“From energy to water limitation: projecting carbon and water cycling in mediterranean beech forests under climate change”

European beech (Fagus sylvatica) is among the most ecologically and economically important tree species in Europe. Climate change impacts on beech forests are already measurable in large parts of its distribution range, with climate-driven growth decline expected across large areas in the near future. Many regions are anticipated to shift from energy- to water- limited functioning, altering in turn forest ecosystem functions. However, previous studies have mostly focused on dendrochronological analyses, while the behavior of beech forests under climate change must be understood as a coupled carbon-water problem, requiring integrated ecosystem scale approaches. This study aims to provide process-based understanding of how and to what extent the carbon and water cycles in beech forests will be affected in the coming decades, analyzing the impact of climate and atmospheric CO₂ on the carbon use efficiency CUE, i.e. the ratio between net and gross primary productivity, and water use efficiency WUE, i.e. the ratio between gross primary productivity and evapotranspiration, as key-indicators of the ecosystem functioning. Therefore, we used a mechanistic, state-of-the-art forest ecosystem model, namely 3D-CMCC-FEM, to simulate carbon and water cycles in ~500 beech stands located across the Italian territory, from the pre-alpine zone to the southernmost region. The sites span thus broad latitudinal and altitudinal gradients, capturing diverse climatic conditions. Additionally, the selected forest stands show different structural characteristics resulting from varying site histories and legacy effects.

Model simulations are carried under current climate conditions and three climate change scenarios from downscaled CMIP6 climate projections, covering the years 2005-2100. Structural data to initialize the model in 2005 are built on measurement of key structural variables from the second Italian Forest Inventory. Taking advantage of in situ measurements and remote-sensing based observations, we evaluate and constrain the ecosystem model processes. We finally analyze how CUE and WUE trajectories covary across the climate space under different climate scenarios taking in to account the role of forest structure, aiming at identifying potential carbon-water trade-offs as forests face changing climatic conditions.