Canopy structure and the impact of drought on a Quercus suber L. woodland in Portugal The thesis covers two topics a performance evaluation of indirect leaf area index L and gap probability Pgap observation with the new digital cover photography DCP with special focus on the application in sparse canopies and the analysis of drought effects on water carbon fluxes in a typical Mediterranean Quercus suber L. ecosystem. L and Pgap are important ecosystem parameters. Their indirect height and angular distributed measurement remains a challenging task in open canopies. DCP was successfully applied here for the first time height and angular dependent. Results show similar Pgap compared to an established method. Clumping index Omega could be successfully derived by DCP for calculating L. DCP yielded precise L matching observations with litter traps. Woody component exclusion by object based image analysis improved results. Ground based crown observations yielded reasonable L height distributions compared with direct measurements. Plant species developed vast structural and functional adaptations to regulate carbon assimilation and respiratory water loss under drought. In the context of the extreme drought year 2012 drought effects on the entire ecosystem functioning are reported in the second part of the thesis. Therefore multi year observations of climate forcing soil properties as well as ecosystem flux observations were conducted and combined stomatal conductance photosynthesis modeling was applied and evaluated. Results show that precipitation effectiveness ET P increased up to 122\\% in the dry year 2012 due to the ground water access of trees. Understorey and overstorey gross primary productivity and were reduced by 53\\% and 28\\% in 2012. Modelling results showed simultaneous reduction of maximum carboxylation rate and stomatal conductance. However the ecosystem remained a carbon sink in both years with with 38\\% reduced sink strength.