Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 8 2009 10.5194/bg-6-1423-2009 http://www.biogeosciences.net/6/1423/2009/ http://www.biogeosciences.net/6/1423/2009/bg-6-1423-2009.html http://www.biogeosciences.net/6/1423/2009/bg-6-1423-2009.pdf 1423 1444 2009-08-05 Improved understanding of drought controls on seasonal variation in Mediterranean forest canopy CO₂ and water fluxes through combined in situ measurements and ecosystem modelling T. Keenan t.keenan@creaf.uab.es R. García A. D. Friend S. Zaehle C. Gracia S. Sabate CREAF, Autonomous University of Barcelona (UAB), 08139, Bellaterra, Barcelona, Spain LSCE, Orme des Merisiers, Gif sur Yvette, Paris, France Department of Geography, University of Cambridge, Cambridge CB2 3EN, UK Department of Ecology, University of Barcelona (UB), Diagonal 645, 08028, Spain Water stress is a defining characteristic of Mediterranean ecosystems, and is likely to become more severe in the coming decades. Simulation models are key tools for making predictions, but our current understanding of how soil moisture controls ecosystem functioning is not sufficient to adequately constrain parameterisations. <br><br> Canopy-scale flux data from four forest ecosystems with Mediterranean-type climates were used in order to analyse the physiological controls on carbon and water flues through the year. Significant non-stomatal limitations on photosynthesis were detected, along with lesser changes in the conductance-assimilation relationship. New model parameterisations were derived and implemented in two contrasting modelling approaches. <br><br> The effectiveness of two models, one a dynamic global vegetation model ("ORCHIDEE"), and the other a forest growth model particularly developed for Mediterranean simulations ("GOTILWA+"), was assessed and modelled canopy responses to seasonal changes in soil moisture were analysed in comparison with in situ flux measurements. <br><br> In contrast to commonly held assumptions, we find that changing the ratio of conductance to assimilation under natural, seasonally-developing, soil moisture stress is not sufficient to reproduce forest canopy CO₂ and water fluxes. However, accurate predictions of both CO₂ and water fluxes under all soil moisture levels encountered in the field are obtained if photosynthetic capacity is assumed to vary with soil moisture. 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