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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 (δ13C) of organic matter are used to reconstruct palaeovegetation composition, namely, the changing abundance of C3 and C4 species, whereas a geochemical weathering index (Fed/Fet ratios) tracks past East Asian summer monsoon (EASM) intensity. The phytolith assemblages and indices and δ13C of the soil indicate that C4 species' abundance has been increasing in the Songnen grasslands since the mid‐Holocene, although C3 vegetation is still dominant. Statistically significant negative correlations between the δ13C data and Fet/Fed ratios suggest that continuous weakening of the EASM since the mid‐Holocene may be responsible for the 13C‐enrichment of the sediments in the Songnen grasslands. Field vegetation surveys, modern topsoil phytoliths and δ13C calibration data indicate that the expansion of C4 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 C4 species more competitive in the grasslands of northeastern China. HighlightsA continuous grassland landscape history is reconstructed from a sand‐palaeosol sequence via phytolith analysis.Multiple independent approaches were used to reconstruct past EASM intensity and palaeovegetation patterns.Phytolith and δ13C analyses indicate an increase in C4 species since the mid‐Holocene.Aridification drives the increase in C4 species within the grassland ecosystem. [ABSTRACT FROM AUTHOR]

• Soil phytoliths can estimate tree cover and separate the forest from grassland. • The ecotone has experienced many expansions and expansions in the Holocene. • East Asian monsoon modulated precipitation was the dominant driver. Robust forecasts of ecological transformations resulting from ongoing and future climate change are urgently needed. Paleoecological records of vegetation dynamics, combined with records of past climate, can contribute to these forecasts, but the relationship between vegetation and climate is often poorly understood. We synthesized a phytolith dataset using >500 topsoil samples and 6 sedimentary profiles from temperate Northeast China. The modern topsoil dataset was calibrated to quantitatively estimate tree cover and grass cover. Based on the threshold of tree cover, we explored the spatial and temporal variations of the forest-grassland ecotone in NE China as well as possible driving mechanisms. Phytoliths provided reliable quantitative estimates of tree/grass cover, accurately distinguishing sites that are currently forest from grassland in this temperate region. The forest-grassland ecotone expanded westward from 7 to 5 ka, retreated eastward from 4 to 2 ka, and then expanded westward again, migrating <100 km throughout this period. The interval of maximum tree cover varied spatiotemporally from east to west during the Holocene. Precipitation from the East Asian summer monsoon was likely the dominant driver of spatial and temporal variations in the forest-grassland ecotone in NE China, with lesser contributions from the Westerly Jet. [ABSTRACT FROM AUTHOR]