Measured and modelled long-term effects of whole-tree harvest : impact on soil and surface water acid-base status in boreal forests

Abstract: This thesis examines the impact of whole-tree harvest (WTH) on soils and surface waters acid-base status in coniferous forests compared with conventional harvesting (CH). A combination of field observations (up to four decades) and dynamic modelling was used to describe the impact on soil calcium (Ca²⁺) pools and surface water acid neutralizing capacity (ANC). The studies were undertaken in northern and southern Sweden at sites belonging to the long-term wood fuel experiment (HELTRAD) and the ICP Integrated Monitoring (IM) programme. One of the most important findings was that WTH and CH caused large depletions (up to 60%) in soil exchangeable Ca²⁺ pools, circa 40 years after harvest. Despite these losses, tree growth and vitality has not yet been impaired. The results also implied that the fastest depletion rates occurred in CH-plots why soil Ca²⁺ pools between CH and WTH have become more similar with time. However, soil Ca²⁺ pools still remained significantly lower after WTH. Measured differences in soil solution showed that the impact of WTH on ANC was small (16 μEq l⁻¹) and temporary but site-specific. This difference was not large enough to counteract the natural recovery from acidification or lead to adverse ecological effects. The results indicate that WTH can have a large impact on soil exchangeable pools without causing surface water acidification in the absence of strong acid mobile anions. The rapidly declining soil Ca²⁺ pools may be of concern for sustainable forest management. However, trees will likely respond to lower Ca²⁺ availability by e.g. adjusting their uptake or develop biological feed-back mechanisms. The model predictions unanimously suggested that tree growth and net accumulation of nutrients in biomass caused large depletions in soil exchangeable Ca²⁺ pools and a decrease in stream ANC during the 19th and 20th centuries, exacerbated by acid deposition. Future predictions also suggested that these Ca²⁺ losses would continue accelerated by more intense harvesting practices. The impact on modelled stream ANC was smaller and depended on the concentrations of mobile anions. The empirical data pointed in the same direction as the MAGIC predictions, but the model exaggerated the impact. Until the discrepancy between measured and modelled impact have been resolved, interpretations changes related WTH using dynamic modelling or mass balance budget calculations should be done with caution.