Drought acclimation of two deciduous tree species of different layers in a temperate forest canopy

Water-use strategies of Populus tremula and Tilia cordata, and the role of abscisic acid in these strategies, were analysed. P. tremula dominated in the overstorey and T. cordata in the lower layer of the tree canopy of the temperate deciduous forest canopy. Shoot water potential (Psi), bulk-leaf abscisic acid concentration ([ABA](leaf)), abscisic acid concentration in xylem sap ([ABA](xyl)), and rate of stomatal closure following the supply of exogenous ABA (v) decreased acropetally through the whole tree canopy, and foliar water content per area (w), concentration of the leaf osmoticum (c), maximum leaf-specific hydraulic conductance of shoot (L), stomatal conductance (g(s)), and the threshold dose per leaf area of the exogenous ABA (d(a)) required to reduce stomatal conductance increased acropetally through the tree canopy (from the base of the foliage of T. cordata to the top of the foliage of P. tremula) in non-stressed trees. The threshold dose per leaf dry mass of the exogenous ABA (d(w)) required to reduce stomatal conductance, was similar through the tree canopy. After a drought period (3 weeks), the Psi, w, L, g(s), d(a) and d(w) had decreased, and c and v had increased in both species. Yet, the effect of the drought period was more pronounced on L, g(s), d(a), d(w) and v in T. cordata, and on Psi, w and c in P. tremula. It was concluded that the water use of the species of the lower canopy layer-T. cordata, is more conservative than that of the species of the overstorey, P. tremula. [ABA](leaf) had not been significantly changed in these trees, and [ABA](xyl) had increased during the drought period only in P. tremula. The relations between [ABA](leaf), [ABA](xyl) and the stomatal conductance, the osmotic adjustment and the shoot hydraulic conductance are also discussed.