title: Optimal co-allocation of carbon and nitrogen in a forest stand at steady state.
abstract: Nitrogen (N) is essential for plant production, but N uptake imposes carbon (C) costs through maintenance respiration and fine-root construction, suggesting that an optimal C:N balance can be found. Previous studies have elaborated this optimum under exponential growth; work on closed canopies has focused on foliage only. Here, the optimal co-allocation of C and N to foliage, fine roots and live wood is examined in a closed forest stand. Optimal co-allocation maximizes net primary productivity (NPP) as constrained by stand-level C and N balances and the pipe model. Photosynthesis and maintenance respiration increase with foliar nitrogen concentration ([N]), and stand-level photosynthesis and N uptake saturate at high foliage and fine-root density. Optimal NPP increases almost linearly from low to moderate N availability, saturating at high N. Where N availability is very low or very high, the system resembles a functional balance with a steady foliage [N]; in between, [N] increases with N availability. Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability. The predicted relationships between biomass density and foliage [N] are in reasonable agreement with data from coniferous stands across Finland. All predictions agree with our qualitative understanding of N effects on growth.
authors.affiliation: Department of Forest Ecology, PO Box 27, 00014 University of Helsinki, Finland.
authors.forename: Annikki
authors.initials: A
authors.lastname: Mäkelä
authors.affiliation: USDA Forest Service, 271 Mast Road, Durham, NH 03824, USA.
authors.forename: Harry T
authors.initials: HT
authors.lastname: Valentine
authors.affiliation: Finnish Forest Research Institute, Vantaa Research Centre, PO Box 18, 01301 Vantaa, Finland.
authors.forename: Heljä-Sisko
authors.initials: HS
authors.lastname: Helmisaari
mesh_terms: D018533:Biomass; D002244:Carbon / Q000378:metabolism*; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D028222:Picea / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism; D041605:Pinus sylvestris / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism; D018515:Plant Leaves / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism; D018517:Plant Roots / Q000254:growth & development / Q000378:metabolism; D014197:Trees / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism*; D014934:Wood / Q000378:metabolism
chemical_list: D002244:Carbon; D009584:Nitrogen