Subarctic Vegetation under the Mixed Warming and Air Pollution Influence

Abstract

In the Siberian Arctic, worldwide largest forest mortality was caused by chronical (since the 1940s) influence of SO2 emissions on the larch-dominant communities. We hypothesized that warming might mitigate SO2 influence by increasing trees’ vigor and growth. We studied trees (larch, Larix sibirica; spruce, Picea obovate; birch, Betula pendula) and bushes (willow, Salix sp., alder, Duschekia fruticosa) growth dependence on SO2, air temperature, soil temperature and moisture, and precipitation. We sampled woods in severely damaged larch and moderately damaged mixed larch, spruce and birch forests. We generated tree ring chronologies and growth indices (GI). We used Terra/MODIS satellite data for mapping trends of vegetation (NDVI) and productivity (GPP, NPP) indexes. In the larch forest, we found a strong decrease in GI and tree mortality, which lasted until the end of 1990s. In the mixed forest, larch and birch were more resistant to SO2 influence compared to spruce. SO2, air and soil temperatures were mediators of all woody species growth. Winter precipitation stimulated trees growth by mitigating spring water stress. Warming onset in the 2000s led to a pronounced increase of all woody species growth. June–July air and soil temperatures, together with a moderate decrease in SO2 emissions, were the primary drivers of that phenomenon. Increasing trends of GPP, NPP, and NDVI were observed within the large part of earlier damaged forests, which was attributed to trees GI increase together with the expansion of SO2-resistant grasses and bushes.