Will Forest Ecosystems Continue to Remove CO2 from the Atmosphere as Climate Changes?.

In almost every forest we look today we find that there is a net removal of CO₂ from the atmosphere and carbon is accumulating in the soil and in the trees. A net gain of carbon by forest ecosystems implies a lack of balance between the processes removing CO₂ from the atmosphere and the processes returning CO₂ to the atmosphere. Ecosystem net primary production (NPP) is the difference between the gross photosynthetic production (GPP) and the losses of carbon resulting from the autotrophic respiration (R_A), i.e., NPP = GPP – R_A. The net ecosystem production (NEP) is the difference between the net primary production (NPP) and the heterotrophic respiration (R_H) associated with mineralisation of organic matter in the soil, i.e., NEP = NPP - R_H. For a forest at equilibrium, we would expect NEP to be zero (i.e., NPP = R_H ). Conversely, when we measure NEP > 0, NPP must exceed R_H. This world-wide disequilibrium in forests today is the reason why forests are a large global carbon sink, removing from the atmosphere close to 40% of CO₂ emissions. It is commonly assumed, and has been shown by some models, that as atmospheric [C0₂] and surface temperature increase in the future, this current disequilibrium will reverse (i.e., RH > NPP). A key question is whether this is likely?
Whilst including a carbon cycle in GCMs and other models has been a major step forward, a carbon cycle without an associated nitrogen cycle is unrealistic. Firstly, correlation across sites shows that NPP is stimulated by the concurrent deposition of atmospheric N. Secondly soil warming experiments show that N released by decomposition of litter and soil organic matter leads to increases in tree leaf area, uptake of CO₂ and tree growth. Thirdly, after an initial enhancement on warming, R_H settles down to the rate prior to the increase in temperature. For these reasons, recent projections by models linking the carbon and nitrogen cycles show NPP of forests increasing over the next 100 years in parallel with R_H, as atmospheric [CO₂] and surface temperature increase. Thus we may expect the ongoing removal of [CO₂] from the atmosphere by forests (NPP > RH) to be maintained.