Species differences in phenology shape coexistence

We parameterized a competition model to estimate within-season niche differences, fitness differences, and coexistence, and to estimate coexistence when year-to-year fluctuations of germination timing occur. Increasing germination separation caused parallel changes in niche and fitness differences, with the net effect of weakening within-year coexistence. Both species experienced a competitive advantage by germinating earlier and a four-day head start allowed the generally inferior competitor to exclude the otherwise superior competitor.

The overall consequence of germination separation was to limit coexistence within a given year, although year-to-year variation in relative timing of germination was sufficient to support long-term coexistence. Our results clarify how phenological differences structure competitive interactions and highlight the need to quantify year-to-year variation in these differences to better understand species coexistence.