Vegetation of the Ulao Swamp, a disturbed hardwood-conifer swamp in southeastern Wisconsin

Ulao Swamp (Grafton, Wisconsin) is a 185-hectare wetland, which was a confer/hardwood swamp before European settlement. Post-settlement disturbances include logging, drainage, flooding, cultivation, grazing, non-metallic mining, and development in the watershed. As a result of these disturbances, very little of the presettlement-type mixed hardwood and cedar/tamarack conifer swamp vegetation currently remains in the wetland. Historically the northern quarter of the wetland had surface drainage to the north, and the southern three-quarters drained to the south. Between 1980 and 1985 a north-south ditch was constructed causing water from the northern quarter of the wetland to drain southward, dramatically increasing water levels in the central portions of the wetland. This dramatic increase in water levels has caused high mortality of the hardwood trees that were established in the central and southern portions of the swamp. Severe flooding continues in some portions of the wetland. During the 2000 and 2001 field seasons, the vegetation of the Ulao Swamp was quantitatively sampled to describe current conditions and serve as baseline data for evaluating future change in the vegetation. Using ordination and classification analysis, six vegetation cover types were recognized in the swamp: cattail marsh, reed canary grass, sedge/shrub, flooded maple forest, open ash forest, and closed ash forest. The distribution of these six types was mapped, and their species composition was described. The closed ash forest vegetation type was found to most resemble the likely preflooding plant community of those portions of the Ulao Swamp that are at slightly higher elevations. The lower elevations of the swamp now have cattail marsh, sedge/shrub, and flooded maple forest vegetation types, and these were dominated by silver maple forest, rather than green and black ash, before they were flooded. The flooded maple forest, sedge/shrub, and cattail vegetation types appear to have been the most severely affected by flooding, with the sedge/shrub and cattail communities having been flooded for sufficiently long to have nearly lost their forest canopies. The presence and densities of standing dead trees and tree stumps, and analysis of aerial photographs, indicates that nearly all of the Ulao Swamp was a closed canopy swamp forest before flooding began in the early 1980’s. We conclude that restoration of a native swamp forest community to the Ulao Swamp would be greatly hastened by closure of the ditch that breaches the drainage divide in the northern portions of the swamp.