Response of vegetation to past climate changes in Central Asia

Quantifying the long-term trend of climate versus land use influence on vulnerable ecosystems is of great importance to identify the threats of landscape modifications on biodiversity and ecosystem services, and therefore on societies. The evaluation of the resilience of ecosystems is particularly important considering the ongoing climate change.

As ecosystems in arid Central Asia are mainly influenced by climate and physical geography and most species are growing near their physiological limit, the predicted increased aridity for this region likely increases the threat on the ecosystems in this region.

Pollen are the main proxy to explore changes in vegetation at different spatial (local to subcontinental) and temporal (decades to millennia) scales. To quantify human- and climate-induced changes in vegetation, past land-cover (pollen-based estimates), land use (human deforestation scenarios and human population size) and climate (variables derived from climate models) data can be combined, as it has been done in Europe (e.g. Marquer et al., 2017).

This study aims at quantifying the effect of past climate changes on vegetation in Central Asia over the past millennia at century time scale. For this purpose, we use 49 pollen data from sedimentary records (lakes and mires) which were transformed into vegetation composition and diversity indices. Pollen data as point estimates and spatial grids of past vegetation are combined with available annually resolved gridded summer temperature and precipitation estimates inferred from tree-ring chronologies in this region. The reconstructed climate and vegetation trends are compared to different transient Earth System model simulations with the help of the biome-model BIOME4 (c.f. Dallmeyer et al., 2017). Statistical analyses have been performed to compare all data.

We found clear spatial pattern in the plant distribution with i) a large abundance of coniferous trees in northernmost areas and to a lesser extend in the mountains (e.g. Tian Shan), ii) steppes in the lowlands and at high plateaus, and iii) semi-deserts and steppes in the lowlands. The vegetation composition and diversity have significantly changed over the past millennia. Those changes are mainly related to modifications in composition and diversity of plant species in steppes and semi-deserts, of coniferous trees in the mountains, and changes in land use. Our results reveal that precipitation is the major driver of vegetation composition and diversity in Central Asia whereas temperature mainly explains the spatial variation, in particular during major climate events, e.g. the Little Ice Age and the Warm Medieval Period. Further studies are now in progress to quantify the relative (to climate) influence of land use (e.g. anthropogenic land-cover change; ALCC) in the region.

This study demonstrates the climate dependency of vegetation composition and diversity in Central Asia, especially during the major climate events over the last two millennia. This opens the discussion about the resilience of vulnerable ecosystems facing severe impacts of ongoing and predicted climate changes in arid Central Asia.      

Dallmeyer et al. (2017) Climate of the Past 13, 107-134. / Marquer et al. (2017) Quaternary Science Reviews 171, 20-37.