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

Abstract

Summary 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 modeling, 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 with the stomatal regulation strategy seem the primary mechanisms driving mortality vulnerability in mixed seedling plantations.

Plant–plant interactions can mitigate (or exacerbate) hot drought impacts

“…In an article published in this issue of New Phytologist , Mas et al . (2024; 1021–1034) deepen our understanding of how species interactions might mitigate future hot drought effects. Mas et al .…”

“…Given the damages already seen during hotter droughts with higher VPD, it is essential that we identify how individual species and ecosystems will respond to hotter droughts in the future. In an article published in this issue of New Phytologist, Mas et al (2024; 1021-1034 deepen our understanding of how species interactions might mitigate future hot drought effects. Mas et al demonstrate how hot drought effects can be mitigated by interspecific neighbors for Quercus pubescens but exacerbated by interspecific neighbors for Fagus sylvatica.…”

PDG-Arena: An ecophysiological model for characterizing tree-tree interactions in heterogeneous and mixed stands

“…Indeed, the direction of the effect seems to depend on the species composition -and particularly on the species respective strategies in reaction to water stress (Pretzsch et al, 2013;Mas et al, 2024;Jourdan et al, 2020) -as well as on environmental conditions (Grossiord et al, 2014;Pardos et al, 2021).…”

“…Regarding the effect of tree species richness on the resistance of stands to drought episodes, the literature shows heterogeneous results (Grossiord, 2018). Indeed, the direction of the effect seems to depend on species composition - and particularly on the species respective strategies in reaction to soil water deficit (Pretzsch et al, 2013; Mas et al, 2024; Jourdan et al, 2020) - as well as on environmental conditions (Grossiord et al, 2014; Forrester et al, 2016; Pardos et al, 2021).…”

Isohydricity and hydraulic isolation explain reduced hydraulic failure risk in an experimental tree species mixture

“…The effects of species mixing on drought resistance could result from different mechanisms, such as competitive reduction for water through resource partitioning or facilitation—for instance hydraulic redistribution ( Grossiord 2020 ). Yet, tree species diversity effects on tree drought resistance are not universal and can change in direction and magnitude according to the sites, the species of the composition of the mixture ( Grossiord 2020 ; Grossiord et al 2014b ; Mas et al 2024 ). Indeed, previous studies showed that tree species diversity effect can have positive ( Lebourgeois et al 2013 ; de- Dios-García et al 2015 ; Ruiz-Benito et al 2017 ), neutral ( Grossiord et al 2014b ; Merlin et al 2015 ), or even negative impacts ( Grossiord et al 2014a ; Vitali et al 2018 ).…”