Primary productivity of managed and pristine forests in Sweden

Abstract

Land use is affecting 70% of global ecosystems and their functioning. Forest management is a regionally dominant land use and affects forest ecosystems by changing both structure and functioning, but its impact on primary productivity is not well known. Here we investigated the effect of forest management on primary productivity by comparing managed secondary forests with relatively pristine unmanaged primary forests in Sweden. As proxy for primary productivity we used the satellite-based vegetation index NIRv which has been shown to be closely and linearly related to primary productivity. We produced a digital map of 390 primary forests across Sweden, and extracted NIRv over these and surrounding secondary forests forming spatially proximate pairs. By comparing the primary and secondary forests NIRv in the pairs we found that secondary forests on average show higher NIRv, but the highest values were found in primary forests. The difference in NIRv between pairs is related to their difference in mean stand age, and at equal stand age the NIRv of primary forests is higher than in their paired secondary forests. Overall, management leads to increased NIRv through regeneration of forests stands that reduce their mean age. However, primary forests show higher NIRv when controlling for age, despite being found on higher altitudes and on steeper slopes with lower soil moisture, which suggests that forest management other than regeneration is not increasing primary productivity of Swedish forests.