A MODEL OF THE LONG-TERM RESPONSE OF CARBON ALLOCATION AND PRODUCTIVITY OF FORESTS TO INCREASED CO2 CONCENTRATION AND NITROGEN DEPOSITION

We present a simple theoretical analysis of the long term response of forest growth and carbon allocation to increased atmospheric [CO2] and N deposition. Our analysis is based on a recent model which predicts that plant light-use efficiency increases with [CO2] but is independen t of plant N supply. We combine that model with simple assumptions for nitrogen fluxes in the soil. A quasi-equilibrium analysis of the shor t term tree and soil pools is then used to develop a simple graphical depiction of the long term carbon and nitrogen supply constraints on t otal growth, stem growth and foliar allocation. Our results suggest th at long-term growth responses to [CO2] and N deposition depend strongl y on the extent to which stem allocation and foliage allocation are co upled. At one extreme ('no coupling'), when stem allocation is fixed a nd independent of foliage allocation, there is no response of total gr owth or stem growth to increased [CO2] unless N deposition increases. At the other extreme ('linear coupling'), when stem allocation is prop ortional to foliage allocation, there is a significant long-term incre ase in total growth following a doubling of [CO2], even when N deposit ion is unchanged, but stem growth decreases because of a long-term dec rease in foliage allocation. For both types of coupling, total growth and stem growth increase with increasing N deposition. In the case of linear coupling, however, the N deposition response of stem growth is significantly larger than that of total growth, because of a long-term increase in foliage allocation. We compare our results with those obt ained previously from an alternative model of canopy light-use efficie ncy involving a dependence on the foliar N:C ratio in addition to [CO2 ]. Our results highlight the need for more experimental information on (i) the extent to which canopy light-use efficiency is independent of N supply, and (ii) the relationship between foliage allocation and st em allocation.