Dealing With Two Stresses: Impact of a Damaging Spring Frost Followed by a Summer Drought on Saplings of Four Temperate Tree Species.

Global warming increases the likelihood that temperate tree species will face damaging late spring frost (LSF) and severe summer drought during the same growing season. However, the interactive effects of these two stresses are barely explored. We investigated the physiological and growth responses of Acer campestre, Fagus sylvatica, Quercus robur and Quercus petraea saplings to artificially induced LSF and drought, focusing on stomatal gas exchange, carbon partitioning, nonstructural carbohydrates (NSCs), phenology and tree growth. LSF depleted NSCs and changed carbon allocation patterns 1 month after the event. Additionally, LSF decreased diameter increment and root growth of A. campestre and F. sylvatica in the current year. Drought affected gas exchange of all species, decreased NSCs of A. campestre, reduced biomass of Q. robur, and exacerbated the detrimental LSF effect on Q. robur's NSCs. Our findings indicate that saplings prioritized canopy restoration immediately after LSF, and favored reserve replenishment before growth until the end of the growing season. Furthermore, we highlight the risk that LSF and drought in the same year could push tree species beyond their physiological limits and we emphasize the importance of studying multiple stressors' interactions to better understand threshold effects that could profoundly alter forest ecosystems.