Huaiyu Yan’s research while affiliated with Lanzhou University and other places

Understanding the vegetation response to climate change and human activities during the Holocene may help predict the future trajectory of vegetation change. Moreover, in the semi-arid region of northern China, it may also provide a scientific basis for addressing ecological problems and achieving sustainable development goals. Based on 5 AMS ¹⁴C dates and 259 fossil pollen samples from the sediments of Lake Gouchi in Shaanxi Province, China, we reconstructed the pattern and process of vegetation succession in the monsoon margin area since the mid-Holocene. Combined with records of regional monsoonal precipitation and archaeological data, we use the results to evaluate changes in the relationship between vegetation, climate change and human activities. The vegetation development during 8–1 ka (1 ka = 1000 cal a BP, BP = before present, where “present” is 1950 AD) was mainly controlled by natural climate change. Optimum vegetation conditions occurred during 7.8–5.3 ka when the monsoon precipitation was the strongest. The vegetation during this interval was mainly grassland with Artemisia. Meanwhile, the percentage of hygrophytes reached its maximum. During 5.3–1 ka, the monsoon precipitation decreased and the proportion of xerophytes increased, while the proportion of hygrophytes decreased. After ∼1 ka, human activities became the dominant driver of vegetation change. The proportion of xerophytes increased during the Medieval Warm Period (MWP, 1000–1300 AD), while the proportion of hygrophytes increased during the early stage of the Little Ice Age (LIA, 1400–1900 AD). This suggests that the shifts between the humid climate of the MWP and the arid climate of the LIA had relatively little influence on the vegetation. The pollen of anthropochores increased substantially after ∼1 ka, together with an increase in the sediment accumulation rate at Lake Gouchi. Thus, it is likely that human activity began to significantly affect the evolution of the regional vegetation as early as ∼1000 years ago. A synthesis of the timing of the transition from natural to anthropogenic forcing of vegetation change in China reveals pronounced spatial differences. In northern China human activity began to dominate vegetation change from ∼2 to 1 ka (especially since 1 ka), while in southern China, a significant human impact on the vegetation occurred much earlier, during ∼5-3 ka. In the future, under the combined effects of a drying climate and the further intensification of human activities in northern China, it will become increasingly important to implement science-based environmental planning in order to restore the regional vegetation and promote the sustainable development of the terrestrial ecosystems of the region.

Holocene variations in precipitation in central and eastern arid central Asia (ACA) have been widely investigated, but the pattern in western ACA remains unclear. We present records of the stable carbon isotope composition of bulk organic matter (δ¹³Corg), magnetic parameters, and sediment color, from five loess‐paleosol sequences in NE Iran, in western ACA, with the aim of reconstructing Holocene precipitation. The Yellibadragh (YE) section (the thickest among the five sequences) was selected for optically stimulated luminescence (OSL) dating of the coarse‐grained quartz (63–90 μm) fraction, and its δ¹³Corg record was used to quantitatively reconstruct mean annual precipitation (MAP). The record indicates a dry early Holocene (~11.8–7.4 ka), with nearly constant MAP (~93 mm), followed by a wetting trend from the mid‐Holocene (~7.4 ka) onward, with the wettest period in the late Holocene (~4.0–0.0 ka, ~390 mm). The stratigraphic observations and environmental proxies support the reconstruction. The other loess profiles show stratigraphic features and trends of environmental proxies, which are similar to those of the YE profile. A dry early Holocene and wetting trend since the mid‐Holocene, with the wettest climate in the late Holocene in NE Iran, are both consistent with records from sand dunes and lake sediments from adjacent areas, and with loess records from central and eastern ACA. Comparison with loess records from monsoonal Asia supports the interpretation of a “westerly‐dominated climatic regime,” which was proposed mainly on the basis of lake sediment records from the region. Changes in solar insolation may have been responsible for the persistent wetting trend during the Holocene in western ACA.