A forest soil vegetation atmosphere model (ForSVA), I: Concepts

A dynamic modelling approach useful for assessing atmospheric ion deposition effects on stand level biomass production and nutrient cycling in upland forests is presented. The approach quantifies biomass production and nutrient transfers among the major stand compartments, namely foliage, wood (which includes branches and medium to coarse roots), fine roots, forest floor, mineral soil and soil solution. The main processes addressed are net primary production, litterfall (including throughfall), litter decay (including nutrient mineralization), within-tree allocation of photosynthate and nutrients (including translocation of nutrients and of photosynthate before foliage fall), nutrient uptake from available nutrient pools within the soil, ion exchange and replenishment of soil bases (Ca, Mg, K, Na) via soil weathering and atmospheric deposition. Nitrogen processes include N fixation, mineralization of organic N (including nitrification) and uptake of nitrate-N and ammonium-N. Sulphur processes include pH-dependent sulphate adsorption as modified by soil Fe and Al oxides/hydroxides. A quantitative representation of these processes leads to stand-structure dependent predictions about (i) ion leaching from the soil (losses of base cations as well as acid cations such as H+ and Alx+), (ii) soil exchangeable bases, (iii) soil pH, (iv) forest biomass, and (v) nutrient distributions within the foliage, wood, fine roots and the forest floor for several stand rotations, based on user-defined regeneration patterns. Information requirements are limited to (i) stand initialisation (initialising biomass and nutrient pools per stand compartment), (ii) specifying annual atmospheric deposition rates for all major ions (Ca2+, Mg2+, K+, Na+, H+, NH+4, SO2−4, NO−3, HCO−3), and (iii) entering numerical values for the various process parameters. Values for most parameters can be obtained from biophysical data sets about biomass and nutrient distributions of mature forest stands. The remaining parameters can be obtained via species-specific model calibration. For a given species, calculations across site-classes and deposition scenarios do not require additional calibration. All calculations are specific to site, species, atmospheric deposition scenario and regeneration pattern, and are based on annual timesteps and transfer rates.