The Effect of Shrubs on the Establishment of an Endangered Perennial (Asclepias curtissii) Endemic to Florida Scrub

Asclepias curtissii (Curtiss' Milkweed) is an endangered perennial herbaceous plant endemic to Florida scrub habitat. Although many scrub perennials are gap specialists, Curtiss' Milkweed is often found growing in close association with woody vegetation. We asked whether seed germination and seedling establishment are enhanced by the microsite conditions created beneath woody shrubs. In addition, we asked whether adult plants occur in association with shrubs more frequently than would be expected by chance and whether this distribution could be explained by seed dispersal patterns. Seeds were germinated, ex situ, in a factorial experiment with leaf litter and shade as main effects. In a separate experiment, to determine the effect of shrub cover on seedling establishment, 144 Curtiss' Milkweed seedlings were planted into a total of twelve fenced plots within Lyonia Preserve, Deltona, FL. Within each plot, six seedlings were planted in sandy gaps, and six were planted in close association with existing woody shrubs. We assessed survival over a five-month period. We also measured the distance to the nearest woody stem from extant adult plant locations and compared these to the distance of random points around those extant plants. Similarly, we released 70 milkweed seeds from extant plant locations and measured the distance from the spot where they landed to the nearest woody stem and compared this to the distance between random points and woody stems. We found that seed germination was significantly enhanced by shade (P < 0.0001) but not by leaf litter, and that seedlings growing in the shade of close neighboring shrubs had significantly higher rates of survival (P < 0.001) than those seedlings planted in gaps. Extant plants tended to grow close to shrubs, and seeds tended to land near shrubs, but neither of these distances were less than would be expected by random chance (P > 0.10 in both cases). The facilitation of seedling establishment by woody plants has been documented in other arid environments, but not in Florida scrub.