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Leaf water relations of competitive mature Fagus and Quercus trees were compared during four seasons with low, moderate, or high soil drought intensities in humid northwestern Germany. Leaf conductances (gl) typically were higher by about 30% in Quercus than in Fagus sun leaves. Predawn leaf water potentials (Ψpd) and osmotic potentials (Πo, Πp) were remarkably similar for the two species. Fagus had significantly lower leaf tissue elasticities (εmax) than Quercus in dry but not in wet summers. Interannual variabilities in water status parameters were large for gl and seasonal bulk leaf turgor (P) minima; moderate for εmax and daily water potential (Ψ) minima; and small for Πo, Πp, and the relative leaf water content at zero turgor. Fagus regulated water loss conservatively with only limited reductions in P and Ψ in wet or moderately dry seasons but large decreases in Ψpd,gl, photosynthesis, and growth during occasional severe droughts. Quercus displayed patterns of a stress-tolerating species (lower εmax, a less drought-sensitive stomatal regulation, and apparently significant drought-induced osmotic adjustment). In temperate humid environments, tree water relations should be studied for at least three or four seasons to account for large interannual variabilities in water status parameters.

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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.