Stomatal regulation and water potential variation in European beech: challenging the iso/anisohydry concept

Abstract

The iso/anisohydric continuum has been used to classify tree species’ drought response strategies. The range over which stomata are regulating leaf water potential (ψl) before turgor loss occurs can be described with metrics such as the dependence of ψl on soil water potential (ψsoil) and the size of ‘hydroscape area’ (HA), but corresponding field data from adult trees are scarce. We examined the stomatal conductance (gs)–ψl relationship in its temporal (diurnal vs seasonal and interannual) and spatial (within-crown vs between-site) variation in European beech, using extensive ψl and gs measurements in the canopy of four beech stands across a precipitation gradient, and complemented the data set by published ψl and gs measurements in further Central European beech stands (including the extreme 2018 drought) in order to cover the full water potential operation space of the species. Both metrics characterize beech as a strictly anisohydric species with δψl/δψsoil >> 1 and HA = 4 MPa2. However, stomates close sensitively in response to increasing vapor pressure deficit, disproving the widely assumed dependence of large ψl variation on looser stomatal control. Characterizing the water status regulation mechanisms of trees requires separating diurnal from day-to-day variation in ψl and gs. The large diurnal and seasonal ψl variation in beech leaves is partly caused by a low leaf tissue elasticity, suggesting that a whole-plant perspective with consideration of osmotic and elastic tissue properties and stem and root hydraulics is needed for fully understanding ψl regulation and the drought tolerance strategy of trees.

Leaf trait modification in European beech trees in response to climatic and edaphic drought

“…2000; Leuschner et al . 2021), which enables beech to reduce stand transpiration to moderate levels under high evaporative demand, even when high leaf area indices (LAIs) >7 developed at the drier sites (Meier & Leuschner, 2008).…”

“…In contrast, a comparison of growing‐season Ψ tlp means among four mature European beech stands covering a reduction in mean annual precipitation (MAP) of 520 mm·year −1 showed no Ψ tlp decrease with increasing climatic aridity (Schipka, 2002), while others observed osmotic adjustment during the summer period in this important Central European tree species (Backes & Leuschner, 2000; Leuschner et al . 2019, 2021). This matches the reduction in Ψ tlp after drought exposure found for sapling and adult beech trees by Tomasella et al .…”

Adjustment of hydraulic, anatomical and leaf traits in Fagus sylvatica (L.) towards drier conditions. A study in mature stands.

“…showed no tlp decrease with increasing climatic aridity (Schipka, 2002), while others observed osmotic adjustment during the summer period in this important Central European tree species (Backes & Leuschner, 2000;Leuschner et al, 2021). This matches the reduction of tlp after drought exposure found for saplings and adult beech trees by , whereas others did not observe any osmotic adjustment in saplings .…”

“…The δ 13 C decrease with increasing CWBS, on the other hand, is in agreement with our assumption and suggests that leaf expansion at the sites with higher climatic aridity is closely linked to a reduction in stomatal conductance, relative to sites with lower aridity. Stomatal regulation in beech has been found to be relatively sensitive to atmospheric saturation deficits (Oren et al, 1999;Aranda et al, 2000;Leuschner et al, 2021), which enables beech to reduce stand transpiration to moderate levels under high evaporative demand, even when high leaf area indexes (LAIs) >7 are developed at the drier sites .…”

“…Both experimental evidence with saplings and observations in adult trees indicate that beech is moderately sensitive to drought (Geβler et al, 2004;Braun et al 2021). Yet, the species follows an anisohydric water potential regulation strategy, encountering considerable drops in leaf water potential (Leuschner et al, 2021). Thus, European beech is dependent on sufficient water supply during summer and likely also high atmospheric humidity, while sensitivity to frost and summer heat are limiting the species to mesic cool-temperate regions .…”