Impact of Oak Wilt on Vegetation and Structure of a Degraded Barrens
Collada, Angela E.
University of Wisconsin-Stevens Point, College of Natural Resources
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This study investigated vegetational changes in oak wilt patches that occurred in late-successional barrens at Necedah National Wildlife Refuge. Oak wilt, a vascular disease that is usually fatal to members of the red oak subgenus (Erythrobalanus), is common on better drained soils south of the tension zone in Wisconsin. Vegetation of oak wilt patches was compared with that in the surrounding woods and to Old Barrens, a 226 ha barrens restored by wildfire in 1936. Allelopathy of Carex pensylvanica, a species that dominates the herb layer of degraded barrens, was examined experimentally to determine if it inhibits germination of savanna species. A significant majority of patches occurred on well-drained Plainfield-Friendship soils compared with less well drained Meehan-Newson soils. Structure of vegetation within patches, classified in terms of the presence of shrub or tree layers, and condition of the oldest dead oaks were correlated with patch size. Total herb cover was significantly higher in oak wilt patches on Plainfield-Friendship soils than woods on either soil association or Old Barrens, and inversely correlated with total tree cover across all sites. Deciduous shrub cover was higher in Old Barrens than in oak wilt patches. The mean number of species per IO m segment in each sampling unit was significantly higher in Old Barrens than oak wilt patches on Plainfield-Friendship soils but not significantly different between patches and woods. Several species displayed distinct patterns in their distributions. Thirty-seven species were significantly more abundant in Old Barrens than in oak wilt patches on the same soil type and 10 species showed significant differences when oak wilt patches were compared to woods. C. pensylvanica and Vaccinium angustifolium, patch size, soil, and shrub cover were correlated with the presence/absence and abundance of the most common species. Two experiments designed to test the impact of C. pensylvanica on germination of savanna species had mixed results. Germination of Solidago juncea and a commercial lettuce was inhibited by Carex leachate. Mean radicle length of Schizachyrium scoparius and the lettuce was lower when treated with the leachate than with deionized water. The species water interaction was also significant for the 2 species. A test of the effect of Carex leachate and litter (burned, minced, and no litter) on seedbank germination was inconclusive.