Increased light exposure reduces drought survival in tree seedlings

Forests are increasingly subjected to drought stress and heatwaves, with recent years showing widespread tree mortality, particularly in monocultures. To improve the climate resilience of forests, a transformation towards mixed-species and more adaptive stands is essential. Thinning is one effective measure to enhance water and light availability for the (natural or artificial) regeneration of diverse tree species. To better understand how microclimatic factors such as light, vapor pressure deficit, and soil moisture influence drought resistance in the main tree species Abies alba, Fagus sylvatica, Quercus robur, and Pseudotsuga menziesii, seedlings were planted under controlled greenhouse conditions, with either full light or shaded conditions provided by shading nets. A simulated drought period was then applied to observe the time until stomatal closure and critical hydraulic failure, defined as the point at which trees reached the water potential at 88% loss of hydraulic conductivity. Predawn water potential and gas exchange measurements were combined and stem diameter change continuously monitored.

Light-exposed seedlings generally exhibited greater height and stem growth than shaded seedlings. However, predawn water potential measurements indicated that these seedlings suffered from drought earlier than shaded trees and consistently died earlier. This was attributed to higher transpiration rates of light-exposed trees, resulting from a higher evaporative demand, larger plant size and total leaf area compared to the shaded trees. Under light conditions, differences in the sequence of drought-induced mortality were more pronounced (Q. robur died first, followed by F. sylvatica and P. menziesii), whereas under shaded conditions, mortality times were more uniform across these species. Overall, A. alba demonstrated the highest drought resistance.

These results emphasize the critical role of total leaf area in determining drought resistance among tree species. While light generally promotes CO₂ uptake and growth in most tree species, it can also exacerbate drought stress under certain conditions. Considering species-specific drought tolerance and implementing adaptive forest management practices, such as promoting mixed-species stands and adjusting thinning regimes, will be key to balancing the benefits of light with the need for drought resistance during the juvenile stage for mitigating the impact of future heatwaves and droughts on European forests.