Establishment of Lithospermum caroliniense on Sand Dunes: The Role of Nutlet Mass

The adaptive significance of variation in nutlet mass in relation to emergence and survival was investigated experimentally in Lithospermum caroliniense (Boraginaceae), a herbaceous sand dune species. In a wetter than average year, moisture availability in the field was varied by supplemental watering. Nutlet mass was positively correlated with emergence and seedling size, but not survival. Supplemental watering had little effect on emergence or survival because of the heavy and evenly spaced rains during the experiment. Size hierarchies established during the 1st yr of growth persisted for the next 2 yr. Survival to the 2nd yr was greatest for seedlings derived from the larger nutlets. In a drier than average, depth of nutlet burial was varied in addition to supplemental watering and nutless mass. Deeply buried nutlets emerged later than shallowly buried nutlets, and emergence was significantly greater in the plots receiving supplemental water. There was no obvious relationship between nutlet mass and emergence when nutlets were buried at shallow depths, but for deeply buried nutlets, larger nutlets were more likely to emerge. Nutlet mass had no effect on survival of seedlings, although survival rates averaged 92.2% in the watered subplots, compared to 43.4% in the unwatered subplots. No interaction occurred between the watering treatment and nutlet mass. In the second experiment, propagule mass had a significant effect on seedling size during the 1st yr, and larger seedlings had higher overwinter survival. By the 2nd yr, size differences related to initial nutlet mass had disappeared. In both experiments, height and leaf number were strongly correlated from year to year. Block effects were pronounced in both experiments due to differences in soil moisture content among the plots. Regardless of nutlet mass, growth was greatest in the driest plots. In each experiment, some flowering occurred in the drier plots by the 3rd yr of growth. Results demonstrated the proximal adaptive significance of nutlet mass in L. caroliniense. Identifying the factors influencing evolution of nutlet mass is more difficult, but there probably has been strong selection for seedlings derived from large nutlets. In addition, the greater ability of large nutlets to emerge when deeply buried has probably been important, since L. caroliniense is most prominent on rapidly accreting sand dunes.