Invasion ecology has traditionally focused on exotic plant species with early successional life‐history traits, adapted to colonize areas following disturbance. However, the ecological importance of these traits may be overstated, in part because most invasive plants originate from intentional introductions. Furthermore, this focus neglects the types of plants most likely to invade established communities, particularly forests – namely shade‐tolerant, late‐successional species. In invasion ecology, it is generally assumed that undisturbed forests are highly resistant to plant invasions. Our review reveals that this assumption is not justified: in temperate and tropical regions around the world, at least 139 exotic plant species are known to have invaded deeply shaded forest understories that have not undergone substantial disturbance. These exotics present a particular management challenge, as they often increase in abundance during succession. While forest invasions may develop comparatively slowly under natural disturbance regimes, anthropogenic processes, including the spread of exotic pests and pathogens, can be expected to accelerate the rate of invasion.

The design of the life cycle of pin cherry (Prunus pensylvanica L.), a successional species common on disturbed sites throughout much of the northern hardwood and boreal forest ecosystems, assures that its occurrence is integrated into the pattern of disturbance in the climax ecosystem. The combination of buried seed strategy and the mobility offered through avian consumption of fruits ensures reasonably large populations of buried, viable seeds in the soils of forests well after the disappearance of pin cherry from a particular site. Soil sampling data indicate that sufficient numbers of viable pin cherry seeds reside in the soils of second—growth forests in central New Hampshire to account for the dense stands frequently observed after cutting or burning. Further, germination of these buried seeds is apparently triggered by some factor(s) associated with formation of a large gap. By age 25 or 30, when pin cherry individuals are dying rapidly, sufficient numbers of seeds have been produced and disseminated in a dormant condition for the cycle to renew itself with the subsequent occurrence of major disturbance. In high density stands pin cherry grows rapidly, with early attainment of canopy closure (high leaf area index), and rapid attainment of high values of net annual production and nutrient accumulation.

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 animal...

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Theoretical models of plant range expansion require the assumption of occasional long‐distance seed‐dispersal events to explain post‐glacial migration rates. For the many forest herbs whose seeds are dispersed primarily by ants, there are few documented mechanisms of occasional long‐distance dispersal, so models of forest‐herb migration have been largely phenomenological. Here we show that viable seeds of Trillium grandiflorum, an ant‐dispersed forest herb in eastern North America, are dispersed via ingestion and defecation by white‐tailed deer. We also use data from the literature on movement patterns and gut retention times to model a deer‐generated seed shadow, showing that most seeds dispersed by deer should travel at least several hundred meters from parent plants, and occasionally >3 km. Our results provide a mechanism of long‐distance dispersal that has likely contributed to rates of post‐glacial migration and post‐agricultural forest colonization. Corresponding Editor: M. L. Cain

Temperate deciduous forests across much of Europe and eastern North America reflect legacies of past land use, particularly in the diversity and composition of plant communities. Intense disturbances, such as clearing forests for agriculture, may cause persistent environmental changes that continue to shape vegetation patterns as landscapes recover. We assessed the long-term consequences of agriculture for environmental conditions in central New York forests, including tree community structure and composition, soil physical and chemical properties, and light availability. To isolate the effects of agriculture, we compared 20 adjacent pairs of forests that were never cleared for agriculture (primary forests) and forests that established 85-100 years ago on plowed fields (secondary forests). Tree communities in primary and secondary forests had similar stem density, though secondary forests had 14% greater basal area. Species composition differed dramatically between the two forest types, with primary forests dominated by Acer saccharum and Fagus grandifolia and secondary forests by Acer rubrum and Pinus strobus. Primary and secondary forests showed no consistent differences in soil physical properties or in the principal gradient of soil fertility associated with soil pH. Within stands, however, soil water content and pH were more variable in primary forests. Secondary forest soils had 15% less organic matter, 16% less total carbon, and 29% less extractable phosphorus in the top 10 cm than adjacent primary stands, though the ranges of the forest types mostly overlapped. Understory light availability in primary and secondary forests was similar. These results suggest that, within 100 years, post-agricultural stands have recovered conditions comparable to less disturbed forests in many attributes, including tree size and number, soil physical properties, soil chemical properties associated with pH, and understory light availability. The principal legacies of agriculture that remain in these forests are the reduced levels of soil organic matter, carbon, and phosphorus; the spatial homogenization of soil properties; and the altered species composition of the vegetation.

Tree and shrub species composition and environmental characteristics were measured in 56 stands representing the range of natural vegetation types of the lower coastal plain of southeast Texas, and vegetation characteristics were related to the physical environment. Most of the stand-tostand variation in species composition was expressed by the first axis of a reciprocal averaging ordination, and stand position along the first axis in turn was most highly correlated with percent sand in the surface soil (0-15 em) (r = -.81, P < .05). The first ordination axis is interpreted as a soil moisture gradient. A more weakly expressed second axis of variation is interpreted as a soil aerationparent material gradient (from high organic matter, low extractable Ca stands to low organic matter, high Ca stands).Overs tory basal area, woody plant species richness of overs tory and understory, and relative basal area of broad-leaved evergreens in both strata all varied in a regular way along the first ordination axis, having low values at the dry end, peaking in the wet-mesic range, and declining somewhat toward the wet end.Comparison of vegetation types in southeast Texas with those for the whole of the southeastern United States coastal plain showed widespread congruence of types. Major discrepancies were the lack of a pine flatwoods type in Texas and the presence in Texas of a flatland hardwood type and a pine-oak type more characteristic of the Piedmont further east.The question of fire, succession, and climax on the coastal plain is considered and it is suggested that the timing of different studies in the last 50 yr may have influenced results and interpretation.