Long‐term processes affecting restoration and viability of the federal threatened Mead's milkweed (Asclepias meadii)

Abstract

Restoration is an important tool for reducing extinction risk of endangered plants. Population viabilities of few plant restorations have been modeled over decadal time periods and linked with genetic and ecological factors that drive restoration processes. We modeled viability of restored populations of Mead's milkweed (Asclepias meadii, Asclepiadaceae), a self‐incompatible perennial herb of eastern tallgrass prairie (TGP), federally listed as threatened in the U.S. From 1994 to 2004, we planted >600 seeds and >800 juvenile plants representing >50 genotypes across seven TGP sites. Propagule type, genotype, seed source, restoration site, precipitation and fire management significantly affected establishment, growth and viability. Plants established from seed had greater mortality and greater genetic and demographic attrition than did juveniles. Seedling growth rates also projected 20–30 yrs to reach flowering stage, and their survivorship provided a metric of site suitability for life cycle completion. Seed germination and juvenile plant size were greater in burned habitat, and juvenile size was also positively correlated with spring precipitation. Seed production required presence of multiple genotypes among flowering plants. Seedlings demonstrated a heterosis effect, with greater germination among seeds derived from inter‐population crosses. However, cumulative growth of planted juveniles as well as population growth (λ) on sub‐optimal habitat conditions tended to be lower for propagules derived from inter‐population crosses, demonstrating outbreeding depression. Although flowering occurred at multiple sites, positive population growth (λ > 1) occurred at only a single site, where increasing fire frequency decreased extinction probability. These results suggest that restoration of viable Mead's milkweed populations is possible in optimal habitat. However, restoration of this species is constrained by high demographic attrition and the long period (20 or more yrs) required to complete its life cycle. Crossing among populations to increase genetic diversity and compatible mating types may result in tradeoffs, with heterosis at early life history stages, but outbreeding depression expressed in older stages. Fire and precipitation are also critical interactive processes driving A. meadii growth and reproduction. They may be most effective when precipitation, a stochastic process, results in greater than average post‐burn rainfall. These constraints may have implications for restoration of other late‐successional plant species.

Mixing source populations increases genetic diversity of restored rare plant populations

“…Thus the decision about which type of germplasm to use may depend on the amount of seed available and the long‐term cost of using seeds versus plugs (Kaye & Cramer ; Guerrant & Kaye ). Transplants are often more reliable at supporting demographically successful reintroductions relative to seeds (Godefroid et al ; Bowles et al ), although this is not always the case (Albrecht & Maschinski ; van Katwijk et al ). In a retrospective analysis of over 40 instances using outplanted plugs at four sites in Washington, only one had succeeded in establishing a new subpopulation that persisted and grew to exceed the number of plugs originally outplanted (Dunwiddie, unpublished data).…”

Effects of life history and reproduction on recruitment time lags in reintroductions of rare plants

“…High recruitment rates in these subtropical woodies might, in part, reflect their generally greater rates of iteroparity and net reproduction relative to temperate herbs, especially geophytes (Salguero-Gómez 2016; Salguero-Gómez et al 2016), which represented a majority of the long-lived herbaceous species in our data set and exhibited much longer RTLs than short-lived woodies. In fact, we observed the longest RTLs in long-lived succulents from arid habitats and perennial herbs of dunes and grasslands, such as Asclepias meadii, a rare obligate-outcrossing herb that has yet to recruit offspring nearly 2 decades after reaching reproductive maturity (Bowles et al 2015). In grassland plants, in particular, trade-offs between delayed reproduction and survival can be correlated with lower lifetime fecundity , which is consistent with the lower recruitment rates we observed in grassland reintroductions compared with other habitats.…”

“…In fact, we observed the longest RTLs in long‐lived succulents from arid habitats and perennial herbs of dunes and grasslands, such as Asclepias meadii , a rare obligate‐outcrossing herb that has yet to recruit offspring nearly 2 decades after reaching reproductive maturity (Bowles et al. ). In grassland plants, in particular, trade‐offs between delayed reproduction and survival can be correlated with lower lifetime fecundity (Burns et al.…”

Adapting population models for application in pesticide risk assessment: A case study with Mead's milkweed

“…Life‐history graph of Asclepias meadii using the stage classifications from Bowles et al (). Nonflowering plants may be immature plants or mature plants not flowering in a given year.…”

“…Plants can produce several stems from their tubers and spread within a limited radius around the location of stems in the previous year. However, recruitment through vegetative reproduction either is very limited or does not occur (Bowles et al 1998(Bowles et al , 2015.…”