Climatic and anthropogenic forcing of vegetation change at different altitudes on the northeastern Tibetan Plateau over the past millennium

The alpine grasslands and meadows of the Tibetan Plateau are critical for maintaining biodiversity, carbon storage, and regional ecological balance. However, the ecological response of the Tibetan Plateau to climate change and anthropogenic activity is poorly understood. We selected two alpine lakes at different altitudes on the northeastern Tibetan Plateau and used n-alkanes from lacustrine sediments to probe the process of ecological response to climate change (temperature and precipitation) during typical climatic periods of the past millennium, as well as impacts on the ecosystem caused by anthropogenic activities. Our results showed that mid- and long-chain n-alkanes in Lake Dalzong and Lake Bihu sediments, which could be used to evaluate plant biomass variations, were mainly derived from terrestrial plants around the watershed. For both the low-altitude Lake Dalzong (3120 m above sea level (a.s.l.)) and the high-altitude Lake Bihu (4360 m a.s.l.), plant biomass of the watershed increased with enhanced temperature and precipitation during the Medieval Warm Period (MWP), whereas they decreased during the Little Ice Age (LIA), under conditions of lower temperature and precipitation. A further comparison with regional climatic records indicates that temperature and precipitation may have played a more important role in plant biomass changes in the low- and high-altitude regions in the northeastern Tibetan Plateau, respectively. In addition, during the Current Warm Period (CWP), when temperature and precipitation were higher than that in the MWP, plant biomass was lower than that in the MWP in both the Lake Dalzong and Lake Bihu watersheds. The intensification of regional grazing activities and the increase in religious rituals were the primary factors contributing to CWP vegetation degradation. This study highlights the fact that a progressively warmer and wetter climate will not offset the negative impacts of anthropogenic activities on vegetation.