CO2 fertilization, transpiration deficit and vegetation period drive the response of mixed broadleaved forests to a changing climate in Wallonia

KEY MESSAGE: The change in forest productivity was simulated in six stands in Wallonia (Belgium) following different climate scenarios using a process-based and spatially explicit tree growth model. Simulations revealed a strong and positive impact of the CO₂ fertilization while the negative effect of the transpiration deficit was compensated by longer vegetation periods. The site modulated significantly the forest productivity, mainly through the stand and soil characteristics. CONTEXT: Forest net primary production (NPP) reflects forest vitality and is likely to be affected by climate change. AIMS: Simulating the impact of changing environmental conditions on NPP and two of its main drivers (transpiration deficit and vegetation period) in six Belgian stands and decomposing the site effect. METHODS: Based on the tree growth model HETEROFOR, simulations were performed for each stand between 2011 and 2100 using three climate scenarios and two CO₂ modalities (constant vs time dependent). Then, the climate conditions, soils and stands were interchanged to decompose the site effect in these three components. RESULTS: In a changing climate with constant atmospheric CO₂, NPP values remained constant due to a compensation of the negative effect of increased transpiration deficit by a positive impact of longer vegetation periods. With time-dependent atmospheric CO₂, NPP substantially increased, especially for the scenarios with higher greenhouse gas (GHG) emissions. For both atmospheric CO₂ modalities, the site characteristics modulated the temporal trends and accounted in total for 56 to 73% of the variability. CONCLUSION: Long-term changes in NPP were primarily driven by CO₂ fertilization, reinforced transpiration deficit, longer vegetation periods and the site characteristics.