Seed dynamics of resprouting shrubs in grassy woodlands: Seed rain, predators and seed loss constrain recruitment potential

Abstract

Measuring the fate of seeds between seed production and seedling establishment is critical in understanding mechanisms of recruitment limitation of plants. We examined seed fates to better understand the recruitment dynamics of four resprouting shrubs from two families (Fabaceae and Epacridaceae) in temperate grassy woodlands. We tested whether: (1) pre-dispersal seed predation affected seed rain; (ii) post-dispersal seed predation limited seed bank accumulation; (iii) the size of the seed bank was related to seed size; and (iv) viable seeds accumulated in the soil after seed rain. There was a distinct difference in seed production per plant between plant families with the legumes producing significantly more seeds per individual than the epacrids. Seed viability ranged from 43% to 81% and all viable had seed or fruit coat dormancy broken by heat or scarification. Pre-dispersal predation by Lepidopteran larvae removed a large proportion of seed from the legume seed rain but not the epacrids. Four species of ants ('Notoncus ectomoides', 'Pheidole' sp., 'Rhytidoponera tasmaniensis' and 'Iridomyrmex purpureus' were major post-dispersal seed removers. Overall, a greater percentage of 'Hardenbergia' (38%) and 'Pultenaea' (59%) seeds were removed than the fleshy fruits of 'Lissanthe' (14%) or 'Melichrus' (0%). Seed bank sizes were small (<15 seeds⁻²) relative to the seed rain and no significant accumulation of seed in the soil was detected. Lack of accumulation was attributed to seed predation as seed decay was considered unlikely and no seed germination was observed in our study sites. Our study suggests that seed predation is a key factor contributing to seed-limited recruitment in grassy woodland shrubs by reducing the number of seeds stored in the soil.