Natural forest dynamics have more influence than climate change on the net ecosystem production of a mixed Mediterranean forest

Climate models predict increased drought conditions for the Mediterranean region, putting forests under increasing risk from direct or indirect effects of droughts. On the other hand, increasing CO2 and longer growing seasons could compensate the negative impacts of more intense droughts on forest productivity and carbon sequestration. Furthermore, large areas of Mediterranean forests are relatively young and unmanaged, and maturing usually tends to decrease carbon sequestration. We conducted a simulation study to explore the interplay between climate, CO2, and maturing, on forest net primary production (NPP) and net ecosystem production (NEP) over the 2000-2100 period. We used the Not Only Tree/Grass (NOTG) model, which is individual-based, process-oriented, and spatially explicit. We applied the model to the Font-Blanche experimental site, a typical mixed Mediterranean forest dominated by Aleppo pines and holm oaks. Simulations under three climate scenarios (current, moderate change, severe change) suggest that NPP could be enhanced by climate change, both due to higher CO2 and longer growing seasons. On the other hand NEP would only be temporarily enhanced by climate change. The largest effect, however, was a decrease of NEP over time for all scenarios, due to forest maturing, with heterotrophic respiration progressively catching up with NPP. This implies that, regardless of climate change, the sink capacity of large unmanaged forest areas in southern Europe could strongly diminish during the 21st century.