Linking demographic processes with ecological gradients to inform restoration of edaphic endemics

Background/Question/Methods

Reintroduction has emerged as an important conservation tool for the recovery of endangered plant species. Yet, many reintroductions are unsuccessful due to a poor understanding of how biotic and abiotic gradients interact to influence the target species population dynamics. One example of an endangered species that has proven challenging to reintroduce is Astragalus bibullatus, whose distribution is restricted to limestone cedar glade ecosystems in the Central Basin of Tennessee. Edaphic endemics may be especially challenging to reintroduce in the wild because of their specialized soil requirements, restricted habitat, and sensitivity to competition. To improve success rates in future reintroductions, we quantified the demographic structure and ecological variation within and among natural populations over multiple years to relate how ecological gradients might influence the demographic vital rates of this rare edaphic endemic. We tested two hypotheses: 1) recruitment rates decline with increasing vegetation cover since many edaphic endemics compete poorly with surrounding vegetation and early life stages are expected to be more sensitive to biotic stress than later life stages, and 2) fitness is suppressed in shaded microenvironments since most edaphic endemics exhibit a high light requirement for maximum growth and reproduction.

Results/Conclusions

Astragalus bibullatus patches were distributed across ecological gradients that varied from rocky areas with sparse vegetation cover, to densely vegetated areas dominated by forbs, and finally to a locally abrupt zone between the open glade and forest edge that is dominated primarily by moss. Recruitment and reproductive niches varied along these ecological gradients. Seedling recruitment significantly decreased as forb cover increased but was unrelated to moss cover or light availability. Although seeds of A. bibullatus are gravity dispersed and long-lived in the soil seed bank, past fruit production in plots was a poor predictor of seedling recruitment. In contrast, fitness measures were unrelated to forb cover but exhibited a weak negative relationship with moss cover. Light availability (photosynthetic photon flux) was the most important predictor of fitness. Of the eight soil predictors, only nitrate exhibited a significant relationship with recruitment densities and fitness. In conclusion, successful reintroduction of Astragalus bibullatus will require glade sites with spatially heterogeneous microenvironments to support contrasting recruitment and reproductive niches.