Desert seasonal rainfall variability and seed retention in Chorizanthe rigida

Background/Question/Methods

Desert rainfall has been characterized as infrequent, low, highly variable, and unpredictable. However, desert regions have seasonal precipitation patterns. For instance, in North America the Mojave Desert has mostly winter rainfall, while the Sonoran Desert has a bi-seasonal rainfall distribution. This translates into contrasting growth seasons of varying length, moisture and temperature. As such, this desert winter-summer precipitation gradient constitutes an important evolutionary driving force.

 Desert plants have adaptations to cope with rainfall variability. Some desert annuals form an above-ground seed bank by retaining their seeds in hardened structures (called serotinous) and releasing them to rain events. A common seed-retaining annual in North American deserts is Chorizanthe rigida, whose dead, hardened, spiny stems persist for several years firmly anchored to the ground. Both C. rigida’s distribution across the Mojave and Sonoran desert regions and its rain triggered seed release mechanism offer an excellent system to study plants adaptations to seasonal precipitation gradients in deserts. The question addressed in this study was: Do seed dispersal mechanisms prevent winter annuals from dispersing their seeds to a summer rain event? Biogeographic and morphometric approaches were used to analyze the dispersal unit (i.e. involucre) and whole plant morphology of different C. rigida populations.

Results/Conclusions

Chorizanthe rigida individuals belonging to ten different populations were collected in the Mojave, Colorado, Sonoran and Baja California desert regions. Observations on dead C. rigida structures collected in the field and grown in the greenhouse showed that this species morphology varies across deserts with different precipitation regimes. Principal component analysis showed that C. rigida’s funnel shaped involucres are composed of three bracts that vary in size, length, width, and shape. Differences were found in the dehiscent tissue at the base of the involucre as well, both within individuals and among populations.

Awn length in the involucre is a key feature of these dispersal units and is seen to vary considerably within this species at the population level. Variability in involucral morphology may result in different dispersal characteristics adapted to local rainfall regimes. This study illustrates selective pressures from the climatic drivers acting on dispersal morphology in desert regions that vary in seasonal rainfall distribution.