Format: ORAL

Francisco J. Cano1,2, Rosana Lpez3, Jesus Rodriguez-Calcerrada3, Charles R. Warren4, Luis Gil3 Ismael Aranda1

1 Instituto de Ciencias Forestales (ICIFOR-INIA), CSIC, Madrid, Spain. 2 ARC Centre of Excellence for Translational Photosynthesis, Hawkesbury Institute for the Environment, Western Sydney University, NSW, Australia. 3 Department of Natural Systems and Resources. Universidad Politécnica de Madrid, Madrid, Spain. 4 School of Life and Environmental Sciences, University of Sydney, Sydney, Australia

Droughts are becoming a recurrent event worldwide, which reduce carbon uptake and increase tree mortality risk threatening several ecosystem functions and services. Global change is increasing the intensity, duration, and frequency of droughts due to a combined effect of scatter precipitations, atmosphere warming and soil erosion. However, droughts do not affect equally to all forest tree species due to species-specific differences in drought resilience and niche partition. Furthermore, leaf physiology, anatomy and morphology acclimate within the tree canopy to changes in light intensity and quality, evaporative demand and soil water availability. However, not many studies analyse within tree canopy inter-specific drought tolerance and plasticity, which in turn may constrain the adaptative strategies of the species shade tolerance. Here, we measured leaf gas exchange and hydraulics within the canopies of four forest tree species differing in shade and drought tolerance. Trees were in transects of increasing soil depth at the Natural World Heritage Site of Hayedo de Montejo, a relict European forest growing under Mediterranean conditions, and measurements were done during very dry and wet years. We show that shaded leaves are more susceptible to water stress than sun-developed leaves, and that decreased photosynthesis is due to the combined effect of reduced stomatal and mesophyll conductances and to a lesser extent by biochemical constrains, while top canopy leaves are mainly limited by the stomatal closure. This results in earlier lower carbon balance of the more shaded leaves within the canopy, what explains the earlier leaf shedding of the more shaded leaves. More shade tolerant species were more affected by drought along the canopy, while more drought tolerant ones were able to maintain modest gas exchange rates but only in sun-exposed leaves. Climate change may promote open forests which may benefit the more drought albeit less shade tolerant species.