Rainfall Variability in Deserts and the Timing of Seed Release in Chorizanthe rigida, a Serotinous Winter Desert Annual.

摘要

Serotiny and delayed seed dispersal are thought to have evolved to cope with environmental variability. A taxonomic survey of serotinous desert species in the Mojave and Sonoran deserts showed that this strategy is widespread among many families in this arid region. Furthermore, results also showed that similar seed retention syndromes such as those of plants belonging to other world deserts have evolved in North American deserts region. The next three chapters of the dissertation explored both the ecological and evolutionary significance of seed retention using Chorizanthe rigida as the study system. First, I explore the small scale distribution pattern of C. rigida, a serotinous desert annual, in a shrub-mound-to-desert-pavement gradient. The consequences of seed retention and desert landscape soil and surface features are discussed. By timing seed release after the winter rains C. rigida's seeds are dispersed by run-off into the desert pavements and transition zones where it forms dead seed-retaining structures that persist for years, releasing seeds to subsequent rain events. Predictions regarding environmental variability and unpredictability were tested by analysing geographic-scale rainfall patterns and serotiny in C. rigida, a strict winter desert annual that retains its seeds and releases them after rainfall events. Results show that populations living in bi-seasonal environment have higher seed retention and are adapted to avoid releasing seeds to a summer rainfall cue. Biomechanical tests were performed to assess the functionality of propagule attachment structures. Moreover, a strong correlation between propagule size within-individual variance and winter rainfall variability was found, as predicted by bet-hedging theory. These results suggest that seed size variability allows this species to cope with different levels of environmental unpredictability by displaying a diversifying risk strategy. In conclusion, despite the extreme variability and unpredictability of desert ecosystems, seed retention and timing seed release to rain cues allows serotinous desert plants to thrive during brief windows of opportunity and to persist through harsh conditions.