Water relations only partly explain the distributions of three perennial plant species in a semi-arid environment

The water relations and stomatal conductances of three perennial plant species, Stipa tenacissima L., Anthyllis cytisoides L., and Retama sphaerocarpa (L.) Boiss., dominant on the upper slopes, mid-slopes and floor of a valley, respectively, in semi-arid south-east Spain, were investigated to test the hypothesis that differences in plant-soil water relations could account for the different distributions of each species in the catena. Diurnal measurements of water potential (psi(w)), relative water content (RWC) and stomatal conductance (g(s)) of leaves were made over one year. Leaf temperature, air humidity, wind-speed and incident quantum flux density were measured concurrently. Soil water content was determined gravimetrically at 0 - 5 cm and 15 - 20 cm depths. Measurements of psi(w), RWC and g(s) were analysed according to meteorological conditions, based on the maxima for daily air temperature and atmospheric saturation water vapour deficit and on soil moisture content. The hypothesis that plant-soil water relations can explain the distribution of the three species along the catena from valley side to valley floor was rejected for Anthyllis and Stipa but confirmed for Retama.