Abstract: Aims Germination timing determines the environmental conditions experienced after germination, holding significant adaptive significance. Plant populations can exhibit distinct patterns of germination timing along environmental gradients, and germination timing within populations responds to environmental change. Spartina alterniflora has rapidly expanded into temperate, subtropical, and tropical regions since it was introduced to China in 1979. At mid and high latitudes, it typically germinates in spring. However, populations along a latitudinal gradient have developed differentiated germination timing by adapting to local environmental conditions. Few studies have examined how seasonal germination characteristics vary among populations along the latitudinal gradient. Methods In this study, we collected seeds from nine populations along the latitudinal distribution range of S. alterniflora and sowed them in multiple common gardens across three different climate zones. Seed germination dates were continuously monitored to explore how seasonal germination characteristics responds to both the planting and seed source environmental conditions. Important findings (1) As the latitude of common gardens decreased, winter temperatures significantly increased. Consequently, the germination window of S. alterniflora expanded from exclusively spring to both winter (earlier) and spring. Accompanied by increased winter germination percentage (P < 0.05) and decreased spring germination percentage (P < 0.05), the proportion of winter-germinated significantly increased (P < 0.05), while the total germination percentage declined (P < 0.05). (2) Within each common garden, winter germination percentage did not change across populations along a latitudinal gradient; however, spring germination percentage decreased significantly with decreased latitude of origin (P < 0.001). Specifically, in the mid-latitude and low-latitude common gardens, the proportion of winter-germinated showed a significant negative correlation with the latitude of origin (P < 0.05). (3) As the temperature at sites of origin increased, the total germination percentage significantly decreased (P < 0.001), and the proportion of winter-germinated significantly increased (P < 0.001). Overall, both increased temperature of the planting environments and the source environments enhanced the likelihood of germinating in the current winter but reduced the total germination percentage, indicating that temperature is a key factor influencing the seasonal germination characteristics across latitudinal populations of S. alterniflora. These findings provide a scientific basis for predicting changes in the germination strategies of S. alterniflora along the latitudinal gradient under climate warming, which is crucial for forecasting shifts in the fitness and distribution range of S. alterniflora.

Key words: germination timing, seasonality, latitude, Spartina alterniflora, life history