MODELING TRANSPIRATION IN HOLM-OAK SAVANNA - SCALING-UP FROM THE LEAFTO THE TREE SCALE

The transpiration of oak-savannas in southern Spain was modelled by sc aling-up from the leaf to the tree during a drought period. Two charac teristics of this ecosystem were found to facilitate the modelling pro cedure. The first was a near-zero decoupling coefficient between the s urface and the surrounding bulk air, which simplifies the transpiratio n formulation. The second was that the surface conductance (g) is main ly related to the vapour pressure deficit of the air (D-a). Based on b oth of these characteristics, the modelling procedure provides a gener al model of transpiration over the time scale appropriate for a drough t period, from days to months. The response of g to D-a was found to f ollow a negative exponential function, such that beyond a minimum valu e, g becomes independent of D-a. This implies a feedback control on g by D-a. The consequences of this control for transpiration were found at different levels of plant water status. This explains the plants' a daptation to long dry periods, even though there is also continuous wa ter loss during these periods. Such an adaptation was corroborated by a seasonal hysteresis found in the relationship between transpiration and D-a as a function of the plants' water status. (C) 1997 Elsevier S cience B.V.