Toward an allocation scheme for global terrestrial carbon models

The distribution of assimilated carbon among the plant parts has a profound effect on plant growth, and at a larger scale, on terrestrial biogeochemis try. Although important progress has been made in modelling photosynthesis, less effort has been spent on understanding the carbon allocation, especia lly at large spatial scales. Whereas several individual-level models of pla nt growth include an allocation scheme, most global terrestrial models stil l assume constant allocation of net primary production (NPP) among plant pa rts, without any environmental coupling. Here, we use the CASA biosphere mo del as a platform for exploring a new global allocation scheme that estimat es allocation of photosynthesis products among leaves, stems, and roots dep ending on resource availability. The philosophy underlying the model is tha t-allocation patterns result from evolved responses that adjust carbon inve stments to facilitate capture of the most limiting resources, i.e. light, w ater, and mineral nitrogen. In addition, we allow allocation of NPP to vary in response to changes in atmospheric CO2. The relative magnitudes of chan ges in NPP and resource-use efficiency control the response of root:shoot a llocation. For ambient CO2, the model produces realistic changes in above-g round allocation along productivity gradients. In comparison to the CASA st andard estimate using fixed allocation ratios, the new allocation scheme te nds to favour root allocation, leading to a 10% lower global biomass. Eleva ted CO2, which alters the balance between growth and available resources, g enerally leads to reduced water stress and consequently, decreased root:sho ot ratio. The major exception is forest ecosystems, where increased nitroge n stress induces a larger root allocation.