PNW 98 Session - Full Abstracts

Full Abstract

SARAH CASSATT (1)
MARK MATTHIES (1)

(1) Jones & Stokes Associates

Let's Take Another Look at Percent Cover Performance Standards for
Vegetation

Wetland and other types of mitigation projects implemented to meet
regulatory requirements must achieve pre-determined performance standards
at the end of the monitoring period to be considered successful.
Vegetation establishment is often a primary monitoring element and
performance standards for vegetation usually focus on survival rates and
percent cover. Some jurisdictional agencies have required that shrub and
tree communities provide from 60% to 100% cover at the end of a 3-year or
5-year monitoring period. After several years of experience in mitigation
design and monitoring, it has become clear that while using cover
measurements may be an effective tool in determining success, these 60% to
100% cover standards are unrealistic for most mitigation sites and,
therefore, warrant review.

To identify percent cover rates, we developed a list of assumptions;
including initial plant size, planting density, growth rates, and survival
rates; and calculated percent cover for each year of a 5-year monitoring
program. The resulting numbers show that to meet the high percent cover
standards required, planting plans must rely on large plants, very high
planting densities, and/or focus on a few species with high growth rates,
such as willows, cottonwood, and alder. Thus, requiring development of a
high percent cover in a relatively short time frame may be discouraging
attempts to mitigate for wetland losses with native communities that
support a variety of species. In addition, utilizing high planting
densities are very costly for larger mitigation sites and, over the long
run, may be a waste of plant material as individual shrubs begin to
compete with each other for space, perhaps 5 to 10 years down the road.

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