Tumbleweed hedges its bets

While most of the more than 250,000 species of angiosperms produce a single morphological type of fruit/seed as the dispersal and germination unit, a few hundred are known to produce two or more fruit/seed morphs. In these cases, the term fruit (seed) heteromorphism is used to describe the morphologically and physiologically distinct types of dispersal units (i.e. seed, fruit or fruit plus accessory parts) on a single individual plant. Relatively little is known about trade-offs between seed dispersal and dormancy for these species. A recent paper in Annals of Botany looks at this relationship.

Ceratocarpus arenarius grows in a diversity of habitats, including sandy deserts, saline–alkaline deserts, gravel deserts and desert grassland. This species is one of the dominant and important components in desert plant communities of this area, and it is ecologically significant in maintaining plant diversity and stability of the cold desert ecosystem. The mature fruit of this species is the dispersal and germination unit. Plants of C. arenarius reach 5–30 cm in height, depending on the conditions in which they are growing. An abscission zone develops at the second node above the soil surface, and the stem is broken at this zone by wind after the plant is mature. The spherical shape of the plant makes it easy for the individual to be dispersed (rolled or tumbled) by wind far away from the site at which it matured – C. arenarius is a typical ‘tumbleweed’.

Theory predicts that natural selection should produce a negative correlation between dispersal and dormancy. So is there?

Lu, J.J., Tan, D.Y., Baskin, J.M., & Baskin, C.C. (2013) Trade-offs between seed dispersal and dormancy in an amphi-basicarpic cold desert annual. Annals of Botany, 112(9), 1815-1827.
Background and Aims: Several studies have demonstrated trade-offs between depth of seed dormancy and dispersal ability for diaspore-dimorphic species. However, relatively little is known about trade-offs between these two life history traits for a species that produces more than two diaspore morphs. The aim of this study was to investigate the relationship between seed dormancy and dispersal in Ceratocarpus arenarius, an amphi-basicarpic cold desert annual that produces a continuum of dispersal unit morphs.
Methods: A comparison was made of dispersal and dormancy breaking/germination responses of dispersal units from ground level (a), the middle of the plant canopy (c) and the top of the plant canopy (f). Various features of the morphology and mass of dispersal units and fruits (utricles) were measured. The role of bracteoles in diaspore dispersal by wind, settlement onto the soil surface and dormancy/germination was determined by comparing responses of intact dispersal units and fruits. Movement of dispersal units by wind and animals, seed after-ripening, germination phenology and the presence of water-soluble germination inhibitors in bracteoles were tested using standard procedures.
Key Results: Dispersal units a, c and f differed in morphology and mass; in the majority of cases, extremes were exhibited by a and f, with c being intermediate. Overall, relative dispersal ability was f > c > a, whereas relative intensity of dormancy was a > c > f. Bracteoles increased dispersal distance by wind, enhanced settlement of diaspores onto the soil surface and mechanically inhibited germination.
Conclusions: The results provide evidence for a model in which there is a continuous inverse-linear relationship between diaspore dispersal ability and depth of dormancy. Thus, dispersal unit heteromorphism of C. arenarius results in a continuum, from no dispersal ability/high dormancy (dispersal unit a) to high dispersal ability/low dormancy (unit f), which may be a bet-hedging strategy in the cold desert environment.