Interactions between beech and oak seedlings can modify the effects of hotter droughts and the onset of hydraulic failure

Mixing species with contrasting resource use strategies could reduce forest vulnerability to extreme events. Yet, how species diversity affects seedling hydraulic responses to heat and drought, including mortality risk, is largely unknown.

Using open-top chambers, we assessed how, over several years, species interactions (monocultures vs. mixtures) modulate heat and drought impacts on the hydraulic traits of juvenile European beech and pubescent oak. Using modelling, we estimated species interaction effects on timing to drought-induced mortality and the underlying mechanisms driving these impacts.

We show that mixtures mitigate adverse heat and drought impacts for oak (less negative leaf water potential, higher stomatal conductance, and delayed stomatal closure) but enhance them for beech (lower water potential and stomatal conductance, narrower leaf safety margins, faster tree mortality). Potential underlying mechanisms include oak’s larger canopy and higher transpiration, allowing for quicker exhaustion of soil water in mixtures.

Our findings highlight that diversity has the potential to alter the effects of extreme events, which would ensure that some species persist even if others remain sensitive. Among the many processes driving diversity effects, differences in canopy size and transpiration associated to the stomatal regulation strategy seem the primary mechanisms driving mortality vulnerability in mixed seedling plantations.

Methods
Water-related physiological traits, including light‐saturated assimilation (Amax), stomatal conductance (gs), water potential at predawn (Ψpredawn) and midday (Ψmidday), water potential at turgor loss point (ΨTLP), minimum stomatal conductance (gmin), individual leaf area (LAleaf) and specific leaf area (SLA) were measured on 96 seedlings of European beech and pubescent oak grown in monoculture and in mixture. These physiological traits were used to calculate the stomatal safety margin (ΨTLP - P50; SMP50) using the P50 extracted from a database, and the leaf safety margin (Ψmidday - ΨTLP; SMleaf). Measurements were done at the late growing season (i.e., September) during three years (from 2020 to 2022). The trees were under a fully crossed combination of two air temperature and soil moisture regimes. The gas exchange measurements were carried out between 9 am and 3 pm when the trees were the most active. Further, the model SurEAU simulated the time to stomatal closure (TSC) and to hydraulic failure (THF) using the physiological traits collected in 2022.