Element Removal in Harvested Tree Biomass: Scenarios for Critical Loads in Wallonia,South Belgium

The critical load concept is nowwidely used as a tool for developing emissioncontrol policies in Europe. As a signatorycountry of the Convention of Long-RangeTransboundary Air Pollution, critical loads foracidity, nutrient nitrogen, nitrogen and sulphurhave been calculated for the Flemish and Walloonregions in Belgium. This paper describes themethodology used for estimating critical loadsfor forest soils in the Walloon region accordingto the Steady-State Mass Balance equations. As anexample the methodology was applied to thecatchment `Waroneu', situated in a sensitive areaof the Haute Ardenne. Main input parameters tothe equations were derived from precipitation andrunoff data of the catchment study. Improvedestimates of nitrogen uptake (Nu) and base cationuptake (BCu) were obtained by intensive samplingof Picea abies and Quercus roburtrees. Nutrient contents (Ca, Mg, K, N) andnutrient to nitrogen ratios of Picea abiesreflected the poor soil quality at a site withhigh N deposition. Quercus robur nutrientcontents increased from stem to higher orderbranches with a high proportion of nutrientslocated in the bark. However the simulation ofstem only harvesting had a minor effect oncritical loads. Measured wood densities werelower than reported literature values with a maineffect on Nu and BCu estimates. The use ofrecommended default values and/or data derivedfrom the experimental site resulted in a widerange of critical loads, some of which werelargely overestimated. Results demonstrated theimportance of site specific data for criticalload calculations.