Transpiration and photosynthesis of holm oak trees in southern Spain

“Dehesas”, the Iberian semi-natural woodlands based on large holm oak (Quercus ilex L.) trees, represent a key agroforestry system in Spain and Portugal sustaining relevant economic activities (e.g. “iberico” pork feeding, truffle) and ecosystem services. Despite of their importance, scarce information is still available regarding both the transpiration (E_p) and photosynthesis of holm oaks growing in dehesas and the effects of irrigation on these fluxes. Two experiments were performed in Espiel and Hinojosa del Duque (southern Spain) where sap flow was measured in large trees between 2009 and 2011 and photosynthesis was measured with a transitory-closed chamber in smaller trees at different moments of 2009. In 2009, half of the trees equipped with sap flow sensors were drip irrigated and half were rainfed. However, the supply of irrigation barely affected E_p and shoot water potential. The seasonal course of E_p peaked in late spring and decreased gradually, reaching minimum values during winter. Vapor pressure deficit (VPD) exerted a strong control over canopy conductance and its seasonal variation was identified as a major driver for the observed patterns of E_p. Photosynthesis showed a similar course as that of E_p but there was a second smaller peak in autumn. Seasonal water use efficiency was remarkably high due to the reduced summer E_p. Our results also indicate that hyperbolic models of canopy conductance and water use efficiency in response to VPD may be used to estimate diurnal and seasonal E_p and net photosynthesis of holm oak if the tree size and leaf area density are known.