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Floristics, zonation and succession of vascular vegetation in fifteen beaver-impacted wetlands in western Massachusetts

Robert Theron McMaster, University of Massachusetts Amherst


The recent population growth of the beaver (Castor canadensis L.) has profoundly influenced the modern western Massachusetts landscape. Trapped to regional extinction in the early 18th Century and reintroduced in the 1930s, beavers have increased rapidly, approaching carrying capacity in some parts of the state. Quantitative investigations of the vegetational dynamics in 15 beaver wetlands in Franklin and Hampshire Counties between 1980 and 1995 revealed five distinctive physiographic zones, each identified with a characteristic plant community. Zone 1, immediately upstream from the beaver dam, supports a community of aquatics such as Najas flexilis and Lemnu minor. Zone 2, immediately upstream from Zone 1, supports plants tolerant of anoxic soils including Leersia oryzoides and Eleocharis obtusa. Zone 3 is often dominated by tussock-forming graminoids such as Calamagrostis canadensis and Carex stricta. Zone 4 contains a mixture of herbs and shrubs, especially Alnus rugosa, Eupatorium maculatum, Spiraea latifolia and Thelypteris palustris. Zone 5 includes some Zone 4 species along with others such as Betula populifolia and Tsuga canadensis that are typical of the surrounding upland. Well-drained sites follow the classic path of hydrarch succession from floating-leaved aquatics to emergents to shrubs and trees. Frequently re-flooded sites often follow a similar progression for five to ten years, but then become dominated by tussock-forming graminoids. Shrubs and trees seldom invade the resulting hummock-and-hollow topography, succession slows or may even cease. Vascular plant diversity is affected by both hydrological disturbance and site age. Maximum diversity normally occurs in the central portion of an abandoned site where levels of hydrological disturbance are intermediate. Lower diversity levels occur close to the dam where hydrological disturbance is greatest or on the upstream margins of the site where hydrological disturbance is least. Maximum diversity occurs in sites that have been abandoned for 10 to 20 years whereas lower vascular plant diversity is found in younger and older sites. These relationships are consistent with Connell's Intermediate Disturbance Hypothesis (1978). Watersheds with beaver-impacted wetlands at different stages of development exhibit greater landscape diversity than watersheds without beavers; hence proper management of beaver populations may be an important aid in promoting biological diversity.

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Recommended Citation

McMaster, Robert Theron, "Floristics, zonation and succession of vascular vegetation in fifteen beaver-impacted wetlands in western Massachusetts" (1997). Doctoral Dissertations Available from Proquest. AAI9737562.