Disentangling morphological, physiological and environmental drivers of hydraulic dysfunction in tree saplings. A modelling approach

Survival in drought-prone environments is challenging during trees’ early life stages, which rely on functional traits to modulate their water balance. Yet the trait–environment interactions that drive sapling drought stress and mortality in field conditions remain unclear, limiting the success of reforestation attempts. We investigated how interactions between plant traits and soil conditions shape drought-induced hydraulic dysfunction in saplings commonly used in reforestation programmes. Three species with contrasting functional strategies – Fraxinus ornus L., Quercus ilex L., and Quercus pubescens Willd. – were monitored over two consecutive years in a restored Mediterranean area. We simulated plant water fluxes using the SurEau-Ecos plant hydraulics model embedded within the MEDFATE framework, with parameters estimated from measured and literature-derived traits. Model outputs were interpreted to identify mechanisms underlying drought vulnerability. We found that variations in water potential causing 50% loss of hydraulic conductivity, explaining more interspecific differences in drought resistance than minimum conductance over the same period. Root elongation and crown area affected modelled interannual variation in percentage loss of hydraulic conductivity (PLC), particularly in the two deciduous species. In older Q. pubescens, increased rooting depth reduced modelled PLC, whereas in older F. ornus, greater crown development increased it. Simulations across different potting growing media revealed soil-specific PLC responses, with lower PLC when soils were characterised by slighter decreases of water retention loss, as described by Van Genuchten parameters. By elucidating key drivers of sapling hydraulic dysfunction, this trait-based modelling approach provided informative insights for reforestation programmes.