Tree and Shrub Seedling Colonization of Old Fields in Central New York

Abstract

The main objective of this study was to determine the important mechanisms regulating woody plant establishment in recently abandoned agricultural fields in central New York. The study focused on the early life stages, from the dispersal of seeds to the fate of seedlings at the end of the second growing season, of common old—field invaders, primarily Acer rubrum, Pinus strobus, Cornus racemosa, and Rhamnus cathartica. We investigated patterns of survivorship and causes of mortality during each of these life stages in microsites dominated by previously established herbs and in experimental openings in the herb canopy. Establishment of a tree or shrub seedling was much less likely from seeds landing among herbs than in openings. During the fall, seed predation by mice, especially under the cover of herbs (median of averages across all species = 6% in open, 85% under herbs), was an important process regulating the relative abundance and spatial distribution of seeds after dispersal. Seedling emergence rates were generally quite low, but for most species tended to be greater in the presence of perennial herbs (12%) and were lower under annuals and biennials (7%) and in bare soil (6%). For cohorts of seedlings followed for two growing seasons (Experiment 6A), some mortality occurred due to frost heaving (17% in open, 4% under herbs) and apparent heat stress or desiccation during the summer (5% in open, 0% under herbs); however, the largest source of mortality, especially under the herb canopy, was predation by rodents, especially meadow voles (6% in the open, 62% under herbs). Competition with herbs, especially aboveground competition, significantly reduced growth of woody seedlings. In spite of growth reduction in the presence of herbs, there was no significant relationship between seedling size and survivorship within any treatment, indicating that mortality could not be directly attributed to competition. For different species, and at different life stages, support was found for all three of the succession models of Connell and Slatyer (1977). The tolerance model was supported in some cases of seed predation (Experiment 2), seedling emergence (Experiments 4 and 5, in part), and seedling predation (Experiment 7). The facilitation model was supported in some cases by increased seedling emergence (Experiments 4 and 5, in part) and increased seedling survivorship in the presence of herbs during periods of environmental stress (Experiments 6A and 8). The inhibition model was also supported in some cases of decreased growth (Experiment 6B) and decreased survival of seeds and seedlings (Experiments 1, 3, 6A, and 8) in the presence of herbs. The lower survivorship under herbs was largely the result of seed and seedling predation, which were the two most important processes regulating tree and shrub colonization of old fields in this study. Reduced growth of later colonists due to competition with earlier colonists is considered direct inhibition, while increased mortality due to predation on later colonists caused by animals whose activity is necessarily associated with the presence of earlier colonists is considered indirect inhibition. Seed and seedling predation are potentially important mechanisms regulating succession and should be explicitly included in studies of succession.