Climatic disequilibrium in tree cover is frequent in protected areas worldwide — implications for conservation and restoration

Many species and ecosystems that diversified and adapted under consumer control in prehistoric times are nowadays highly threatened. Nature protection areas (PAs) form a major conservation strategy to avoid their losses. We argue that many PAs across Earth are in disequilibrium with current climatic conditions. At the same time, the main consumers of woody vegetation keeping these systems in climatic disequilibrium, that is, large-bodied herbivores and/or fire, have strongly declined or changed in occurrence in (pre-)historic times. Without active intervention, this lack of consumer control will cause the systems to approach climate equilibrium with major implications for baseline-focused approaches in species and nature protection and restoration. In a global analysis we quantified the prevalence of climatic disequilibrium in PAs for all terrestrial biomes. We calculated climatic disequilibrium in PAs as the difference between actual tree cover and the potential tree cover under current climatic conditions (i.e., mean annual temperature and annual precipitation sum). We show that climatic disequilibrium conditions in tree cover are a widespread phenomenon in PAs across all biomes with highest values for the temperate grassland, tundra and taiga biomes. We argue that trophic rewilding, notably the restoration of functionally diverse large-herbivore assemblages, would not only help maintain climatic disequilibrium states, but also reduce labour and costs for management.