Abstract: Understanding modes of response to environmental variability is paramount to gaining predictive power regarding organismal population dynamics. Biological bet hedging may be an ecologically important and ubiquitous mechanism for fitness maximization amidst unpredictable environmental variability. Additionally, as environmental variability is seldom completely unpredictable, bet hedging and adaptive plasticity may be combined by organisms. Here we investigate bet hedging in a seed-heteromorphic species, that is, one that produces multiple morphologically different seeds (diaspores) simultaneously. We collected specimens across an aridity gradient in the southwestern United States to test whether the ratios of the different seeds produced follow expectations from bet-hedging theory. Furthermore, we germinated and raised a subset of the field collected offspring in a common-garden environment to examine germination differences between populations as well as plasticity associated with the ratio of basal to aerial seeds produced. Across the aridity gradient we found that, in agreement with bet hedging theory, the ratio of basal to aerial seeds significantly increases with increasing aridity. However, when we grew F1 individuals from six extreme populations in a common-garden environment, the seed ratio pattern was reversed. In addition, the observed reversal in seed ratios was greatest for the most variable populations suggesting greater plasticity in these populations. An allometric analysis of this data shows that larger plants produce relatively fewer basal seeds. In addition, plants in more arid, variable habitats tend to be smaller, and this size difference operating through the allometry contributes greatly to the among population seed ratio pattern observed in the field. In a non-water limited greenhouse environment all plants grew much larger and plants from more variable sites responded more strongly, growing bigger than their counterparts from 68 less variable sites. Regarding germination patterns we found that germination time in the growth chamber was fastest for plants from more variable rainfall sites, though their germination fractions were lowest. These results suggest that plants from more variable sites may have the lowest field germination rates, but this may be masked in germination trials with ample water because they are much more responsive to water availability. Overall, we provide strong evidence for bet hedging exhibited by population differentiation in phenotypic expression.
