A FOREST SOIL VEGETATION ATMOSPHERE MODEL (FORSVA) .1. CONCEPTS

A dynamic modelling approach useful for assessing atmospheric ion depo sition effects on stand level biomass production and nutrient cycling in upland forests is presented. The approach quantifies biomass produc tion and nutrient transfers among the major stand compartments, namely foliage, wood (which includes branches and medium to coarse roots), f ine roots, forest floor, mineral soil and soil solution. The main proc esses addressed are net primary production, litterfall (including thro ughfall), litter decay (including nutrient mineralization), within-tre e allocation of photosynthate and nutrients (including translocation o f nutrients and of photosynthate before foliage fall), nutrient uptake from available nutrient pools within the soil, ion exchange and reple nishment of soil bases (Ca, Mg, K, Na) via soil weathering and atmosph eric deposition. Nitrogen processes include N fixation, mineralization of organic N (including nitrification) and uptake of nitrate-N and am monium-N. Sulphur processes include pH-dependent sulphate adsorption a s modified by soil Fe and Al oxides/hydroxides. A quantitative represe ntation of these processes leads to stand-structure dependent predicti ons about (i) ion leaching from the soil (losses of base cations as we ll as acid cations such as H+ and Alx+), (ii) soil exchangeable bases, (iii) soil pH, (iv) forest biomass, and (v) nutrient distributions wi thin the foliage, wood, fine roots and the forest floor for several st and rotations, based on user-defined regeneration patterns. Informatio n requirements are limited to (i) stand initialisation (initialising b iomass and nutrient pools per stand compartment), (ii) specifying annu al atmospheric deposition rates for all major ions (Ca2+, Mg2+, K+, Na +, H+, NH4+, SO42-, NO3-, HCO3-), 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 distrib utions of mature forest stands. The remaining parameters can be obtain ed via species-specific model calibration. For a given species, calcul ations across site-classes and deposition scenarios do nor require add itional calibration. All calculations are specific to site, species, a tmospheric deposition scenario and regeneration pattern, and are based on annual timesteps and transfer rates. (C) 1997 Elsevier Science B.V .