Duplicating the Mound-and-Pool Microtopography of Forested Wetlands

Abstract

of wetland forests. A common characteristic of forested wetlands in non-floodplain areas throughout the northeastern United States is their so-called "mound-and-pool" microtopography, also referred to as "pitand-mound" or "hummock-and-hollow" (Golet et al., 1993). Typically, these wetlands are dominated by red maples (Acer rubrum) which grow on the mounds, thus avoiding the anaerobic soil conditions associated with prolonged flooding or saturation. The precise origin of such microrelief is open to question, although the most likely explanation appears to be windthrows (Golet et al., 1993). Tree species growing in wetlands are typically shallow-rooted and easily toppled by wind. This results in a raised mass of root and soil, which, as the woody material decays, setties into a mound, creating a relatively dry site for seedling establishment. The growth of the root systems of the young trees, and the accumulation of trapped soil and leaf litter probably also contributes to the initial formation of a mound. Regardless of its origin, the mound-and-pool microrelief increases the probability that some individuals of woody species will survive seasonal or even multi-year periods of high water. The mounds also increase the diversity of the plant community, since they provide establishment sites with a relatively wide range of flooding frequencies and moisture conditions (see Golet et al., 1993 for a li{erature review). With the current high level of interest in wetland restoration, enhancement, and creation, it is surprising that more attention has not been given in the literature to duplicating the mound-and-pool microrelief. This is especially true considering the regulatory emphasis on in-kind replacement of wooded wetlands in New England. Mounds are an important feature of these ecosystems, whether from an ecological or aesthetic point of view, and should be included in any attempt to re-create authentic versions of them. Equally important, by providing small-scale topographic relief, mounds increase the margin of tolerance for plants placed in fluctuating hydrological conditions. This increases the shortterm chances of success in a mitigation project, and also favors a natural pattern of species recruitment and community development as a project matures. Despite this, the only previous report of an attempt to replicate the mound-andpool microtopography in the Northeast appears to be a 1990 report by by D.J. Crispin and A.D. Randall. Their effort to create mound-and-pool topography on a 66.8-ha (165-acre) site in the Great Cedar Swamp in southeastern Massachusetts met with mixed success. In attempting to restore the site to its original con~tition, they encountered a number of problems, foremost of which was the miring of heavy construction equipment in the clay and muck soils. Other difficulties included low survival rates for woody species in rootmats transplanted from natural wetlands (called "strip mulching") and a highly altered site hydrology that necessitated construction of a 2,440-m (8,000-foot) clay dike and several dams to control groundwater and stream flows (Crispin and Randall, 1990; Valiton, 1995). This paper describes a design and method for constructing mounds and pools