A forest soil vegetation atmosphere model (ForSVA), II: Application to northern tolerant hardwoods

Abstract

The ForSVA (forest soil-vegetation-atmosphere) model of the preceding paper was used for simulating biomass growth and nutrient cycling (Ca, Mg, K, N, S) in three northern tolerant hardwood sites in eastern North-America: one north of Lake Superior (Turkey Lakes site), one in the Harp Lake watershed near Dorset, Ontario, and one in the Adirondack Mountains of New York State (Huntington Forest). The basic model calibration was done for the Turkey Lakes site. Model performance testing, as well as testing the general portability of the model formulation beyond that which is valid for the initial calibration site, was done with the other two sites. Model simulations included the evaluation of biomass and nutrient contents of all major stand compartments (foliage, fine roots, wood including bark and coarse roots, forest floor, ion exchange complex, soil solution, soil matrix within the rooting zone) over several tree life cycles, or stand cycles. Also addressed were carbon and nutrient flows into and out of these compartments in conjunction with the potential impacts of acidifying atmospheric ion deposition (SO 2− 4, NO − 3, NH + 4, H +). It was found that current atmospheric deposition rates of soil acidifying ions should increase the leaching of nutrient base cations (Ca 2+, Mg 2+, K +) from each of the 3 sites. The corresponding decrease in base cation availability should also lead to a decrease in soil pH, a decrease in base cation uptake, a decrease in foliar base cation concentrations, and a decrease in biomass growth. The latter would occur once the base cation availability, as calculated, becomes growth limiting. Altogether, stand-level biomass productivity was calculated to decrease under current acid sulphate/nitrate loads, and this decrease reduced the calculated longevity of the trees within the stands. The decrease in productivity under current atmospheric deposition rates would be the highest at the Harp Lake site (site of high SO 2− 4 deposition with low number of cation exchange sites in the soil) and the lowest at the Huntington Forest site (site of low SO 2− 4 deposition). Major uncertainties in this modelling effort relate to (1) actual relationships between foliar nutrient concentrations and nutrient availability, (2) the influence of foliar nutrient concentrations on productivity, (3) the rate of base cation replenishment on the cation exchange sites in soils.