Shrub—carr vegetation over its full range of variation in southeastern Wisconsin was quantitatively analyzed and the influence of origin, environmental factors, and disturbance on the vegetation was investigated. Plant composition was sampled in 76 stands distributed over 13 counties. A list of common shrub—carr species was derived by combining presence with frequency or intercept data. Salix petiolaris and Cornus stolonifera were the most common of 38 shrub species. The vegetation had three distinct layers, an upper dominant shrub layer, an intermediate tall herb, grass, and sedge layer, and a low diminutive herb layer, but there was no stratification within the shrub layer. The pattern of plant distribution was very heterogeneous, due to irregularities in the soil surface and to disturbance. The response of common species to disturbance was indicated by arranging stands along a disturbance gradient. Most shrub—carrs in southeastern Wisconsin originated in the 1930's when shrubs colonized abandoned mowing meadows. The relative stability of the shrub—carr as a community appeared to be due to shrub resprouting after disturbance. Lowland forest undoubtedly invades the shrub—carr, although fire may kill seedlings and saplings and thus retard three invasion.