The focus of the work reported here is the impact of severe heat stress conditions on orchards' carbon dioxide exchange rate (NEE, Net Ecosystem Exchange). NEE was monitored by means of the eddy-covariance technique over an irrigated orange-tree orchard during summer 2009 in Southern Spain. In that period, severe heat spells occurred (maximum air temperature and vapour pressure deficit up to 38degreesC and 5 kPa, respectively). Under these conditions, orange trees maintained their transpiration rates at levels similar to those observed for normal sunny days, while canopy stomatal conductance and NEE were strongly reduced, thereby leading to a marked decrease in water use efficiency. The experimental results are discussed in the context of (i) stomatal and non-stomatal limitations to CO 2 exchange and (ii) orchard respiration loss. As the frequency of extreme events is expected to increase in the Mediterranean Basin, our results suggest that water productivity of irrigated orchards may be significantly affected by climate change.
