A Natural History of Millbrook Marsh,
A Wetland In An Urbanizing Setting

Discussion

The majority of shrub cover is Tartarian honeysuckle and Multiflora rose. These two, along with Autumn olive, are among the non-natives that are listed by the Pennsylvania Fish and Boat Commission as having negative impact on native plants and animals (Thorne et al. 1995). In addition to being non-native invasive species, they are facultative upland species, indicating the probability of a somewhat drier site than in the past. In contrast, Silky dogwood, a native wetland shrub, is dominant along Slab Cabin Run in the northeast area of Millbrook Marsh. Evident from the photographs is that much of the vegetation that was replaced by the shrubs in the wetter areas was herbaceous emergent wetland vegetation in the past. Much of the previously farmed area has not reverted to wetland vegetation, but to woody upland vegetation instead. There are some areas of probable fill. Most likely a decrease in soil moisture in addition to the disturbance by farming has contributed to the increase in upland shrub species. Although drainage tiles were found in parts of the Farm 12 pastures along Puddintown Road near Bathgate Spring Run, hydric soils, evidence of wetland conditions in the past, were not found in the vicinity of the tiles (Brooks, pers. comm.). A question arises, therefore, as to why the drainage tiles were used.

Any one or a combination of several factors could be contributing to the drying of some areas in the marsh. One possibility is that the high velocity stormwater has lowered the stream beds, causing the lowering of the water table. Another possible factor is removal of water by numerous wells in the vicinity including the ones between Millbrook Marsh and Spring Creek Park. Excessive quantities of water removed by wells had lowered the water table in other parts of the State College Area by 1965 (Clark 1965).

The shrub invasion seems to be very prevalent along the stream banks of Slab Cabin Run. As evident in the aerial photography from 1948 to 1994, farming occurred in that area in the past, which has probably contributed to the shrub invasion. It is not uncommon that woody vegetation will mature along riparian corridors after farming activities cease. Some areas may have been covered with fill on the east side of Slab Cabin Run due to the construction of the Mt. Nittany Expressway. Another possible reason for greater shrub cover in those areas is scouring and lowering of the stream beds. In addition, frequent flooding by sediment laden stormwater could have raised the soil levels along streambanks by depositing more silt, sand and gravel than normal (without concentrated stormwater) when the waters overflow the banks, resulting in drier areas adjacent to and along Slab Cabin Run and Thompson Run. This action would cause the soil levels to be higher, farther from the water table level. In 1980, the edges of Slab Cabin Run were described as seasonally flooded wetland with oldfield herbs and grasses intermixed. The vegetation was found on relatively high ground that appeared to be "human or flood deposited areas of silt and sand that have accumulated into a berm along the stream." (PennDOT 1981, p. III-14). At that time, those areas became flooded only during times of high water in Slab Cabin Run. This is most likely also occurring along Thompson Run (Figure 5-11).

Another change that has occurred is an increase in the proportion of non-native species on the site. Though different areas in Millbrook Marsh were inventoried during the various studies, and a more controlled study would be preferable, the change can still be regarded as notable. The 1980 plant inventory (PennDOT 1981) resulted in finding 20% non-native species where as the inventories from 1994 to 1997 (Brooks et al. 1998) yielded 32% non-native species.

Some of the endangered threatened or rare species in Millbrook Marsh have disappeared and others have been recently discovered there. Three species of special concern have been discovered since 1994. Four of the special concern species that were on vegetation inventories in 1980 or before that are not on the inventory lists of 1994 or later. This is not conclusive evidence that these species have disappeared, though it is a possibility. Extensive vegetation study should prove their existence or extirpation from within Millbrook Marsh.

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Figure 5-11 Gravel deposit on Thompson Run bank in Millbrook Marsh July 1998.

On to Chapter 6