FOREST-BGC, A general model of forest ecosystem processes for regional applications. II. Dynamic carbon allocation and nitrogen budgets

TLDR

The updated FOREST‑BGC model dynamically adjusts leaf, root, and stem carbon allocation fractions each year based on stand water and nitrogen limitations. Water deficit is quantified by annually integrating daily soil water deficit fractions, while nitrogen limitation is defined relative to an optimum foliar N pool, and leaf/root N concentrations and maximum photosynthetic capacity are recalculated annually as functions of nitrogen availability to drive allocation changes. Simulations for Madison and Missoula forests show that lower water or nitrogen availability reduces the leaf/root carbon partitioning ratio, yielding leaf area indices from 4.5 to 11, stem carbon biomasses from 31 to 128 Mg ha⁻¹, and nitrogen incorporation rates from 34 to 109 kg ha⁻¹ yr⁻¹, confirming the model’s ability to capture dynamic annual carbon partitioning.

Abstract

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.