EVALUATION OF FORESTED WETLANDS CONSTRUCTED FOR MITIGATION IN COMPARISON TO NATURAL SYSTEMS
USGS Patuxent Wildlife Research Center
11410 American Holly Drive
Laurel, MD 20708, USA
Intensive research on six constructed forested wetlands in Central Maryland was conducted in 1993-1996 to determine success of these habitats as functional forested wetlands, especially as suitable wildlife habitat. These sites were constructed as mitigation to offset losses of natural forested wetlands from highway construction and other human activities. Areas studied ranged in size from 2 to 35 acres and were constructed by private companies under contract with three mitigation agencies (Maryland State Highway Administration, Baltimore Gas and Electric Company, and Prince George's County Government). Adjacent natural forested wetlands were used as reference sites where similar data were collected.
All sites were gridded into 25x25 meter sections. Within each section one-meter square quadrats were randomly selected and permanently marked with stakes to facilitate annual vegetation sampling. All plants within the quadrats were identified in August and percent ground cover estimated. Nursery-stock trees and shrubs, which were transplanted to constructed forested wetlands by contractors, were individually marked by researchers. Mortality of transplants was checked in the spring of each year and growth (height, diameter, canopy) measured in the fall. Wildlife use of the sites and adjacent reference areas were monitored every two weeks for birds.
Photo Plate 1: All trees and shrubs were tagged so they could be identified in subsequent years.
Photo Plate 2: Measurements of all woody transplants including height, canopy, and diameter.
Photo Plate 3: Meadow
voles were the most commonly trapped mammal and did extensive
damage to trees
Based on data from the first four years of this study it appears that it will take 35-50 years before these areas have forested wetland vegetation and wildlife similar to that found on mature forested wetlands. This long-time period is based on the high mortality and slow growth of nursery-stock trees and shrubs transplanted on the areas.
Figure 1: Mortality of shrubs, like trees, was high and in general was related to excessive surface water.
Figure 2: Mortality of trees after three years averaged 55 percent, with green ash having lowest mortality.
Mortality and slow growth resulted mostly from excessive surface water on the sites, although damage to woody transplants by wildlife (especially trunk girdling) was also a problem. The level of ground water did not appear to be a factor in regard to transplant mortality. Green ash (Fraxinus pennsylvanica) was the woody transplant species that had the least mortality. Sampling of vegetative ground cover with one-meter square quadrats showed the predominance of grasses and herbs.
Figure 3: Grass was the dominant plant cover at all sites. Woody vegetation comprised less than five percent.
Numerous mammals and amphibians were caught on both constructed and reference sites and there was range overlap of most species for the two sites. Wood frogs (Rana sylvatica) and salamanders were uncommon or absent on the constructed sites, and probably represent the best wildlife species to evaluate the maturation of a forested wetland from a wildlife perspective.
Figure 4: Average Well Measurements for March to May 1995 at BGE (10 Wells). Computer generated
contour maps of well data revealed that large sections of some sites had low ground water.
Photo Plate 4: Pitfall and funnel traps associated with drift fences revealed a diverse population of amphibians and reptiles.The present high costs ($100,000-200,000 per acre) and the questionable results to construct forested wetlands in Maryland, suggest that managers consider alternative methods to effectively mitigate for loss of natural forested wetlands. Restoration of previously destroyed wetlands or the purchase and protection of existing wetlands may be more appropriate techniques to slow the loss of forested wetlands. If these methods are not an option within the watershed where the original wetland was destroyed, than an increased ratio of mitigated wetlands to destroyed wetlands should be required to compensate for the long development time and the potential for only partial success with constructed forested wetlands. If woody transplants are used they should be planted during the second or third growing season when the site is vegetated with herbaceous ground cover. Areas on the site that have surface water, or have plants that suggest surface water (e.g., Typha spp.), should not be planted with woody transplants.