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 independent of plant N supply. We combine that model with simple assumptions for nitrogen fluxes in the soil. A quasi-equilibrium analysis of the short term tree and soil pools is then used to develop a simple graphical depiction of the long term carbon and nitrogen supply constraints on total growth, stem growth and foliar allocation. Our results suggest that long-term growth responses to [CO2] and N deposition depend strongly on the extent to which stem allocation and foliage allocation are coupled. At one extreme ('no coupling'), when stem allocation is fixed and independent of foliage allocation, there is no response of total growth or stem growth to increased [CO2] unless N deposition increases. At the other extreme ('linear coupling'), when stem allocation is proportional to foliage allocation, there is a significant long-term increase in total growth following a doubling of [CO2], even when N deposition is unchanged, but stem growth decreases because of a long-term decrease 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 obtained previously from an alternative model of canopy light-use efficiency 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 stem allocation.