Assessing carbon and water fluxes in a mixed Mediterranean protected forest under climate change. An integrated bottom – up and top – down approach

This paper proposes a framework based on the integration of bottom-up and top-down approaches to assess and quantify the effect of local climate change on carbon and water fluxes in a protected Mediterranean mixed forest. The forest, with an extent of 50 ha, is in a coastal zone and is in decline because of severe summer drought stress. Since 2018, it has been involved in the LIFE PRIMED ecological restoration project.The bottom-up approach used gas exchange measurements and the MOCA model (MOdelling for Carbon Assessment) to quantify photosynthesis and transpiration of the main species in the forest (P. latifolia, P. lentiscus, Q. cerris, and F. angustifolia sub. oxycarpa). The top-down approach used the satellite models C-fix and NDVICWS, to quantify and spatialise gross primary production (GPP), net primary production (NPP), and evapotranspiration (ETA) at the forest stand scale. The up-scaling process from leaf to forest stand was made possible by calibrating two key parameters: light use efficiency (epsilon) and vegetation transpiration coefficient (Kcveg). The results indicate a decline in GPP, NPP, and ETA during the study period (2018-2022) due to the summer drought conditions caused by rising temperatures (16.60 degrees C in 2018 vs. 16.98 degrees C in 2022), increased vapor pressure deficit, reduced soil water content, and rainfall (1328 mm in 2018 vs. 468 mm in 2022). In 2018, the annual GPP was 1186.83 +/- 351.40 gC/m2y, which decreased to 1128.67 +/- 331.21 gC/m2y in 2022. During the dry summer period, GPP ranged from 401.76 +/- 119.54 gC/m2y in 2018 to 347.39 +/- 101.62 gC/m2y in 2022. Annual ETA rates varied between 1184.86 +/- 351.86 mmH2O/m2y (2018) and 1304.92 +/- 384.07 mmH2O/m2y (2022). The summer ETA rates were significantly influenced by drought conditions, decreasing from 638.73 +/- 190.67 mmH2O/m2y in 2018 to 390.35 +/- 108.83 mmH2O/m2y in 2022.Based on the observed trend in other Mediterranean areas, species-specific responsiveness to local climate change suggests that P. latifolia and P. lentiscus, which are better adapted to water-limited environments, could gradually replace Q. cerris and F. angustifolia subs. oxycarpa. This study provides useful quantitative indications for the management and restoration goals of the area and suggests a methodology that can be applicable in several management practices, particularly for small heterogeneous and fragmented forested areas such as the Mediterranean ones.