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The role of fruits and fires in the germination of a rare subshrub, Amorpha georgiana (Fabaceae)1

Abstract. Seed dormancy is a trait that has evolved to maximize germination when environmental conditions are optimal for successful recruitment of new individuals into a population. Physical dormancy is one such trait common in legumes and release from this condition is often associated with disturbance events, such as fire. Changes to the specific conditions (e.g., climate, fire regimes) that release seeds from dormancy can have negative effects on populations, especially of rare species. Therefore, understanding the species-specific requirements needed to release seeds from dormancy and induce germination provides insight into ecological processes, effects of changing environmental conditions, and potential conservation actions. Amorpha georgiana (Fabaceae) is a rare subshrub with indehiscent fruits (pods) found in fire-maintained longleaf pine habitats in the southeastern United States. The germination ecology of this species is largely unknown. The goals of this study were to determine the presence of physically dormant seed in A. georgiana and then, through a series of laboratory-based experiments, resolve the fire related factors that effectively overcome dormancy and promote germination. The seeds of A. georgiana exhibit physical dormancy and show significantly higher germination after brief exposure to hot water (94 °C) compared to all other temperatures and treatments assessed. Regardless of treatment, seeds have low germination when retained in their pods, but the pod is not the cause of physical dormancy. We found enclosure within an indehiscent pod and exposure to pyrogenic temperatures (> 80 °C) both affect dormancy release and germination of A. georgiana. The 3.5-fold increase in germination at pyrogenic temperatures suggests the response is highly fire-adapted and fits the definition of obligate pyrogenic dormancy release. While season of fire exposure was not significant in our study, we did find consistent negative effects on the germination response. Our results suggest pulses of heat from periodic growing season fires are likely necessary to promote recruitment and maintain populations of this rare species.