Morphological variability and divergence in seed characters of a winter desert annual (Chorizanthe rigida) across a precipitation gradient

Background/Question/Methods

Deserts environmental variability constitutes an important selection factor shaping life history evolution and morphological diversity. Desert rainfall has been characterized as infrequent, low, highly variable, and unpredictable. However, desert regions have different seasonal and intra-seasonal precipitation patterns and so display a combination of predictable and unpredictable environmental variance. Desert annuals may cope with environmental uncertainty showing plastic germination triggered by seasonal cues and seed-banking bet-hedging strategies that respond to rainfall variability. Chorizanthe rigida is a  North American Winter desert annual that forms an above-ground seed bank by retaining seeds on its lignified dead skeletons for several years and whose seed dispersal is triggered by rainfall. The objective of this study was to analyze the relationship between seed retention, seed morphological variability and rainfall gradients in C. rigida populations. Specifically, we were interested in exploring which adaptations would allow an above-ground seed-banking Winter annual to expand its distribution range to desert regions with different seasonal rainfall distribution? Furthermore, we were also interested in exploring if an above-ground seed banking desert annual would exhibit a diversified bet-hedging strategy to cope with rainfall variability. Individuals from six populations were collected along two precipitation gradients in the North American desert region: a Winter to bi-seasonal (i.e. Winter and Summer) precipitation gradient and a Winter precipitation gradient with varying predictability. We used a morphometric approach to analyze propagule variation. A Winter effective precipitation index and Percent Summer rain was calculated for each site from long-term precipitation and temperature data.

Results/Conclusions

As predicted, we found that C. rigida populations occurring at sites with a bi-seasonal rainfall distribution had larger propagule attachment base areas to avoid releasing seeds to Summer rainfall occurring at a time of high evapo-transpiration caused by high temperature. As predicted, we found that C. rigida populations occurring at sites with higher within-season Winter rainfall variability displayed more propagule-size variability thus acting as a diversifying bet-hedging trait. We found high and positive significant correlations between Winter effective precipitation coefficient of variation and involucre size variance as predicted by classic bet-hedging theory. Our study provides an example of an adaptation of a desert annual to among-season rainfall variation patterns and of a diversifying bet-hedging trait that responds to rainfall variability among desert regions.