PROCESS BASED FOREST MODELLING: A THOROUGH VALIDATION AND FUTURE PROSPECTS FOR MEDITERRANEAN FORESTS IN A CHANGING WORLD. La modelización basada en procesos de los ecosistemas forestales:validación y posible evolución futura de los ecosistemas mediterráneos bajo escenarios de cambio climático

Predicting the effects of climate change on the future performance of current forest stands can benefit from highly mechanistic process based models which can accurately describe the responses of forest stands under a variety of conditions. The lack of understanding of the effect of water stress on ecosystem function leads to big discrepancies when modelling in these conditions but recent advances allow us to successfully reproduce water and carbon balances during periods of stress. This will be of particular importance for modelling future scenarios, where water stress is expected to increase in Mediterranean ecosystems. The availability of new high precision field data, and highly developed models allows for large scale validation. A forest ecosystem model, GOTILWA+, is developed and validated against FLUXNET data in a water stressed Mediterranean site. The recently released level 4 Euroflux data uses detailed gap filling techniques to provide high quality half hourly measurements of the carbon and water fluxes at many sites throughout Europe. This work presents the validation process and results from the model application at a site in Puechabon, France, and an in depth investigation into forest ecosystem function under stressed conditions. Particular attention was paid to the modelling of water and carbon fluxes, and in assessing different approaches to modelling the effect of water stress on the photosynthetic apparatus, autotrophic and heterotrophic respiration. We focus on the techniques used to confront problems which arose in the validation process. In particular the interaction of the photosynthetic apparatus with soil water availability, through stomatal conductance, mesophyll conductance and the hydrological stress induced changes in physiological capacities of photosynthesis. The inversion of latent heat fluxes allowed for the reconstruction of the annual evolution of soil water, and the decoupling of the hydrological cycle from the photosynthetic model. Further techniques such as the inversion of the McNaughton-Black equation for evapotranspiration to back-calculate