Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 1 2007 10.5194/bg-4-137-2007 http://www.biogeosciences.net/4/137/2007/ http://www.biogeosciences.net/4/137/2007/bg-4-137-2007.html http://www.biogeosciences.net/4/137/2007/bg-4-137-2007.pdf 137 154 2007-02-22 Topography induced spatial variations in diurnal cycles of assimilation and latent heat of Mediterranean forest C. van der Tol tol@itc.nl A. J. Dolman M. J. Waterloo K. Raspor Dept. of Hydrology and Geo-Environmental Sciences, Vrije Universiteit Amsterdam, The Netherlands Dept. of Civil and Geodetic Engineering, University of Ljubljana, Slovenia The aim of this study is to explain topography induced spatial variations in the diurnal cycles of assimilation and latent heat of Mediterranean forest. Spatial variations of the fluxes are caused by variations in weather conditions and in vegetation characteristics. Weather conditions reflect short-term effects of climate, whereas vegetation characteristics, through adaptation and acclimation, long-term effects of climate. In this study measurements of plant physiology and weather conditions are used to explain observed differences in the fluxes. A model is used to study which part of the differences in the fluxes is caused by weather conditions and which part by vegetation characteristics. Data were collected at four experimental sub-Mediterranean deciduous forest plots in a heterogeneous terrain with contrasting aspect, soil water availability, humidity and temperature. We used a sun-shade model to scale fluxes from leaf to canopy, and calculated the canopy energy balance. Parameter values were derived from measurements of light interception, leaf chamber photosynthesis, leaf nitrogen content and ¹³C isotope discrimination in leaf material. Leaf nitrogen content is a measure of photosynthetic capacity, and ¹³C isotope discrimination of water use efficiency. For validation, sap-flux based measurements of transpiration were used. The model predicted diurnal cycles of transpiration and stomatal conductance, both their magnitudes and differences in afternoon stomatal closure between slopes of different aspect within the confidence interval of the validation data. Weather conditions mainly responsible for the shape of the diurnal cycles, and vegetation parameters for the magnitude of the fluxes. Although the data do not allow for a quantification of the two effects, the differences in vegetation parameters and weather among the plots and the sensitivity of the fluxes to them suggest that the diurnal cycles were more strongly affected by spatial variations in vegetation parameters than by meteorological variables. 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