Intraspecific Variation Along an Elevational Gradient Alters Seed Scarification Responses in the Polymorphic Tree Species Acacia koa

Physical dormancy in seeds can challenge restoration where scarification conditions for optimal germination and seedling vigor are unknown. For plant species that occur along wide environmental gradients, optimal scarification conditions may also differ by seed source. We examined intraspecific variation in optimal scarification conditions for germination and seedling performance in koa (Acacia koa), which occurs across a wide range of environmental conditions. To evaluate scarification responses, we recorded imbibition percentage, germination percentage, germination time, seedling abnormalities, early mortality, seedling growth, and seedling survivorship, and developed a scarification index (SI), which accounts for these processes simultaneously. We hypothesized that seeds from lower elevation sources in wetter environments would have harder seed coats and would require more rigorous scarification treatments. To test this hypothesis, we repeatedly exposed seeds to hot water differing in temperature and time until seeds imbibed. Supporting the hypothesis, seeds from lower elevation sources generally required more rigorous scarification, although substantial variation existed among sources. Koa seeds germinated in about a week following imbibition. Boiling seeds (i.e., maintaining at 100°C) was effective for imbibing seeds but also reduced germination percentages substantially. Repeated exposure to 90°C to 100°C water did not reduce germination percentage but decreased seedling performance and increased early mortality. No seeds remaining unimbibed after six attempts of boiling germinated whereas seeds remaining unimbibed after 15 attempts of exposure to 90°C to 100°C water showed high germination percentages. Abnormalities in seedling development were rare but increased with treatment intensity. Exposure to 100°C water for 1 minute overall generated the best SI values but the best treatment differed by elevation, and the treatment with the best SI was rarely predicted from highest germination percentages. Seeds that imbibed without any treatment germinated at the same level as manually filed seeds but produced poor seedling quality. Variation in mother tree environments along an elevational gradient can lead to differences in seed coat characteristics, which may explain the differing responses to treatments. Scarification treatments affected processes beyond imbibition and germination, and using an index like SI will improve efficiency while reducing seed waste by helping to identify optimal scarification treatments.