Small, Forested Wetlands
Starting in 2004, we examined wetland features along
headwater streams as part of a larger study focused on
the effects of forest management in headwater riparian
areas. We were interested in the frequency, surface area,
and other attributes associated with the many small, cryptic
wetlands observed along the headwater streams of the
Jack Janisch, Washington Department of Ecology
Washington Department of Natural Resources
We lacked applicable methods for wetland delineation on features of this size and character, so we devised and compared two methods for our sampling. One method used three criteria, including the characterization of wetland soils, the hydrologic footprint, and hydrophilic vegetation. The other method used only the hydrology and vegetation criteria. We collected information about wetland character, association with catchment soils and geology, stream aspect, gradient, and large woody debris. We recorded observations of sensitive animals that were in or near the wetlands.
We found that small, forested wetlands are common (average 2.3 per channel across 30 surveyed channels), and usually have surface areas less than 100 m2 — well below sensitive site area thresholds currently used in Washington for planning timber sales. Wetlands are most frequent on north-facing aspects, along streams with spatially continuous, perennial flow and large woody debris.
The small wetlands and seeps are important to sensitive species like amphibians and mollusks, possibly affecting their distribution and movements. We also observed that the wetlands are responsive to hydrologic change over time and may ‘flicker’—that is, develop or fade at a time scale of several years, suggesting that they may be sensitive to both short- and long-term effects of environmental change. Headwater wetlands may play a role in stream temperature, flow regimes, and water quality. In a broader perspective, given the vast forested area implicated, headwater wetlands warrant further evaluation not only relative to forest management, but also for their collective significance in headwater ecosystems and linkages to downstream areas.
Janisch, J.E., A.D. Foster, W.J. Ehinger. 2011. Characteristics of small headwater wetlands in second-growth forests of Washington, USA. Forest Ecology and Management 261(7) 1265-1274.