Vitis aestivalis var. bicolor Michx.
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Vitis aestivalis var. argentifolia
Basis for Listing
Vitis aestivalis var. bicolor (silverleaf grape) is a native component of hardwood forests in three southeastern Minnesota counties (Wabasha, Winona, and Houston) (The Blufflands). These counties mark the northwestern limit of the species' range. Quite often, a species becomes rare as it approaches the limit of its range, which is the case with V. aestivalis var. bicolor in Minnesota. Extensive searches by botanists of the Minnesota Biological Survey have found only a few occurrences. Although these occurrences have not been thoroughly studied, they appear to consist of small populations in isolated forest fragments.
Vitis aestivalis var. bicolor was listed as special concern in Minnesota in 1996. Although it was recognized as a rare species at that time, the degree of its rarity had not yet been realized. During subsequent botanical surveys, only three additional populations were found; that brought the total number of historical and recent populations to 13. Furthermore, at least one of the populations is known to have been partially destroyed, presumably in a misguided effort to manage the forest for the benefit of individual canopy trees, rather than the forest community as a whole. For these reasons, its status was changed to threatened in 2013.
Vitis aestivalis var. bicolor is a high-climbing or sprawling vine, with tendril-bearing stems up to 20 m (66 ft.) tall and 20 cm (8 in.) in diameter at breast height (dbh). The bark is brown or reddish brown, exfoliating in thin strips or shreds. Leaf blades are 3-5 lobed, 10-25 cm (4-10 in.) long, and 10-25 cm (4-10 in.) wide. The lower surface of the leaf is silver-gray to green-gray and hairy on the veins and in axils of the veins. The lower surface also has scattered tufts of reddish, cobwebby hairs. The flowers are 5-merous, unisexual, and greenish. The fruit is a blue-black, glaucous berry, 7-9 mm (0.28-0.35 in.) in diameter (Smith 2008).
Vitis aestivalis var. bicolor looks something like V. riparia (wild grape), except the undersides of the leaves of V. aestivalis var. bicolor are conspicuously whitened in comparison to the dark green upper surface. Three varieties of summer grape have been described from the eastern United States, differing mostly in the size of the fruit, degree of pubescence, and other minor characters (Moore 1991). The Minnesota plants have been identified as var. bicolor, which is a widespread northern form with small, sour tasting fruit.
Vitis aestivalis var. bicolor is restricted to mesic hardwood forest habitats in the southeastern corner of the state, where the terrain is defined by deep valleys and tall bluffs (Blufflands Subsection). Relatively little is known about the habitat preferences of V. aestivalis var. bicolor in this region, except that it seems to prefer Quercus spp. (oak) forests, especially the margins of oak forests, where there is more sunlight. It also occurs in brushy habitats and in pioneering stands of young forest trees, but it is absent from floodplains, where the more common V. riparia is often abundant.
Biology / Life History
In a forest interior, V. aestivalis var. bicolor may manage to climb into the canopies of the tallest trees. However, under these conditions, its overall presence and effect on the forest community are limited by the intense competition for light. The situation is different in highly disturbed habitats, where competition for light has been reduced like after a logging operation or a major storm. Under such conditions, an established population of V. aestivalis var. bicolor can grow rampantly, clambering over shrubs and tree saplings. In extreme cases, the weight of the vines is enough to deform the branches of a host, and the large, closely spaced leaves can smother shrubs and saplings. It is also possible that the tendrils could cause some minor damage by girdling small twigs of the host (Lutz 1943). However, potential damage caused to the host must not be overemphasized; in Minnesota, V. aestivalis var. bicolor is a rare and benign member of the natural forest community, to which it is well adapted.
Conservation / Management
There has been at least one situation where a number of mature V. aestivalis var. bicolor vines, growing in a stable forest in a public park in Minnesota, were intentionally destroyed, presumably in a misguided effort to manage the forest for the benefit of individual canopy trees. This is just one example of what can happen when a native forest is managed as a collection of component species, instead of as a complex, biotic community dependent on the interaction of all the component species. There really are no situations in Minnesota where the ecological health of a mature, functioning forest is improved by attempting to remove native vines; the vines do no harm in a forest community and should be left alone.
Best Time to Search
Vitis aestivalis var. bicolor can be reliably identified by characteristics of the leaves, which are fully developed by the middle of May.
Welby Smith, MN DNR, 2008 and 2017
Lutz, H. J. 1943. Injuries to trees caused by Celastrus and Vitis. Bulletin of the Torrey Botanical Club 70:436-439.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.
Minnesota Department of Natural Resources, Division of Ecological Resources. 2008. Rare species guide: an online encyclopedia of Minnesota's rare native plants and animals [Web Application]. Minnesota Department of Natural Resources, St. Paul, Minnesota. Accessed 1 July 2009.
Minnesota Department of Natural Resources, Division of Fish and Wildlife. 1995. Statement of need and reasonableness in the matter of proposed amendment of Minnesota Rules, Chapter 6134: endangered and threatened species. Minnesota Department of Natural Resources, St. Paul, Minnesota. 336 pp.
Moore, M. O. 1991. Classification and systematics of eastern North American Vitis L. (Vitaceae) north of Mexico. Sida 14:339-367.
Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis, Minnesota. 320 pp.
Smith, W. R. 2008. Trees and shrubs of Minnesota. University of Minnesota Press, Minneapolis. 703 pp.