The importance of feedback processes and vegetation transition in the terrestrial carbon cycle

The C. fluxes are influenced by the direct and indirect response of ecosystems to changing atmospheric CO₂ concentrations and climates. Each ecosystem is characterized by its NNP, which depends on environmental conditions. Incorporation of grid cell specific feedback processes is an innovation in this model. Implemented feedback processes are CO₂ fertilization, effects of climate change on photosynthesis, plant respiration and decomposition and shifts in vegetation patterns due to climate change and changes in water use efficiency. This paper presents an evaluation of the importance of feedback processes on global and regional scales .

Temperature feedback on plant growth is the most important feedback process at the global scale, while CO₂ fertilization is of lesser importance. However, CO₂ fertilization is the most significant feedback in low latitudes, while temperature change most strongly influences the C. cycle in the high latitudes. The C. dynamics of land cover change, including feedbacks, depend on characteristic of the former and actual land cover type. The shifts in land cover are parameterized so that they mimic succession. The course and duration of the transitional phase strongly affect the C fluxes. We applied both an equilibrium approach with instantaneous transitions and dynamic approaches with gradual type-dependent transtion. Transitions in natural vegetation between 1990 and 2050 involve about one-third of all grid cells, especially in the higer latitudes. The impact of vegetation transition on C. fluxes differs depending on the instantaneous or gradual transtion strategy. The latter results in higher and delayed NEP fluxes, resulting in significantly different atmospheric CO₂ concentrations. The dynamic approach is more realistic and should be included in integrated assessment models that project future atmospheric CO₂ levels.