ABSTRACT
Forests contain 70–80 percent of terrestrial carbon (C) and represent a global C sink, despite of continued deforestation. On the other hand, forests and land-use change (LUC) account for the largest uncertainties within the global C balance. The future role of forests is even more uncertain within the context of global change. The question if forests will shift from C sinks to sources is fundamental for conservation and management. We aim to synthesize current knowledge on the role of the major forest biomes in the global C cycle. Furthermore, we evaluate management options, based on their potential to maximize greenhouse gas (GHG) mitigation and other benefits from forest ecosystems. Whereas boreal and temperate forests currently act as net sinks, there is a potential for increased insect outbreaks, fires, and droughts. Pressure from infrastructure development and the demand for biomass fuels is increasing. Tropical forests account for the largest gross C sink, but face deforestation and degradation. The future role of tropical forests depends mostly on LUC dynamics. Uncertainties persist about deforestation rates and impacts from global change. It is clear, however, that changes in the C balance of tropical forests will affect atmospheric CO2 levels on a global scale. In managed temperate and boreal forests, aboveground carbon (AGC) can be maximized by optimizing rotation cycles in even-aged stands. Mixed-species stands may increase productivity and resilience to disturbances. Research is needed on the impacts of silvicultural activities on soil organic carbon (SOC). In the tropics, reforestation and deforestation avoidance are the most important GHG mitigation pathways. Lessons learnt from REDD (reduced emissions from deforestation and forest degradation) pilot projects may help to establish GHG mitigation strategies in the future. The majority of timber harvest in the tropics is unsustainable and could be improved by extending rotation cycles, reduced impact logging, and long-term management planning. International support is needed to ensure capacity building and strong institutions, and to create incentives for global markets. The extension of planted forests is growing worldwide. Plantations sequester important amounts of C, although afforestation may initially lead to SOC losses, and AGC storage over multiple rotation cycles is low, compared to natural forests. Conservation of old-growth forests is the most effective way to secure C storage.
