Furthermore, it should be recognized that, as it is currently developed as an assessment tool, HGM is principally a diagnostic method, not a prescriptive “cookbook.” In this respect, the HGM models do not specifically lay out design parameters that guarantee the likelihood that hydrology, desired wetland vegetation, and desired animals will be reestab
site higher on mitigation wetlands (p=.0006). However, the species composition of mitigation wetlands less than 3 years in age differed from that of mitigation wetlands more than 3 years in age due to the influx of introduced species, averaging 11 additional species per site as the site aged. So the mitigation wetlands may not maintain native plant species over time, especially in the face of changes occurring with urbanization of the landscape.
In the case of the organic matter content of the soils, naturally occurring wetlands and mitigation wetlands are significantly different. There is less organic matter in the top 5 cm of the soil of mitigation wetlands (p=.0001) and at 15 to 20 cm (p= 0.0551) than in naturally occurring wetlands. There was no substantive relationship between soil organic matter concentration and the age of mitigation wetlands (r2=.0232, p=.6003). This suggests that development of a soil organic matter content similar to that of naturally occurring wetlands may not be achieved for a very long time, if ever. Finally, the hydrological characteristics of the mitigation wetlands differed from the naturally occurring wetlands. As mentioned above, mitigation wetlands had more open water than naturally occurring wetlands. On average, 57% of the area of the mitigation wetlands was flooded, while 28% of the area of the naturally occurring wetlands was flooded during the year (p < 001). The predominance of deep open water on mitigation wetlands was indicated by higher mean annual water levels (0.85 m) on mitigation wetlands than on naturally occurring wetlands (0.25 m, p < 001). Hydrological variability also differed between naturally occurring wetlands and mitigation wetlands. The mean difference between the 10th and the 90th percentiles of water levels was 0.60 m for naturally occurring wetlands and 0.32 m for mitigation wetlands (p < 01). The difference between the 10th and the 90th percentiles was used to represent conditions commonly found in the wetlands as it minimizes the effects of extreme storm events.
Given the above analysis, conclusions about the performance of the mitigation wetlands would depend on whether only vegetation characteristics were considered and on how one viewed the predominance of alien plant species. In addition, there is evidence that the conclusion might change with time, especially with time periods longer than the 5-year monitoring requirement often associated with permits. Regardless, these kinds of analyses do not overcome the inherent problems of using structural similarity to infer functional equivalence, let alone determining the effects of permit decisions on the resource as a whole. Faced with this dilemma, it was found that the concept of hydrological equivalence as exemplified in HGM classification brought important insights to the evaluation of mitigation wetlands.