Holocene environments and the forest dynamics enigma in the arid Altai highlands

The objective of this study is to reconstruct the ecological history of the arid highlands of Central Asia during the Holocene. The study of Lake Khindiktig-Khol, located at the watershed between the outflow into the Arctic Ocean and the closed Central Asia, and its surrounding environment provides valuable insights into the ecological and climatic history of the area. By analyzing multiple paleoenvironmental proxies, pollen, chironomids, cladocerans, and non-pollen palynomorphs, this research has reconstructed the complex interplay between climate, vegetation, biodiversity and aquatic ecosystems over the past 11.4 ka. Our findings, in accordance with numerous other paleoreconstructions based on various proxies, reveal that the early Holocene was characterized by cold and arid conditions, with limited forest cover and low biodiversity. However, conifers such as Pinus sibirica, Picea obovata, and Larix sibirica were present in the region, even at elevations above the modern treeline, suggesting that moisture alone was not the sole limiting factor for forest development.

The mid-Holocene (approximately 7.5–3 ka BP) marked a period of increased humidity and warmth, often referred to as the Holocene climatic optimum. During this time, forest cover expanded, and biodiversity reached its peak, as evidenced by the dominance of cold deciduous and evergreen coniferous biomes in the pollen record. The lake ecosystem also responded to these favorable conditions, with increased productivity and the establishment of diverse aquatic communities. However, by the late Holocene, forest cover in the region gradually declined, culminating in the near-treeless landscape seen today. This shift was likely driven by a combination of cooling temperatures, permafrost expansion, and increased waterlogging—all of which hindered tree growth. The disappearance of forests in the late Holocene remains a topic of debate, but evidence suggests that permafrost expansion and waterlogging may play a significant role. While human activity in the late Holocene may have contributed to local deforestation, the widespread absence of trees in areas that are now uninhabited points to natural climatic and environmental changes as the primary drivers. These findings provide crucial insights into the long-term dynamics of high-altitude ecosystems and their responses to climatic shifts, offering valuable perspectives for predicting future changes in similar environments under current global warming trends. The unresolved questions about early Holocene conifer persistence highlight important knowledge gaps in our understanding of alpine vegetation resilience.