Q. robur compared to the other two species.