Desert annual pants are frequently the subjects of coexistence studies focusing on the role of environmental variation. Germination rates are observed to vary substantially over time forming the basis of the storage-effect coexistence mechanism. Laboratory germination studies have reproduced some of this variation, but do not replicate the full range of conditions found in the field. Here we discuss one aspect of field variation that potentially has a major effect on plant species coexistence, but has been neglected in the past. Experimental and observational data from the desert annual plants at our field site near Portal, AZ show distinct species-specific differences in the speed of germination following rainfall. This indicates an important and previously unexplored opportunity for covariance between competition and the environment, a key quantity for coexistence by the storage effect. Most seeds are at or near the surface, and so moisture at the soil surface is critical for germination. Duration of this moisture varies greatly due to duration and spacing of rainfall events, and also due to solar radiation, temperature, humidity and wind following these events.

Results/Conclusions

Field observations and laboratory experiments show that the observed differential germination speed is strong enough to lead to different species dominating due to different durations of soil moisture. Thus, variation in duration of surface soil moisture potentially contributes substantial yearly variation in the species composition of the annual plant community. Using data on germination speed from field observations and germination experiments in the lab, we built a model linking fluctuations in surface soil moisture and species’ germination speeds to predict the crop of plants resulting from specific rainfall patterns. Our model implies that the interaction of species-specific germination speed with variation in soil moisture makes a major contribution to the storage-effect coexistence mechanism in this community. For example, species that germinate quickly have greater opportunity to take advantage of short-duration rainfall and surface soil moisture, while other species require a longer period of surface soil moisture to successfully germinate, but may be superior competitors once germinated. Incorporating the interaction between germination speed and the duration and spacing of rainfall greatly expands the opportunities for species coexistence in this community. This increased understanding of the factors controlling community composition also provides a more refined understanding of the long-term stability of species composition.