Germination speed modulates priority effects: Evidence from a large-scale field study.

Priority effects, where species that arrive first influence later arriving species, are often considered in terms of seed arrival time. However, the timing of seedling emergence may play a more critical role, as this defines when plants start interacting. Further, initial seed density may also be important, allowing early-arriving species with low initial seed density to overcome seed limitation, while also potentially allowing late-arriving high-density species to overcome the disadvantage of arriving late. In this large-scale, multi-site field experiment, we manipulated species arrival and emergence timing by sowing fast- and slow-germinating meadow species in various arrival orders and seed densities across two climatically contrasting sites in Sweden. Our findings demonstrate that germination speed modulates the strength and direction of priority effects: fast-germinating species were less affected by both early- and late arrival. Conversely, slow-germinating species were disadvantaged by late arrival and benefited significantly from early arrival, particularly at the more productive, northern site with shorter growing seasons. Contrary to expectations, initial sowing density had limited and inconsistent effects on priority effect outcomes. These results highlight that emergence timing, not just seed arrival, is a key aspect of priority effects, influencing plant competition and community structure. Furthermore, the context dependency across sites emphasizes the importance of environmental conditions in modulating priority effects, with implications for predicting vegetation dynamics under climate change.