Water limitation may restrict the positive effect of higher temperatures on weathering rates in forest soils

Abstract. Climate change is generally expected to have a positive effect on weathering rates, due to the strong temperature dependence of the weathering process. There are, however, a number of feedback effects, both positive and negative, that can affect the weathering response to climate change, but that have not been fully taken into account in previous estimates. Important feedback mechanisms are the direct effect of changes in soil moisture, and the indirect effects through tree growth and decomposition on weathering rates. In this study, the dynamic forest ecosystem model ForSAFE, with mechanistic descriptions of tree growth, decomposition, weathering, hydrology and ion exchange processes, is used to investigate the effects of future climate scenarios on weathering rates in a more holistic way than has been done before. 544 productive coniferous forest sites, part of the Swedish National Forest Inventory, are modelled, and differences in weathering responses to changes in climate from two Global Climate Models are investigated. The study shows that weathering rates are likely to increase, but not to the extent predicted by a direct response to elevated air temperatures. The simulations show that increases in soil temperatures are less evident than those in air temperature, thereby dampening the effect of warming on weathering. Most importantly, the study shows that soil moisture availability has a strong potential to limit the expected response to increased temperature. While changes in annual precipitation may not indicate further risk for more severe water deficits, seasonal differences show a clear difference between winters and summers. Taking into account the seasonal variation, the study shows that reduced soil water availability in the summer seasons will strongly limit the expected gain in weathering associated with higher temperatures.