Caracterització, modelització de l'impacte del canvi climàtic i els processos dinàmics de boscos mediterranis

Resumen

Ongoing climate change affects forests worldwide, but how and how much so? Recent widespread forest die-off events highlight potentially dramatic outcomes brought by overall warmer and drier conditions, but large uncertainties remain regarding forest responses. Characterizing and forecasting forest responses to climate is critically needed in order to enable mitigation and adaptation strategies. Process-based models are an essential tool in this task, yet their use is hindered by knowledge gaps regarding the key mechanisms driving forest responses and their representation within models. The general objective of this thesis is to advance the understanding and modelling of the effects of water and temperature on forest structure and functioning. To do so, plant hydraulics and biophysical principles are leveraged within a mechanistic perspective, so that the integration of processes across scales yields emergent forest behavior. Results from this thesis cast new light on the role of water availability and transport in governing the processes underpinning wood formation, tree growth and forest structure. Namely, water potential is suggested to modify cell turgor pressure and subsequently drive cell expansion, yielding variations in conduit dimensions in the xylem and cell division in the cambium. The effect of temperature on cell extensibility is further proposed to modulate cell expansion and division. According to this biophysical model, seasonal fluctuations in water potential and temperature directly influence the amount and structure of wood formed within a year, independently of carbon assimilation. Model validation is carried out at two study sites, including xeric and cold species distribution margins. At the inter-annual scale, results suggest that biophysical drivers interact with carbon assimilation to determine tree growth. Besides, a theoretical model is formulated where forest below- and aboveground structure is assumed to tend towards an equilibrium with precipitations and atmospheric water demand. The model is used to predict root distribution from hydraulic traits and simulated water fluxes and is validated using several study sites, including contrasting climatic conditions in Catalonia (NE Spain). Application to forest inventory data in Catalonia yields estimates of root distribution at the regional scale. Overall, this thesis highlights the range of forest responses to climate from the cellular to regional scale and the hydraulic and biophysical mechanisms underlying them. New modelling solutions integrating evidenced processes and bridging scales are proposed. Their implementation within vegetation models and a greater focus on water pools and fluxes could help reducing uncertainties regarding the fate of forests in the face of climate change.