Distributional Limits of Old-field and Woodland Annual Herbs: The Relative Importance of Seed Availability and Interference from Herbaceous Vegetation

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Abstract
Summer-flowering old-field and woodland annuals are not sympatric, even though they occur in adjacent habitats. This study examined the relative importance of seed availability and of interference by herbaceous plants in determining the local distribution of an old-field annual, Chenopodium album L., and of a woodland annual, Pilea pumila (L.) Gray. Seedling emergence, survival and the incidence of reproduction were monitored for each species in a 1-year-old field, a 3-year-old field and woodland. Chenopodium seedling emergence was significantly higher in the 3-year-old-field than in the other sites, whereas Pilea seedling emergence increased with increasing successional age of the site. Removal of herbaceous vegetation and litter had a positive effect on Chenopodium emergence in the 3-year-old field and the woods, and for Pilea in the 3-year-old field, but in the 1-year-old field Pilea had significantly lower emergence in cleared plots than in vegetated plots. Chenopodium had high survivorship, flowered and set seed only in the 1-year-old field, whereas Pilea seedlings survived to flowering and reproduced only in the woodland site. With or without herbaceous vegetation, seeds of each species germinated and seedlings were established, but they did not reach flowering size in each other''s habitat, regardless of time of emergence. Thus, lack of a seed source or interference from herbaceous vegetation alone are not responsible for restricting each of these annual species to its particular habitat, but apparent species'' differences in physiological tolerance over light and soil moisture gradients are partly responsible for their environmentally distinct distributions.