PATTERNS AND CAUSES OF RESISTANCE TO TREE INVASION IN RIGHTS-OF-WAY

This study was designed to identify the critical life-history stages a nd ecological processes responsible for variation in rates of tree inv asion in utility rights-of-way in the Hudson Valley of New York. We us ed repeated censuses to estimate natural rates of tree seedling establ ishment, growth, and survival in the 12 major plant communities found along rights-of-way in the region. An individual-based model was used to integrate our field data and calculate measures of the rate of tree invasion in each plant community. The most common tree invaders were species with large, wind-dispersed seeds (i.e., Acer rubrum and Fraxin us americana) although these were rarely the most common tree species in the forests bordering the corridors. In narrow (approximate to 30-5 0 m wide) rights-of-way there was little relationship between seedling density and distance to the nearest seed sources; however, in wide co rridors, seed dispersal clearly limited tree invasion. Annual variatio n in seed production in adjacent forests was also a major factor in th e high spatial and temporal variability in tree seedling establishment . Growth rates of newly established seedlings did not vary significant ly among the different communities; however, seedling growth rates inc reased dramatically once the seedlings emerged above shrub or herb-dom inated canopy. Thus, the duration of competition (i.e., the number of years required by a seedling to emerge above the shrub or herbaceous c anopy) appears to have been more important than the initial intensity of competition in determining variation among communities in resistanc e to tree invasion. As a result, shrub communities generally had high resistance to invasion. Among herbaceous communities, the highest resi stance to invasion occurred in communities dominated by the grass Schi zachyrium scoparium (little bluestem) on poor soils.