Ecological significance of serotiny and timing of seed dispersal in desert regions with varying seasonal rainfall distribution

Background/Question/Methods

Desert regions have different annual, seasonal and intra-seasonal precipitation patterns related to their location and geologic characteristics. For instance, in North America the Mojave Desert has mostly winter rainfall, while the Sonoran Desert has a bi-seasonal rainfall distribution. In these deserts, decadal precipitation variability is associated to weather phenomena like ENSO-events while seasonal rainfall variability is caused by the Pacific frontal systems, or monsoon systems in the Gulf of Mexico and Gulf of California. Desert annuals have developed adaptations to cope with rainfall variability, including dispersal strategies allowing them to scatter seeds to reach new ephemeral resource patches, or retaining seeds in protected structures and releasing them to precipitation pulses. Chorizanthe rigida is distributed in North American Deserts and is a serotinous desert winter annual whose dead spiny structures persist on the ground for several years, strongly holding involucres that get dispersed by rainfall. Biogeographic information and morphometric techniques were used to analyze C. rigida populations occurring in desert regions with varying seasonal rainfall distribution to understand the ecological significance of serotiny and timing of seed dispersal to rain events. The question addressed in this study was: Does C. rigida dispersal morphology vary in desert regions with differing rainfall distribution?

Results/Conclusions

Chorizanthe rigida is distributed from the southern Great Basin Region, to the Mojave, Sonoran, and Baja Californian Deserts which are regions dominated by winter and bi-seasonal rainfall distribution patterns. C. rigida’s funnel shaped involucres are composed of three bracts that vary in size, length, width, and shape. Awn length in the involucre is a key feature of these dispersal units and is seen to vary considerably within this species. Preliminary results on the morphometrics of C. rigida involucres indicate that at the population level there are adaptations to local rainfall regimes. Variability in involucral morphology may result in different dispersal characteristics related to varying precipitation regimes occurring in the Mojave and Sonoran deserts. This study illustrates selective pressures from the climatic drivers acting on dispersal morphology in desert regions that vary in seasonal rainfall distribution.