The role of soil seed banks and leaf litter in the regeneration of native and exotic tree species in urban forests.

摘要

The objectives of this research were to determine the effects of urbanization on seedling recruitment in forests. Seedling recruitment is a function of availability of seeds and "safe sites" which provide the conditions needed for germination and establishment.; I determined seed availability in urban deciduous forests by conducting a soil seed bank study in two parks in New York City. I found little significant difference in the seed bank between soil depths, parks, or sampling years. The seed banks of these urban forests differed from those reported for non-urban forests in their higher density estimates and in the presence of exotic woody species.; Leaf litter, by modifying the forest floor microenvironment, can alter safe site availability. To determine the effect of urbanization on safe site availability, I surveyed leaf litter quantity and composition in ten forests along an urban-rural gradient originating in NYC and extending to northwestern Connecticut. Mean leaf litter depth, mass, and density increased significantly with increased forest distance from NYC. In these forests, seedlings of small-seeded species occurred more frequently in litter shallower than the mean forest litter depth. The litter depth in which seedlings of large-seeded species occurred did not differ from the surrounding forest.; I conducted lab experiments to test the effect of litter depth on germination and seedling growth of native and exotic tree species. Germination and seedling biomass of the small-seeded native species (Betula lenta and Liquidambar styraciflua) were reduced, while for the large-seeded native species (Quercus velutina and Quercus rubra) these were either unaffected or increased by increased litter depth. Germination of the intermediate-sized exotic species (Ailanthus altissima) was unaffected but seedling biomass was reduced by increased litter depth.; I conducted similar experiments on the effects of litter depth of urban and rural litter types. Germination of the two native species (Acer saccharum and Quercus rubra) was greater in rural litter than in urban litter while germination of the exotic species (Ailanthus altissima) was unaffected by litter type. Litter type had little effect on seedling biomass except for the large-seeded native species which had increased robustness in response to increased litter depth.