干旱区生态环境知识资源中心

Ecological responses to past climate change as determined from palaeorecords offer insights into responses that may accompany future climate change. In arid and semi-arid lands, the interactions between regional vegetation and climate change are not yet well understood, partly due to a lack of suitable palaeovegetation proxies that can provide accurate and continuous tracers for past vegetation dynamics. To gain a better understanding of long-term vegetation dynamics, this study employs a multiproxy approach applied to sand-palaeosol sediments of northeastern China's Songnen grasslands. Phytolith analyses and data on the stable carbon isotope composition (delta C-13) of organic matter are used to reconstruct palaeovegetation composition, namely, the changing abundance of C-3 and C-4 species, whereas a geochemical weathering index (Fed/Fet ratios) tracks past East Asian summer monsoon (EASM) intensity. The phytolith assemblages and indices and delta C-13 of the soil indicate that C-4 species' abundance has been increasing in the Songnen grasslands since the mid-Holocene, although C-3 vegetation is still dominant. Statistically significant negative correlations between the delta C-13 data and Fet/Fed ratios suggest that continuous weakening of the EASM since the mid-Holocene may be responsible for the C-13-enrichment of the sediments in the Songnen grasslands. Field vegetation surveys, modern topsoil phytoliths and delta C-13 calibration data indicate that the expansion of C-4 species since the mid-Holocene is mainly due to their ability to cope with aridity when growing season temperature is not undergoing a significant decrease. Future precipitation decreases in arid and semi-arid lands should make C-4 species more competitive in the grasslands of northeastern China.