INSIGHTS INTO THE HABITAT AND BREEDING BIOLOGY OF THE FEDERALLY ENDANGERED HERBACEOUS PLANT, THALICTRUM COOLEYI, AHLES

Abstract

Thalictrum cooleyi Ahles (Cooley's meadowrue, Ranunculaceae) is a dioecious species, endemic to fire-dependent, wet pine savannas of the southeastern US. T. cooleyi was listed as federally endangered in 1989; knowledge of habitat and breeding biology are crucial to conservation. I studied NC populations of T. cooleyi and examined (1) woody species associates, (2) breeding system, (3) sex ratios, and (4) potential pollen limitation. I also assessed seed germination requirements and vegetative propagation potential for T. cooleyi. To document woody species associates of Thalictrum cooleyi, I sampled 104-3 m² quadrats and measured percent canopy closure, identified woody understory species, and estimated their percent cover. Logistic regression showed that T. cooleyi presence decreased as canopy closure increased. In contrast, T. cooleyi was more likely to occur as woody shrub cover increased. Canopy closure and woody understory cover were not associated with density, total number, and flowering to vegetative ratio of T. cooleyi plants. Only Morella cerifera L. of the woody understory was a significant predictor of T. cooleyi presence (Wald = 13.823, df = 1, P < 0.001). Deviations from strict dioecy are uncommon in Thalictrum cooleyi; only 5% of female plants surveyed had hermaphroditic flowers (n = 375). Sex ratios differed significantly from 1:1 and were male-biased. Despite male-bias, my supplementation experiments found evidence for pollen limitation to seed set. Thalictrum cooleyi seeds are dormant at maturity, requiring cold stratification at 1 °C to break dormancy. Germination rates were higher for seeds that received warm then cold stratification, compared to cold only (33 ± 0.67% vs. 26.7 ± 1.76%, respectively). Gibberellic acid can break dormancy without cold stratification. Light exposure during incubation is not a requirement for germination. Overall, germination rates were low ([equal to or less than] 33.3 ± 0.67%), likely due to low seed viability. Thalictrum cooleyi can be propagated vegetatively from caudex and rhizome divisions. My work supports concerns that habitat loss (i.e., transition of open pine savanna to dense forest), pollen limitation to seed set, and poor seed viability threaten the recovery of Thalictrum cooleyi. Conservation efforts will benefit from this new knowledge on seed germination and vegetative propagation protocols for T. cooleyi.