Abstract
In the context of a probable increase in intensity and frequency of extreme summer drought events, a better understanding of the key processes involved in water relations is needed to improve the theoretical foundations of predictive process-based models. This paper aims to analyse how temperate deciduous trees cope with water shortage. The exceptional summer drought of 2003 in Europe provided an opportunity to monitor stomatal conductance and twig water potential in European beech (Fagus sylvatica L.) at predawn and midday and to analyse variations with respect to leaf height within the canopy. By comparing our field measurements of twig water potential to values found in the literature, we confirmed the strong impact of soil water shortage on crown water relations. This paper shows that (1) the vertical gradient of stomatal conductance within the crown disappeared under extreme soil water depletion; (2) at maximum drought intensity, predawn twig water potential (ψ pd) reached −2.3 MPa at a height of 14 m in the crown and −2.0 MPa at a height of 10 m. The significant differences in ψ pd between the two measurement heights in the canopy may be due to night transpiration; (3) there was a close relationship between predawn twig water potential and relative extractable soil water; (4) as drought conditions intensified, there was a close relationship between canopy radiation interception and predawn water potential, as estimated daily from relative extractable soil water.
Highlights
The water potential gradient between the soil, the tree and the atmosphere allows water to be transported through the xylem elements in trees
The seasonal progression of soil water depletion corresponded to a continuous decrease in relative extractable soil water (REW), which was calculated with the BILJOU water balance model (Figs. 1 and 2)
The adult European beech trees in our study showed lower values of predawn water potential in Lorraine (N–E France) during summer 2003 than other previous studies
Summary
The water potential gradient between the soil, the tree and the atmosphere allows water to be transported through the xylem elements in trees This thermodynamic process is beneficial for trees, when xylem water potential exceeds the maximum capillary pressure sustainable without air entry (Cochard 2006), xylem embolism results (Sperry and Tyree 1988). In European beech trees, stomatal conductance increases from the base to the top of the crown (Aranda et al 2000, Cochard et al 1999, Herbinger et al 2005; Lemoine et al 2002a; Wieser et al 2003), whilst diurnal leaf water potential decreases (Aranda et al 2000; Lemoine et al 2002a, b). Little is known about the spatial variation of stomatal conductance in crown leaves under extreme soil water depletion
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