Drought responses at leaf, stem and fine root levels of competitive Fagus sylvatica L. and Quercus petraea (Matt.) Liebl. trees in dry and wet years

The inter-annual variability of four growth-related parameters (light-saturated photosynthesis A,,, leaf area index L, annual stem diameter growth S, and fine root production P) was investigated together with various leaf water status parameters in adult Fagus sylvatica and Quercus petraea trees during five summers with weak, moderate or severe soil water deficit. Study aims were (i) to identify differences in drought sensitivity among the tree organs (leaf, stem or root) and between the tree species, and (ii) to test the hypothesis that differences in whole-tree drought response are relevant for competition between Fagus and Quercus in Central European mixed stands. Drought had only weak or no influence on A,, and L of the two species during the 5 years; the only exception being a severe drought in 1990 with an A,,, reduction of Fagus by 30%. Stem diameter growth of Fagus was significantly smaller in dry than in wet summers; in contrast, no clear stem growth/moisture relationship was detected for Quercus during the study period. Soil water deficits apparently stimulated fine-root,growth of Fagus during the dry mid-summer 1995, thus compensating for root biomass losses due to high root mortalities in this period. Quercus fine-root biomass and productivity were not significantly influenced by this drought. A higher drought sensitivity of stem diameter growth, fine-root production and photosynthesis in Fagus corresponded to a larger inter-annual variability of the predawn leaf water potential between wet and dry summers and generally smaller leaf conductances of this species compared to Quercus. Quercus was less sensitive to drought in its growth processes, but showed larger inter-annual variabilities of the osmotic potentials of the leaf symplasm and of the seasonal minima of bulk leaf turgor compared to Fagus. Thus, Fagus is competitively superior in mixture with Quercus despite its higher sensitivity to soil water deficits. It is concluded that a whole-plant perspective rather than a leaf-centred view of drought response is needed to predict the consequences of water shortage for ecosystem-level processes, such as tree competition and succession driven by climate change. (C) 2001 Elsevier Science B.V. All rights reserved.