Quantifying Carbon and Water Use Efficiencies of Forest Ecosystems in Wallonia, Belgium: Insights from Species-Specific Responses to Thinning and Climate Change

Optimizing carbon use efficiency (CUE) and water use efficiency (WUE) is a critical challenge for temperate forests worldwide, particularly under changing climatic conditions. CUE refers to the proportion of carbon assimilated during photosynthesis that contributes to biomass, while WUE quantifies the carbon gained per unit of water lost through transpiration. The region of Wallonia, Belgium, with temperate forests covering 33% of its land, serves as an exemplary case for analyzing the relationship between CUE and WUE under varying ecological and climatic conditions. Globally, the coupling of CUE and WUE remains insufficiently understood, especially at the species level. This study investigates the dynamics of CUE and WUE across several dominant tree species in Wallonia. It utilizes outputs from the CARAIB dynamic vegetation model to evaluate species-specific responses to thinning practices and climate scenarios (RCP 8.5 and RCP 2.6) over the period 1980 to 2070.

Our analysis distinguishes between the isohydric and anisohydric behaviors of tree species, emphasizing their contrasting long-term responses to climatic changes and their influence on ecosystem efficiency. Trees such as Abies and Picea tend to be isohydric. They conserve water by closing their stomata early during drought. They benefit from thinning practices initiated at 40 years, with intervals of 3–9 years designed to manage competition as they mature. Conversely, trees like Quercus and Populus tend to be anisohydric. They maintain photosynthesis under stress by keeping their stomata open. Populus requires earlier thinning interventions, typically starting at 30 years, with shorter regrowth periods of 15 years to optimize light penetration and nutrient availability. In contrast, Quercus thinning is initiated at 40 years, with regrowth periods of 30 years, to support their growth and optimize resource utilization. Thinning reduces competition and reallocates resources, modulating trade-offs between WUE and CUE while supporting species-specific growth under varying climatic stressors. Tailored thinning practices enhance resource availability for both isohydric and anisohydric species. Isohydric species gain from improved water availability, complementing their inherent drought resilience, while anisohydric species benefit from increased carbon assimilation through enhanced access to light and nutrients.

These findings underscore the importance of aligning species composition and management strategies with localized environmental conditions to bolster forest resilience. With this study, we investigate species-specific management strategies to support sustainable forestry, identifying species that are better adapted to changing climatic conditions and capable of maintaining vital ecosystem services.