Leaf water relations of competitive Fagus sylvatica and Quercus petraea trees during 4 years differing in soil drought

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 (Ypd) and osmotic potentials (Po, Pp) were remarkably similar for the two species. Fagus had significantly lower leaf tissue elasticities (emax) 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 emax and daily water potential (Y) minima; and small for Po, Pp, and the relative leaf water content at zero turgor. Fagus regulated water loss conservatively with only limited reductions in P and Y in wet or moderately dry seasons but large decreases in Ypd,gl, photosynthesis, and growth during occasional severe droughts. Quercus displayed patterns of a stress-tolerating species (lower emax, a less drought-sensitive stomatal regulation, and apparently significant droughtinduced 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.