Divergent shifts in flowering phenology of herbaceous plants on the warming Qinghai–Tibetan plateau

Abstract

Both advanced and delayed shifts of different magnitude in flowering phenology, based on limited controlled experiments and ground-based observational studies, have been reported on the Qinghai–Tibetan Plateau (QTP). To clarify flowering phenological changes, we collected 1646 ground-observed phenological records of 21 species, from 27 stations, for 78 time series, and daily average air temperature data from 1983 to 2017 on the QTP. Trends in the first flowering date (FFD) and changes with temperature increase were analyzed using regression models. The FFD of early flowering time series showed an overall trend for significant advance (-0.371 ± 0.149 days/year, p < 0.001), whereas the mid-to-late-flowering showed a delayed trend (0.158 ± 0.193 days/year, p = 0.108). This finding is in contrast with the stable trend for early flowering species and the advanced trend for midsummer and late-flowering species in high latitude and Arctic regions, where the low-temperature environment is similar. The growing degree days from 1 January to the day of the observed occurrence of the FFD for early flowering time series kept stable (p = 0.531) and FFD showed a positive regression with chilling days; however, the growing degree days of mid-to-late-flowering time series increased significantly at a rate of 5.375 ± 2.519°C-days/year (p < 0.001), indicating the limitation on FFD from other related climatic factors except for forcing temperature may strengthen, and FFD showed no relationship with chilling days. The mid-to-late-flowering time series showed vernalization sensitivity in the spring of the flowering year, whereas the early flowering time series did not. This indicated that retarded vernalization completion may be responsible for the delayed flowering phenology of mid-to-late-flowering species. The FFD of early flowering species, with flower preformation in the preceding autumn, was mainly controlled by forcing temperature in spring; decreased chilling days indicating warmer winter were also beneficial for development and fitness of preformed floral organs, and thus, temperature increase significantly advanced the FFD. These findings suggest that divergent flowering phenology shifts may alter the species composition and ecosystem services of alpine ecosystems, causing challenges in grassland management.