Hemlock relicts are small patches of mesic forest composed mostly of species that are disjunct from and generally far south of their usual Wisconsin ranges. Most documented relicts occur in southwestern Wisconsin's Driftless Area, within the Western Coulees and Ridges Ecological Landscape. Typical sites are deep, steep-sided, moist ravines, with cool northern or eastern slope exposures. Exposures of bedrock, most often Cambrian sandstones, are typically present and contribute to the ability of this community to develop and persist in areas that would otherwise be vegetated with deciduous hardwood forests. The porous sandstone has a high capacity to hold water, which slowly moves through the rock and keeps conditions humid and cool. The dominant tree is most frequently eastern hemlock (Tsuga canadensis). Eastern white pine (Pinus strobus), yellow birch (Betula alleghaniensis), and paper birch (Betula papyrifera) are common northern canopy associates. Other trees present are usually those present in the adjoining hardwood forests. Shrubs and herbs with northern affinities are also important, and may include mountain maple, Canada yew (Taxus canadensis), showy mountain ash (Sorbus decora), blue-bead lily (Clintonia borealis), rose twisted-stalk (Streptopus roseus), shining club-moss (Huperzia lucidula), and spinulose wood fern (Dryopteris carthusiana). The dense shade of the hemlock combined with the northern aspect of many stands means that very little light reaches the forest floor causing a very sparse groundlayer.
Hemlock relicts are highly localized in southwestern Wisconsin. They are concentrated in a few areas, such as the drainages of the upper Kickapoo and Baraboo Rivers, and in a few of the deep gorges that cut into the flanks of the Baraboo Hills. Extremely isolated outliers are known from a few sites south of the Wisconsin River. Unusual plants and animals have been documented in several stands. These include periglacial relicts, that for various reasons were unable to keep pace with changes to the vegetation as the climate changed following the last glacial advance, and a number of habitat specialists that are rare elsewhere in southern Wisconsin.
