COS 49-4 - CANCELLED - Seed traits underlie tradeoffs in spatial and temporal dispersal across an aridity gradient

Background/Question/Methods

Plants are rooted in place, but they are able to move through time and space via seeds. Traits associated with this early life stage may be critical as they shape which species contribute to the next generation of plants, ultimately determining the trajectory of a community. Both spatial and temporal dispersal are essentially means of seeking out environments favorable to growth and reproduction, and although their tradeoff is often predicted theoretically, empirical support is mixed. Annual plants in arid systems only have one chance to reproduce, thus locating a favorable environment via dispersal is crucial to their persistence. In this study, we asked which seed traits are related to spatial and temporal dispersal strategies, whether these strategies trade off, and how the prevalence of these strategies in plant communities varies along an aridity gradient. To do this, we collected seeds from 299 annual species from grasslands of northern California to the Sonoran Desert in Arizona and measured a suite of 10 seed traits on each species. To then link these traits with dispersal strategy, we estimated the spatial and temporal dispersal probabilities of a subset of species in four long-term monitoring sites across our aridity gradient using hidden Markov models.

Results/Conclusions

Preliminary results indicate that the majority of variation in seed traits among our species can be reduced to two dimensions – the first explaining variation in mass and seed settling time and the second explaining variation in shape, dispersal vector, and appendages. Overall, we found a strong trade-off between temporal and spatial dispersal, but the strength of this tradeoff varied across sites. Species at the drier end of our gradient have higher temporal dispersal and lower spatial dispersal than those from the wetter end of our gradient, while those at the wetter end displayed more variable strategies. Furthermore, these strategies are linked to key seed traits: species that specialize on temporal dispersal have rounder seeds that lack appendages and are predominantly dispersed via ants, wind, or are unassisted whereas species that specialize on spatial dispersal have the opposite traits and are dispersed via large mammals, humans, or birds. High aridity may thus constrain seed dispersal strategies whereas strong competition in less arid systems may allow for a range of adaptive seed dispersal strategies. Further, this study provides empirical evidence for the theoretical tradeoff between spatial and temporal dispersal but suggests that the strength of this tradeoff may shift with aridity.