Napaea dioica L.
Basis for Listing
Napaea dioica is a large and distinctive plant belonging to a monotypic genus that is endemic to the Midwest. It is the only strictly dioecious species in the Malvaceae family that is native to the Western hemisphere, and it has no close relative living today. For these reasons, it appears that the species is quite old. Napaea dioica is threatened over much of its range, and in Minnesota, it survives locally and occurs primarily on alluvial terraces and floodplains. It was originally listed as a state endangered species in 1984, but was reclassified as threatened in 1996 based on a better understanding of its habitat preferences.
Because N. dioica has no close relatives, it should not be confused with any other species. It is a large, robust plant, often standing 2 m (6.5 ft.) tall with several flowering stems. Flowers are white, dioecious, and arranged in a panicle with 5 petals and 5 sepals. Fruits are depressed-globular in shape, eventually separating into many one-seeded indehiscent locules. Palmate leaves are diagnostic and are alternate on the stem. Lower leaves are quite large and have 9 pointed lobes and short hairs on the lower surface.
Most populations of this species in Minnesota are located on stream banks and floodplains in the valleys of small- to medium-sized streams. Napaea dioica may occur in full sun, under a canopy of trees in full shade, or in partial shade in canopy openings. Plants growing in full shade do not appear to be as robust as plants growing in only partial shade. Commonly associated species include Angelica atropurpurea (angelica), Silphium perfoliatum var. perfoliatum (cup plant), and Rudbeckia laciniata var. laciniata (tall coneflower). Populations often extend several miles or more along river segments but these linear metapopulations consist of scattered patches and individuals. Occupied riverbank segments often share a common geology related to the underlying bedrock. In the Root River drainage of Fillmore and Olmsted counties, meta-populations extend sporadically up to 48-64 km (30-40 mi.), where the streams cut through a set of bedrock strata dominated by the Galena Group, Prairie du Chien Group, and St. Lawrence and Franconia Formations. Populations end rather abruptly near the Houston County line, where the valleys become quite broad as the river cuts into the Ironton and Galesville Sandstones. All habitats are likely to be flooded in spring, but would be only moist by mid-summer (Smith 1989).
Biology / Life History
In Minnesota, N. dioica emerges in May and by late June is producing inflorescences. The large panicles of numerous showy white flowers are present in July and fruits are produced during August and September. With large and rather corky roots, N. dioica is well-adapted to its streamside habitat. When dislodged by floods, pieces of the rootstock will resprout if deposited in suitable habitat.
Conservation / Management
Although recent surveys have located additional occupied sites in the southeastern part of the state, N. dioica is still considered quite rare because its geographic range in Minnesota is very limited and most of its habitat has been destroyed by agricultural activities. In addition, a number of existing populations consist of only a few plants in small remnant habitats that are still threatened. It is unclear how upstream land use could impact the habitat of this species. Unimpeded flow of streams or rivers and the accompanying natural processes of flooding and bank scouring may be necessary for seedling establishment (NatureServe 2006).
Conservation Efforts in Minnesota
Though a number of important sites occur in State Parks or on State Forest land, most N. dioica plants occur on private land. No known conservation plans have specifically addressed the needs of this species.
References and Additional Information
Iltis, H. H. 1963. Napaea dioica (Malvaceae): whence came the type? American Midland Naturalist 70:90-109.
Minnesota County Biological Survey. 1994. Natural communities and rare species of Houston County. Biological Report No. 50. Minnesota Department of Natural Resources, St. Paul, Minnesota.
Minnesota County Biological Survey. 1995. Natural communities and rare species of Goodhue County. Biological Report No. 44. Minnesota Department of Natural Resources, St. Paul, Minnesota.
Minnesota County Biological Survey. 1995. Natural communities and rare species of Houston County. Biological Report No. 51. Minnesota Department of Natural Resources, St. Paul, Minnesota.
NatureServe. 2006. NatureServe Explorer: An online encyclopedia of life [web application]. Version 5.0. NatureServe, Arlington, Virginia.
Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis, Minnesota. 320 pp.
Pusateri, W. P., D. M. Roosa, and D. R. Farrar. 1993. Habitat and distribution of plants special to Iowa's driftless area. Journal of the Iowa Academy of Sciences 100(2):29-53.
Smith, W. 1989. Status report on Napaea dioica (Glade Mallow) in Minnesota. Minnesota Department of Natural Resources Biological Report No. 6, St. Paul, Minnesota. 11 pp.
Utech, F. H. 1970. Preliminary reports on the flora of Wisconsin. No. 60 Tiliaceae and Malvaceae - Basswood and Mallow families. Transactions of the Wisconsin Academy of Sciences, Arts and Letters. 58:301-332.