© 1991 Heron Publishing—Victoria, Canada
FOREST-BGC, A general model of forest ecosystem processes for regional applications. II. Dynamic carbon allocation and nitrogen
budgets
Steven W. Running (1) and Stith T. Gower (2)
1. School of Forestry, University of Montana, Missoula, MT 59812, USA / 2. Department of Forestry, University of Wisconsin, Madison, WI 53706, USA /
Summary
A new version of the ecosystem process model FOREST-BGC is presented that uses stand water and nitrogen limitations to alter
the leaf/root/stem carbon allocation fraction dynamically at each annual iteration. Water deficit is defined by integrating
a daily soil water deficit fraction annually. Current nitrogen limitation is defined relative to a hypothetical optimum foliar
N pool, computed as maximum leaf area index multiplied by maximum leaf nitrogen concentration. Decreasing availability of
water or nitrogen, or both, reduces the leaf/root carbon partitioning ratio. Leaf and root N concentrations, and maximum leaf
photosynthetic capacity are also redefined annually as functions of nitrogen availability. Test simulations for hypothetical
coniferous forests were performed for Madison, WI and Missoula, MT, and showed simulated leaf area index ranging from 4.5
for a control stand at Missoula, to 11 for a fertilized stand at Madison, with Year 50 stem carbon biomasses of 31 and 128
Mg ha–1, respectively. Total nitrogen incorporated into new tissue ranged from 34 kg ha–1 year–1 for the unfertilized Missoula stand, to 109 kg ha–1 year–1 for the fertilized Madison stand. The model successfully showed dynamic annual carbon partitioning controlled by water and
nitrogen limitations.
