Process-based models are important tools for understanding the dynamics and functioning of forest ecosystems. In nearly all models, forest productivity is largely driven by carbon (C) input via photosynthesis, which reacts strongly positive to elevated CO2 (eCO2). The simulated increases in forest productivity in terms of wood production, however, have been much higher than experimental and observational evidence suggests, at least in those models that did not account for nutrient limitations. Experimental results show that in most investigated systems, eCO2 leads to increasing allocation of C below ground, without sustained enhancements of wood production. This translocation of C below ground, which also enhances nutrient uptake, appears to be larger than simulated in our current models. However, because of a lack of experimental data from real ecosystems, in particular from the tropics, the big question remains unresolved whether increasing levels of CO2 have and will lead to substantially increased forest productivity in terms of stand-level wood increment. To reduce the uncertainties, it will be necessary to develop new theory, implement that in models, and rigorously compare model results with observations. They range from physiological measurements, results from ``free air CO2 enrichment'' experiments to forest inventories, tree rings and 13C analyses~(as a proxy for changes in water use efficiency). Key issues that should be addressed in the models include the coupling of C and nutrient cycles, limitations to the C sink capacity of trees, dynamic changes in allocation, and potential feedbacks from tree population dynamics on stand productivity.     «

Process-based models are important tools for understanding the dynamics and functioning of forest ecosystems. In nearly all models, forest productivity is largely driven by carbon (C) input via photosynthesis, which reacts strongly positive to elevated CO2 (eCO2). The simulated increases in forest productivity in terms of wood production, however, have been much higher than experimental and observational evidence suggests, at least in those models that did not account for nutrient limitations. E...     »