Varied responses of forest at its distribution margin to Holocene monsoon development in northern China

Abstract

Based on pollen analysis and other evidence in a sediment profile from Huangqihai Lake, located at the limit of Asian monsoon influence, the local vegetation development and climatic change were reconstructed for the past 8600years. From 8600 to 7200cal. yr BP, the local vegetation was dominated by steppe and tree cover was low. Then pine increased gradually, and the local vegetation was pine and oak mixed forest with variable ratios of the two dominant tree taxa from 7200 to 3600cal. yr BP. From 3600 to 3000cal. yr BP, the forest was replaced by shrubland, which was attributed to a period of intense drought. From 3000 to 600cal. yr BP, the tree cover declined to its lowest levels, probably due to long-term further drought. After 600cal. yr BP, steppe has become dominant again. We found that the changes of vegetation type indicated by pollen percentages did not match existing evidence for monsoon dynamics. Changes in tree pollen concentration, however, well captured the changes in the Asian monsoon, implying different sensitivity of vegetation cover and vegetation type (e.g. replacement of steppe by forest) to the monsoon development. In addition, a shrubland-dominated stage which has not been evident in previous studies was also clear in our pollen sequences. Our study implied that the responses of marginal forest to drought were variable, depending on the drought regime. The different responses of forest to climate drying in this study provide insights into the reliable prediction of forest dynamics in the semi-arid regions.