Phytolith records of flourishing early Holocene Pooideae linked to an 8.2 ka cold event in subtropical China

The grass subfamily Pooideae originated in a temperate niche during the late Cretaceous; it is the largest Poaceae subfamily, consisting of almost 4,000 species, which are distributed worldwide. Pooideae responses to climate changes at different time scales, and different ecological zones are thus important in understanding Poaceae evolutionary processes and their relationship with climate change. In the study described in this article, we reconstructed Pooideae variability during the early Holocene, as inferred by a phytolith sequence from the Lower Yangtze in subtropical China. The phytolith assemblage was marked by three increases in Pooideae phytoliths, dated to ca 8.4–8.0, 7.8–7.6, and 7.4–7.2 ka BP (before present, 1950 AD), with each representing pronounced increases in Pooideae extent and distribution. All these increases were within age ranges that agreed well with the timing of weak Asian Monsoon events, at 8.2, 7.7, and 7.3 ka BP. The first Pooideae flourishing period in subtropical China was the most significant, lasting for approximately four centuries and being characterized by a double peak, which equated with an event at 8.2 ka. This suggested that cold and/or dry conditions—which occurred over a period of several hundred years and were linked to weakening of the Asian monsoon—probably caused Pooideae to flourish in the Lower Yangtze region. Comparison of two diagnostic trapezoid phytolith types—namely wavy and wavy narrow—which showed different changes between ca 8.4 and 8.0 ka BP, suggested that they responded differently to the climate change represented by the 8.2 ka event. Our phytolith records have provided not only new data clarifying the detailed Pooideae response to the 8.2 ka event but also a reliable index for past cold climates in subtropical China.