Vegetation change at the southern boreal forest margin in Northeast China over the last millennium: The role of permafrost dynamics

Abstract

To better understand the nature and causes of boreal forest change and, in particular, the role of permafrost dynamics, we reconstruct the recent vegetation history of the Greater Hingan Mountains of northeast China based on a lake sediment core. This core is located in an area of boreal forest underlain by permafrost, and spans a 1000-year warming-to-cooling cycle. Palynological assemblages indicate that, throughout most of its history, vegetation was characterized by changes in the relative proportions of taxa, including the cold-resistant boreal conifer species Larix and Pinus, and warmth-adapted temperate broadleaf shrub species Corylus and Rhododendron, corresponding to variations in temperature. However, after ~1950 CE, rapid warming led to the breakdown of this relationship between vegetation and climate, and the proportion of conifers increased in the short term, most likely due to permafrost thawing. This effect was most pronounced in lowland areas whose frost table depth was initially ~20 cm closer to the surface than the corresponding frost table on mountain slopes. The increase in frost table depth due to warmer temperatures provided greater root space for shallow-rooted conifers in lowlands, as well as meltwater from thawing permafrost. We speculate that the coupled system of vegetation, climate and permafrost was stable before ~1950 CE; however, most recently, there has been a transition due to warming-induced permafrost thawing. As the southern boundary of permafrost moves poleward, it is suspected that lowland rather than slope sites will witness the sharpest vegetation shifts.