Forest structure matters: Vertical metrics mitigate vapor pressure deficit in Scots pine-dominated temperate forests during dry periods

Temperate forests face increasing drought risk, with longer and more frequent events further limiting water resources in already dry regions. A promising climate-adaptive management strategy is to promote greater structural complexity, as forest structure shapes microclimate in ways that can alleviate high vapor pressure deficit (VPD) during dry periods. However, the mechanism by which structural attributes modulate subcanopy VPD, and whether such effects involve trade-offs with topsoil water availability, remain poorly understood. With this study, we ask (1) whether greater VPD reduction comes at the cost of topsoil desiccation across stand types, (2) which structural variables best explain VPD reduction, and (3) if these metrics can mitigate macroclimate-driven VPD increases. We conducted our study in 40 plots representing four stand types (single- and multilayered stands of Scots pine and conifer-deciduous mixtures) in Brandenburg, one of Germany’s driest regions. Forest structure was quantified using hemispherical imaging and mobile terrestrial laser scanning. Furthermore, microclimatic conditions (temperature, humidity) and soil moisture in 10 cm depth were monitored over two growing seasons. We found that (1) mixed-species and understory-rich stands experienced lower VPD than pure pine stands without increasing topsoil desiccation. Within-stand variation in VPD reduction was best explained by (2) structural metrics describing vertical layering, in particular those reflecting understory vegetation density. These attributes (3) counterbalanced up to 24.5 % of the macroclimate-driven VPD increase (1.67 kPa) observed during hot and dry periods, highlighting their substantial mitigation potential for climate-adaptive forest management. While future research should consider deep soil water depletion, our study identifies key metrics derived from three-dimensional forest structure that drive subcanopy VPD. This advances our understanding of VPD variation at local scales, which will be critical for sustaining regeneration potential and ecosystem resilience in a changing climate.