Stomatal response of Quercus pyrenaica Willd to environmental factors in two sites differing in their annual rainfall (Sierra de Gata, Spain)

Abstract

Quercus pyrenaica natural forests located in the Sierra de Gata (Salamanca Province, Spain) were studied. Two permanent sampling sites were selected at the two extremes of a rainfall gradient in this area. Diurnal courses of transpiration rate, stomatal conductance and leaf water potential were determined approximately every 2-3 weeks in 1991 and 1992 during the active leaf period at different levels in the tree canopy. Current variations in photosynthetically active radiation (PAR) incident to the leaf surface, air and leaf temperature, vapour pressure deficit (VPD) and soil moisture were also measured. Boundary-line response curves between leaf conductance and four variables were studied to determine the general stomatal response patterns. Leaf conductance increased rapidly at first, with small increases in PAR. Above 50 mu mol m(-2) s(-1), no additional increases in conductance were observed. The optimum temperature ranged between 18 and 22 degrees C. Conductance remained constant at low and moderate VPD values and strongly decreased after a given threshold value (2.4 KPa). The response was sharper at the humid site. Beyond a threshold leaf water potential level (-2 MPa), stomatal conductance decreased rapidly as water potential continued to decline.