Geographically mediated differences in exposure to disturbance agents that operate at different scales impose demographic contrasts between plant populations. We examined such effects on the stand-scale population structure of the two varieties of Pinus clausa (sand pine). Pinus clausa var. immuginata stands of the Florida panhandle exhibited reverse-J diameter and age structures, with ongoing stem recruitment and relatively high seedling and sapling densities in canopy gaps. Seedling radial growth rates were relatively slow in P. c. var. immuginata stands, reflecting the partially closed character of the forest canopy. Radial growth-release events among trees were recorded in 35 -65% of trees in P. c. var. immuginata stands. Timing of releases matched exposure to hurricanes and extratropical storms in these coastal settings. Wind damage provides opportunities for recruitment and growth of stems, so that stands of P. c. var. immuginata do not require stand-destroying disturbances to persist in the landscape. Mature stands of Pinus clausa var. clausa on the Florida peninsula were broadly unimodal in diameter form and narrowly even-aged, with virtually all trees established within a decade following stand-initiating disturbances in the 1920s and 1930s; seedlings were absent. Regeneration of P. c. var. clausa in our study was limited to stands that have experienced crown fire since 1970. Seedlings in the recently burned P. c. var. clausa stands exhibited rapid growth rates. Growth-release events were uncommon in P. c. var. clausa stands, except for an Atlantic coastal population that experienced repeated exposure to hurricanes from 1947 -1950. Crown fires appear essential to long-term maintenance of naturally seeded populations of Pinus clausa var. clausa on most sites.

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Invasion by exotic plants often is restricted by processes, such as seed predation, acting on early life-history stages; however, the relative importance of these processes might vary among habitats. Modern human land use has created a mosaic of habitats in many landscapes, including the landscape of the northeastern United States. European buckthorn (Rhamnus cathartica) is an exotic plant that has achieved varying success in North American habitats. We studied dispersal, seed survival, germination, and seedling survival of buckthorn populations at four plots in each of sugar maple (Acer saccharum) forests, old fields, and abandoned conifer plantations in central New York State. Dispersal was low in maple forests, as evidenced by low collection rates of R. cathartica seeds in seed traps. Rates of post-dispersal seed predation were highest in maple forests and lowest in old fields, suggesting greater use of maple forests by granivorous rodents. Germination rates did not vary among seeds planted in soils of these habitats when studied in the laboratory despite differences in soil pH. Survival of transplanted seedlings was low in maple forests relative to old fields and plantations. Buckthorn invasion of old fields and abandoned plantations was not strongly constrained by factors we considered, and the buckthorn populations in these habitats were large. A combination of low dispersal by frugivores, low seed survival due to predation, and low seedling survival due to dim light conditions apparently prevents R. cathartica from invasion of intact maple forests of our area. Native fauna and canopy closure may act synergistically to reduce success of invasive plants in natural habitats.

Dispersal by frugivorous birds facilitates invasion by many exotic plants. We measured the seed rain of ornithochorous native and exotic plants at three habitats of a fragmented landscape of the northeastern United States for 1 year. We studied maple-beech forests, old fields, and abandoned conifer plantations. Across all sites we collected 2,196 ornithochorous seeds, including seeds from six exotic species and 10 native species. The majority (90%) of collected seeds were from exotic species. Seed dispersal was broadly similar among habitats, though seed rain of exotic species was higher in old fields than forested habitats. Seed rain was not strongly influenced by artificial perches for most species. However, seeds of exotic species were more commonly found in traps under an artificial perch in old fields. Seed rains for the exotic Elaeagnus umbellata, Rhamnus cathartica, and Rosa multiflora were positively associated with local density of mature plants. Seed rain of R. cathartica was positively associated with abundance of seedlings but not saplings, suggesting that post-dispersal mortality was important. Seed dispersal of the exotic Lonicera spp. was high in all habitats, accounting for 66% of all seeds collected. With the exception of Lonicera spp., seed rain of common exotic invaders was affected by the abundance of seed sources, and these species might be effectively controlled by elimination of local fruiting plants. Fruits of Lonicera morrowii, which has extensively invaded our area, are apparently a common component in the diet of frugivores.

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