Argonne Experimental Forest features the largest contiguous tract of old-growth hemlock hardwood forest on drumlinized ground moraine. These types of protected rich mesic hardwood forests are very scarce in northern Wisconsin and on this landtype (Argonne Outwash Plains). Situated on a gently sloping, unpitted outwash fan is a northern mesic forest dominated by sugar maple, hemlock, yellow birch, and basswood. Sugar maple is reproducing best but basswood seedlings and white ash saplings are also present along with occasional ironwood poles. The shrub layer is sparse with widely scattered leatherwood, and elderberry. Characteristic herbs include oak fern, ground pine, wood ferns, rosy twisted stalk, Canada mayflower, running club-moss, lady fern, and sweet cicely. Only minor amounts of downed logs, snags and tip-ups are present but numerous residual den trees (especially hemlock and yellow birch) add significantly to the stand structure. Also present are pole hardwoods that grade down into a remnant hemlock dominated forest with pockets of upland northern white cedar. This is surrounded by a good to excellent quality conifer swamp, which has several rare species. The conifer swamp (northern wet forest) is dominated by black spruce, tamarack, and white cedar. Understory species include Labrador tea, leather-leaf, balsam fir, yellow bluebead lily, false mayflower, creeping snowberry, Canada blueberry, and sphagnum. Rare species include small round-leaved orchis, sparse-flowered sedge, and northern bog sedge. Several old raptor nests were located within the mesic forest. The extensive habitat of Argonne Experimental Forest supports uncommon boreal bird species such as boreal chickadee and gray jay. In addition, the rare three-toed woodpecker (Picoides tridactylus) has been seen in the conifer swamp. Other birds include broad-winged hawk, blackburnian warbler, and scarlet tanager. Argonne Experimental Forest is owned by the US Forest Service and was designated a State Natural Area in 2007.