Latitudinal trends in flowering phenology of the invasive Spartina alterniflora in China

Phenology plays an important role in the responses of plants to the environmental changes, especially in latitudinal scale, because temperature influences the timing of development. Flowering time is a key event for plants, which affects their chances of pollination, and determines the timing of seed ripening and dispersal and so on. The distribution of Spartina alterniflora covered a wide latitudinal gradient in its native (USA, 27-45^o N) range, where many studies have indicated that the flowering time of S. alterniflora decreased with increasing latitude. As S. alterniflora has spread over 19ᵒ of latitude ( 20-39^o N) since introduced into China in 1979, which provide an opportunity to study the flowering phenology of S. alterniflora in its invasive range. Here, we ask if patterns of latitudinal flowering phenology found in the native range of S. alterniflora have developed in China. If so, we ask if that variation has a genetic basis or is driven by a direct response to differences in the environment. We distinguish between these possibilities using geographic field survey across broad latitudinal gradients, incorporating a common garden experiment.

In the field, our study showed that there is a hump-shape relationship of the flowering time of S. alterniflora with latitude in China in a 3 year's field observation, which is different from the linear relationship in its native range. The flowering initiation date was earliest at the lowest latitudinal site, but not at the highest latitudinal site. However, the flowering time of S. alterniflora decreased with increasing latitude in the common garden (24^o N) for 2 growing seasons. By calculating the growing degree days to onset the flowering both in the field and common garden, we found that growing degree days in flowering time both decreased with increasing latitude in field and common garden, which indicated that the growing degree days likely to be the mainly factors that shape the flowering phenology of S. alterniflora. And latitudinal variation in flowering time persisted for two years in the common garden, suggesting that plants may be evolving genetic clines for this trait. Based on our results in China, we assumed that global warming might induce an earlier flowering initiation date of S. alterniflora in the lower latitudinal sites in US.