Modelling effects of climate change and forestry on weathering rates and base cation cycling in forest soils

Sammanfattning: Base cations are important in forest soils as nutrients for vegetation and protection against soil acidification. Today, base cations have been lost from soils through acidification and forest harvesting. With low levels in soils, temporary situations of deficiency could occur. One of the largest inputs of base cations to soils is minerals weathering, which makes it an important process. It is, however, hard to measure. By using process-based models, weathering rates can be calculated and used in estimates of the sustainability of forestry with regards to base cation stores. In a changing environment, dynamic process-based models can give important insights about changes in weathering and base cation cycling. In this thesis, the two process-based models, PROFILE and ForSAFE, have been used to describe the effects of a changing climate, acidification and intensified forestry on weathering and base cation cycling. Based on the same weathering process descriptions, PROFILE is a simpler steady-state model, while ForSAFE is a more complex dynamic model with feedback, for example between vegetation and soil chemistry. These results contribute to the existing knowledge about sustainable forestry and add new knowledge about the seasonal effects of weathering.The studies show that weathering rates modelled using PROFILE and long-term averages from ForSAFE are approximately the same size, if soil-moisture levels are equal. However, ForSAFE studies can provide results with high temporal resolution. Depending on research question and data availability, both models can be useful. ForSAFE can increase understanding of processes and dynamics in a changing environment at well-investigated sites. PROFILE can be used for areal upscaling and to assess long-term base cation sustainability of forestry. ForSAFE results show that weathering rates have a strong seasonal dynamic, with low winter rates and high and highly variable summer rates, depending on temperature and soil moisture. Weathering increases temporarily after clear-cutting, as both concentrations of weathering products in the soil water and the soil moisture levels are affected. Removal of branches and tree-tops, in addition to stems, has only a small effect on weathering, much smaller than the loss of base cations through the harvesting. Climate change increases weathering in the Swedish climate. In southern Sweden, rates increase throughout the year, though most in spring and summer. Recurrent low soil moisture during summers already has an inhibiting effect on weathering. This is projected to continue. In northern Sweden, winter temperatures will still mostly be below zero, and weathering rates will only increase in the warmer seasons. The increasing weathering in spring might become increasingly important for vegetation. Vegetation period lengthens in a warming climate and dry summers (especially in the south) might push growth and nutrient uptake to earlier in the year. PROFILE-modelled weathering, with harvest removal of base cations, was used to calculate exceedance of critical biomass harvesting. This, together with acidification status of the soil, was used to assess if whole-tree harvesting would be detrimental to the acidification situation of the soils. Results show that soils generally are more at risk in southern Sweden, with a couple of exceptions where high weathering provides enough base cations. These studies confirm that soil moisture is one of the bigger uncertainties in weathering calculations and an important factor in modelling future biomass growth. Soil moisture is not measured at the modelled sites. Future studies would be needed, to measure soil moisture and compare it with ForSAFE-modelled soil moisture to reduce uncertainties related to soil moisture in PROFILE and ForSAFE. For ForSAFE, this is needed in areas with flat surroundings, where runoff is not as efficient as ForSAFE models it to be. For PROFILE, it is most relevant in sites with differing textures in different soil layers.

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