Seed Banks in Desert Annuals: Implications for Persistence and Coexistence in Variable Environments

Abstract

It is widely believed that desert annual plants maintain between—year seed banks, yet few field studies actually have measured the proportion of the viable seed bank that remains dormant through a season. Dormancy and germination fractions were quantified for a guild of winter annuals on a creosote flat in the Sonoran Desert for three years. Predictions from two types of theoretical models applicable to temporally variable environments were examined: (1) the evolution of life history traits promoting persistence in the face of temporal variation and (2) the role of temporal variation in mediating species coexistence. The density of ungerminated seeds was estimated by collecting soil samples after germination, but prior to new seed set. Seedlings were followed in nearby plots to estimate the density of germinated seedlings and their reproductive success. Long—term data collected from permanent plots over a 10—yr period were used to calculate temporal variation in reproductive success for each species. Species with higher temporal variation in reproductive success had lower germination fractions and smaller seeds, consistent with the theory that seed dormancy and large seed size are partially substitutable bet—hedging strategies. The data also suggested that this system possesses traits that are necessary for temporal variation to promote coexistence. First, between—year seed banks, necessary to buffer populations in unfavorable years, were documented for 17 species. Second, there was a strong tendency for year—to—year variation in germination fractions to vary among species. Finally, plants germinated more in years of higher reproductive success. We discuss how a correlation between germination and reproductive success enhances the role of temporal variance in success hierarchies in promoting species coexistence.

Coexistence and Relative Abundance in Annual Plant Assemblages: The Roles of Competition and Colonization

“…C, Density (number/m 2 averaged over 10 yr and two sites) of annuals in the Holkham dune system studied by Grubb et al (1982) and Rees et al (1996). D, Density (number/m 2 averaged over 3 yr) of winter annuals in an Arizona desert community (Pake and Venable 1996). E, Summer and (F) winter annual plant density (average number of individuals/m 2 over 18 yr) in the Arizona desert (Guo et al 2000; different sites than Pake and Venable 1996). seed mass shown in figure 1.…”

“…A second community pattern, which is more specific to these systems, is the negative correlation between seed size and abundance ( fig. 2; Grubb et al 1982;Maranon and Grubb 1993;Rees 1995;Pake and Venable 1996;Rees et al 1996;Guo et al 2000;Coomes et al 2002). The smallest-seeded, bestcolonizing species tend to be the most common, while the largest-seeded, competitive species are the least abundant.…”

“…D, Density (number/m 2 averaged over 3 yr) of winter annuals in an Arizona desert community (Pake and Venable 1996). E, Summer and (F) winter annual plant density (average number of individuals/m 2 over 18 yr) in the Arizona desert (Guo et al 2000; different sites than Pake and Venable 1996). seed mass shown in figure 1. The per capita rate of increase or colonization rate (l) in the absence of competition is given by…”

Quantifying and testing coexistence mechanisms arising from recruitment fluctuations

“…Indeed, when variable recruitment occurs, there is often some relatively persistent stage in the life cycle that has the effect of buffering recruitment variation. For example, in annual plants, a persistent seedbank often accompanies highly variable recruitment of seeds into it (Pake and Venable, 1996). In trees, both seedlings and adult trees may show high levels of persistence.…”

Climate variability has a stabilizing effect on the coexistence of prairie grasses

“…In large part, this uncertainty reflects the scarcity of long-term datasets suitable for testing the prerequisites of the storage effect. Existing empirical studies typically document species-specific responses to the environment for taxa with long-lived life stages, showing the potential for the storage effect to operate (12)(13)(14)(15)(16). However, to take the next step and prove that the storage effect is actually helping to stabilize coexistence, evidence for the first two conditions must be combined with the critical third condition: more severe competition in favorable years.…”