Strategies for difficult times: physiological and morphological responses to drought stress in seedlings of Central European tree species

Drought is an important factor in ecological change and species distribution shifts. We conducted a greenhouse experiment with seedlings of four Central European tree species: Pinus sylvestris (PS), Picea abies (PA), Fagus sylvatica (FS), and Quercus robur (QR) to investigate their response to drought. We monitored maximum quantum yield of photosystem II (F-v/F-m) during a 60-day drought treatment and measured above- and below-ground characteristics as morphophysiological responses to drought stress. Due to the fast, juvenile growth of the deciduous species (FS and QR), they had higher soil water uptake and suffered more quickly from severe drought than conifers (PS and PA). The deciduous species maintained a higher F-v/F-m, until volumetric water content (VWC) was very low (< 5%), oscillating within a narrow safety margin. Both conifers PA and PS conserved soil water; photoinhibition in these species occurred at VWC of 14.5% and 5.5%, respectively. There were no differences in height between drought-stressed and irrigated seedlings, while drought reduced all root characteristics of the deciduous seedlings. Our study revealed trade-offs between different water management strategies, growth rate, and photoinhibition during the juvenile growth stage of our focal species. For climate change adaptation, anisohydric deciduous tree species seem to be more suitable. However, PS, with its water-conserving management and low photoinhibition threshold, holds promise for successful regeneration on drought-prone sites. Since species selection is critical for forest sustainability, our study contributes to the broader discussion of tree species' drought resistance during the vulnerable juvenile phase in the face of climate change.