Life history strategy and extinction risk in the warm desert perennial spring ephemeral Astragalus holmgreniorum (Fabaceae)

This study of Astragalus holmgreniorum examines its adaptations to the warm desert environment and whether these adaptations will enable it to persist. Its spring ephemeral hemicryptophyte life history strategy is unusual in warm deserts. We used data from a 22-year demographic study supplemented with reproductive output, seed bank and germinant survival studies to examine the population dynamics of this species using discrete-time stochastic matrix modeling. The model showed that A. holmgreniorum is likely to persist in the warm desert in spite of high dormant-season mortality. It relies on a stochastically varying environment with high inter-annual variation in precipitation for persistence, but without a long-lived seed bank, environmental stochasticity confers no advantage. Episodic high reproductive output and frequent seedling recruitment along with a persistent seed bank are adaptations that facilitate its survival. These adaptations place its life history strategy further along the spectrum from ‘slower’ to ‘faster’ relative to other perennial spring ephemerals. Extinction risk for small populations is relatively high even though mean λs >1 because of high variance in year quality. This risk is also strongly dependent on seed bank starting values, creating a moving window of extinction risk that varies with population size through time. Astragalus holmgreniorum life history strategy combines the perennial spring ephemeral life form with features more characteristic of desert annuals. These adaptations permit persistence in the warm desert environment. A promising conclusion is that new populations of this endangered species can likely be established through direct seeding.

Methods
This is a long-term demographic data set (22 years) for a federally endangered plant. It was collected via yearly visits to the field study plot, where marked plants were monitored once in late spring for survival, size, and flower number and new recruits were marked and measured. These raw demographic data were processed along with supplementary data on seed bank persistence and reproductive output per flower as described in thepublished paper to arrive at 39 vital rates to be used to parameterize the resulting population matrix model. This data set includes yearly values over the 22 years of the study for the 19 vital rates in the model that are variable, to show how the means and variances for these variable vital rates were calculated. These means and variances along with values for the invariant vital rates are included in the published paper.