Shrubland longevity and rare plant species in the northeastern United States

Thickets in the northeastern United States typically consist of opportunistic, early successional plants, a category that includes few if any rare species. Nonetheless, some of the region's globally rare plants and many regionally rare, disjunct and edge-of-range species are shrubs or herbaceous plants with high fidelity to shrublands. Some shrub-dominated ecosystems harbor clusters of rare plant taxa. Shrubland longevity seems to be the key. Because shrublands tend to be isolated and small in the Northeast and dispersal of rare plants is further limited by the sparseness of propagule-exporting populations, multiple rare species are likely to accumulate and persist only in shrublands that are stable over long time periods. Zones at the two extremes of the soil moisture gradient, at the highest elevations, and exposed to marine salt spray often are occupied by relatively stable shrublands that include rare plant populations. In a few cases, shrublands also have persisted on mesic, fertile soils for long enough to accumulate clusters of rare plant species. High rates of human disturbance might provide an explanation, but it is unlikely that such disturbance regimes would have been sustained uninterrupted over centuries and across the transition from Native American to European land-use practices. Alternatively, mesic shrublands could persist with only moderate rates of disturbance and through extended disturbance-free periods if the plants themselves are slowing succession or changing its trajectory. Evidence is reviewed that some shrubs may alter the environment and ecosystem processes in ways that favor their own ability to persist and reproduce while making a site less hospitable to the establishment of forest trees. Plants ordinarily thought of as early successional species do the opposite, changing the environment in ways that ultimately hinder their own ability to persist by incidentally making the site more hospitable to species that outcompete them. Hypothesized processes that sustain northeastern shrublands on mesic soils involve fire, frost pockets, soil hardpans, soil acidification, nitrogen sequestration, and allelopathy. They can account for shrubland persistence if dominant shrub species (in some cases with a boost from human activity) foster one or more of these processes to the greater detriment of their potential competitors than to themselves. Subordinate shrubland species, including rare plants, do not need to be active in fostering such processes to benefit from them. (C) 2003 Elsevier B.V. All rights reserved.