Non-linear response of mid-latitude Asian dryland vegetation to Holocene climate fluctuations

Understanding the response of mid-latitude Asian (MLA) biomes to Holocene climate fluctuations is important for the conservation of fragile dryland ecosystems under future climate warming. Here, we use fossil pollen records from two cores recovered from Balikun Lake, eastern Tien Shan, to reconstruct regional Holocene vegetation dynamics and environmental change. These records are combined with present-day pollen assemblages measured in samples from airborne pollen traps and soils across the catchment. We find that halophytes from saline-alkali habitats dominated the traps and near-shore surface samples, implying that halophytes are the primary local pollen source when the lake-shore environment becomes drier. A high relative abundance of Chenopodioideae pollen during the early Holocene in our near lakeshore (BLK06) core implies significant shrinkage of the lake area, linked to increased aridity across the MLA. When combined with inferred low temperatures, we propose a widespread expansion of desert vegetation under cold-and-dry conditions. A consistent abundance of Pinus, Picea, Betula pollen in both cores implies a common source areas characterized by widespread forests, in addition to locally-derived pollen, during the mid-Holocene. We infer that these changes correspond to increasing growing degree days, promoted by ice sheet retreat and increased winter insolation during the mid-Holocene. This allowed alpine forests to expand extensively along the Tien Shan Mountain. In the late Holocene, tree cover gradually shrunk as temperate grassland expanded in response to increased effective moisture, associated with increased intensity of westerlies coupled with lower temperatures linked to decreased insolation. We propose that rising temperatures in the recent past and near future will act to enhance the glacial meltwater supply in the MLA drylands, yielding an increase in both the growing degree days and the moisture conditions, which will drive expansion of the mountain forest and grassland biomes.