Tree cover increase mitigation strategy: implications of the “replacement approach” in carbon storage of a subtropical ecosystem

Worldwide tree cover increase (TCI) is one the most important climate change mitigation strategies, given the role of the ecosystems as carbon (C) reservoirs. Reforestation could be key to achieving TCI. However, there is still a knowledge gap about its real mitigation effects when it is applied through its replacement approach (REP-app). We evaluate the REP-app effects (replace native forest cover—NFC—by forestry plantation cover—FPC) on magnitude and direction of ecosystem C stock changes. By means of a field study, we quantified the C stock through a space-for-time substitution method, using eight paired sites (NFC/FPC) in a northern Argentinian subtropical forest (SAY). We studied C in saplings, lianas, above and belowground biomass, dead organic biomass, and soil. Through a metaanalysis, we evaluated the reliableness within the response of C sequestration (gain or loss) in international study cases and, therefore, the robustness of outcomes analyzing some main moderator variables that may influence the result. A significant effect of C loss was revealed by both, the field study (− 50%) and the metaanalysis (− 20.4%), and this response was consistent through stands of ages and species different. SAY sequestered 285 MgC ha−1. Only 35-year-old Eucalyptus stand reached similar values, but they are reservoirs of doubtful permanence with functional attributes that could affect long-term ecosystem functioning. Given that the TCI strategy seek to achieve interconnected efforts in relation to climate, biodiversity, and sustainable development, the REP-app must not be considered a mitigation strategy in subtropical ecosystems. Global programs promoting TCI must be redirected urgently.