Is weathering sufficient to keep forest harvest from exceeding acidification critical loads in Sweden: Conclusions from QWARTS – Quantifying Weathering Rates for Sustainable Forest Management
Kevin Bishop1, Cecilia Akselsson2, Salim Belyazid3 and Stefan Lofgren4
Sweden has ambitious plans for increased primary forest biofuel harvest at rates that are not sustainable according to official Critical Load calculations based on the PROFILE/FORSAFE modeling platform (PFMP). But are those estimates reliable? This presentations summarizes a 7-year multi-investigator project aimed at disproving the hypothesis that the PFMP was appropriate for assessing forest management alternatives. We compared PFMP with other, independent approaches. We also investigated specific process representations, in particular whether there were biological and chemical feedbacks missing from the PFMP that might result in underestimates of weathering. Base cation and aluminium binding reactions, mineralogical determination, and integration of weathering to the catchment scale, were some of the fundamental science questions investigated.
The project found that the major biological effect, removal of weathering products by root uptake, were adequately represented in the PFMP, so a major driver of weathering had not been missed. Refinements were suggested for representation of several processes. The most far-reaching process change was strengthening of weathering “brakes” in the saturated zone. This relates to delivery of weathering products to streams.
In conclusion, the project was able to improve, and suggest further improvements, in the PROFILE/FORSAFE modeling platform, but not able to disprove its essential validity for estimating weathering rates of relevance to national environmental goals. The overall tendency of these changes, and the comparison with other models suggested that there will be a net loss of weathering products from forest soils under many future scenarios in large areas of Sweden unless the intensity of forest harvesting is reduced or weathering products are replaced. This is a major concern, and the export of weathering products by forestry is comparable to the remaining acid deposition.
The reduction of sulfur deposition from the atmosphere means that the transport of sulfate from soil to surface water, with accompanying cations, is lower than before. So while the landscape will continue to lose its overall store of weathering products, and recovery from acidification will be delayed under many future scenarios, the effects on tree health and surface water acidity will be limited, except in areas with high anion fluxes (e.g. sea-salt deposition events or mobilization of nitrogen), particularly where soils are thin.
1Swedish University of Agricultural Sciences, kevin.bishop@slu.se
2Lund University, Sweden, cecilia.akselsson@nateko.lu.se
3Stockholm University, Sweden, salim.belyazid@natgeo.su.se
4SLU Dept. of Aquatic Sciences and Management, stefan.lofgren@slu.se