The influence of natural variability in hydrology, soil, and vegetation on wetland delineation: A case study from the Maryland coastal plain.

摘要

Manuals have been published by various federal agencies over time to guide the practice of wetland delineation. All rely on some combination of hydrologic, pedologic and botanic data to distinguish wetlands from uplands. Practicality requires that such data be collected with minimal sampling over a short time span of hours or days. However, the data being collected is subject to spatial and temporal variability.; In Many Forks Valley, Maryland, we measured the substantial natural variation in expression of hydrologic indicators, spatial distribution of soil redoximorphic features, and temporal composition of plant communities.; The degree of soil variability within a pedon as a function of position on a hillslope, as estimated by depth to redoximorphic features, decreases as one progresses from the higher hillslope pedons (45 ± 12.4 cm) to the wetland pedons (1 ± 0.8 cm). Six replicate soil descriptions from a single pedon can contain enough morphological variability to affect soil drainage class, taxonomic classification, and/or hydric soil determinations.; Seasonal variation in plant community composition is caused by the life cycles of the herbaceous species. This temporal diversity causes related shifts in the wetland index value (WIV), a measure of “hydrophyticness” calculated for the plant community. Alien plants strongly influence the seasonal behavior of the WIV at some releve's. Herbaceous plant community activity is inconsistent with the agricultural concept of growing season presently used in the definition of wetland hydrology (Williams, 1992). The degree to which seasonal variation affects wetland determinations depends on the method used to analyze the vegetative data, with a presence/absence approach (99%) producing the highest percentage of correct conclusions for near-boundary releve's.; In Many Forks Valley delineations near steep slopes are easily made. Where more subtle grades gave a gentler transition from upland to wetland, variability in the availability of hydrologic indicators, soil morphology, and plant community composition has substantial influence on wetland determination.