Pods as sails but not as boats: dispersal ecology of a habitat-restricted desert milkvetch

Premise: Adaptive seed dispersal mechanisms are fundamental to plant fitness, but dispersal advantage is scale-dependent. We tested the hypothesis that informed dispersal in response to an environmental cue enables dispersal by wind on a local scale for Astragalus holmgreniorum, a desert species restricted to swales and wash skirts with overland flow, but prevents longer-distance dispersal by water into unfavorable wash habitats.

Methods: Pod biomechanics in A. holmgreniorum lead to major shape modifications with changes in moisture content. We performed laboratory experiments to examine the interaction of pod shape with wind and water, and conducted field experiments in A. holmgreniorum habitat evaluating the roles of wind, water, and seed predators on dispersal.

Results: Dry pods exhibit a flattened crescent shape with partial dehiscence that facilitated wind dispersal by ground tumbling and seed scattering in laboratory experiments. Rain simulation experiments showed that even small precipitation events returned wetted pods to their cylindrical shape and opened the dorsal suture, exposing the seeds. In the field experiments, dry pods were moved locally by wind, whereas rain caused pod opening and washing out of seeds in place. Seed predators had minimal effect on pod movement.

Conclusions: Astragalus holmgreniorum exhibits pod structural remodeling in response to environmental change in a striking and novel demonstration of informed dispersal. Wind-driven movement of dry pods facilitates local seed dispersal, but rain causes pods to open and release seeds, ensuring that they are not transported out of suitable habitats and into active washes where they would be lost from the seed bank.