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Two permanent sampling sites were selected at the two extremes of a rainfall gradient in natural forests of Quercus pyrenaica Willd. located in the Sierra de Gata (Salamanca Province, Spain). During 1991 and 1992, diurnal courses of transpiration rate (E), stomatal conductance (g(s)) and leaf water potential (Psi(l)) were studied at different levels in the tree canopy. Leaf water potential fell from a high value in the early morning to a minimum at midday and then increased again in the afternoon. Values of Psi(l) in the early morning and at midday were lower at Fuenteguinaldo (dry site) than at Navasfrías (wet site) and were related to soil water availability. Stomatal conductance increased during the morning and maximum g(s) occurred about 3 h before Psi(l) was at its lowest value. The highest values of g(s) were found at Navasfrías during July. The lowest Psi(l) values were found in the upper parts of the canopy and differences among canopy levels were greatest when soil water availability was low. In August, minimum values of Psi(l) differed among canopy levels by 1.0 MPa at Navasfrías and by 0.8 MPa at Fuenteguinaldo. The maximum difference in g(s) among canopy levels was about 150 mmol m(-2) s(-1) at Navasfrías and about 300 mmol m(-2) s(-1) at Fuenteguinaldo. The results indicate a nonconservative pattern of water use in Quercus pyrenaica.

Summary -Aboveground biomass and nutrient content, litterfall and nutrient return to the soil were studied in Castanea sativa Mill forests near Salamanca (Spain), Montpellier (France)

Summary — 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

Abstract. Seasonal variation in nutrient concentration in leaves and branches of Quercus pyrenaica was studied in natural Q. pyrenaica forest in the Sierra de Gata (Salamanca Province, Spain). Two permanent plots were established at the two extremes of a rainfall gradient in this area: annual mean precipitation from 720 mm at Fuenteguinaldo (granite bedrock) to 1580 mm at Navasfrias (schists and graywackes). Leaf and branch samples were collected every three weeks during the growing season from May to October, at three height levels of the tree canopy. Seasonal changes and internal nutrient dynamics were investigated for N, Ca, Mg, K, Na, Mn, Fe and P during a two‐year period. The concentrations of all nutrients varied among the seasons; these variations were related to nutrient mobility and the annual physiological cycle. Nutrient concentrations decrease in the case of K and P, while the sparse mobile nutrients Ca, Mg, Mn and Fe gradually accumulated during each growing season. In Navasfrias a considerable resorption of P from senescing leaves was detected. Different patterns were found for the other nutrients studied (Na and N).