RETRIEVING LEAF CONDUCTANCES FROM SAP FLOWS IN A MIXED MEDITERRANEAN WOODLAND - A SCALING EXERCISE

Xylem sap-flux densities were monitored continuously using Granier-typ e sensors on five Quercus ilex four Arbutus unedo and one Quercus pube scens from June 1993 to October 1994. Half-hourly measurements of inco ming solar radiation, air temperature and humidity, horizontal wind sp eed and precipitation were carried out at the top of a tower at a heig ht of 12 m, about 2 m above the canopy. Leaf physiological measurement s (stomatal conductance, water potential) on individual sunlit leaves from each of the three tree species were obtained on seven complete or partial diurnal lime courses. For these three species, to estimate le af stomatal conductance, we used the big-leaf approach of Penman-Monte ith. We have divided the leaves into sunlit and shaded. The model sums the individual-leaf model for only the sunlit fraction to produce the whole-canopy predictions. Transpiration was deduced from sap flux thr ough a transfer function taking into account stem water storage. Stoma tal conductance for a given species was evaluated half-hourly from tra nspiration and microclimate data inverting the Penman-Monteith equatio n. An empirical model was identified that related stomatal aperture to simultaneous variations of microclimate and plant water potential for the 1993 period. The predicted leaf conductances were validated again st porometer data and those of the 1994 period. The diurnal patterns o f predicted and measured transpiration indicated that stomatal conduct ance was accurately predicted. The leaf conductance models were also c ompared with already published literature values from the same tree sp ecies. In spite of the simplifications inherent to the big-leaf repres entation of the canopy, the model is useful for predicting interaction s between Mediterranean mixed woodland and environment and for interpr eting H2O exchange measurements. ((C) Inra/Elsevier, Paris.).