Morphological variability in propagules of a desert annual as a function of rainfall patterns at different temporal and spatial scales.

Organisms living in highly variable environments have to display integrated strategies to deal with both systematic and random variation occurring at different temporal and spatial scales. Two predictions were tested by analysing geographic-scale patterns of seed size and seed retention (serotiny) in Chorizanthe rigida, a strict winter desert annual that delays seed dispersal and releases propagules after rainfall events: (i) adaptation to systematic environmental cues occurs by means of changes in morphology, and (ii) within-individual variation in seed size allows a differential response to rainfall cues: while some seeds germinate rapidly others are retained for future rainfall events., We quantified morphological variation and performed germination experiments on C. rigida propagules (involucres + achenes) from six populations distributed throughout the Mojave and Sonoran deserts covering: (i) a systematic, west-to-east, winter-to-bi-seasonal (summer and winter) precipitation gradient and (ii) a winter-rain unpredictability gradient inferred from long-term climatic data., The propagule retention structure (i.e. base area of the pedicel) of C. rigida individuals experiencing bi-seasonal rainfall is double the size of those that have evolved under a strict winter rainfall regime, showing that populations living in bi-seasonal environments have higher seed retention, which allows them to avoid releasing seeds to a summer rainfall cue., Within-individual variance of propagule size varied significantly between populations and was correlated with winter rainfall variability in each site., Germination varied as a function of propagule size; smaller seeds germinated more readily than larger seeds. Increased variability in propagule size might result in a more variable germination response., Under common experimental conditions germination varied significantly among sites and was negatively correlated with mean winter effective precipitation, suggesting that propagules from populations in drier sites have lower germination moisture thresholds., Synthesis. C. rigida propagules have larger bases in deserts with biseasonal rainfall, which allows them to avoid seed release during summer rainfall cues, and display within-individual seed variance associated to rainfall unpredictability, a trait often interpreted as a bet-hedging strategy. Our study provides empirical evidence of an integrated strategy that allows to cope with both random and systematic rainfall variation. [ABSTRACT FROM AUTHOR]