Napaea dioica L.
Basis for Listing
Napaea dioica (glade mallow) is a large and distinctive plant belonging to a monotypic genus 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; in Minnesota, it survives locally and occurs primarily on terrace forests and floodplains in the southeastern part of the state (The Blufflands, Rochester Plateau and Oak Savanna subsections). 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, floodplains, and terrace forests 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.
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. Because the species is dioecious, both male plants and female plants must occur within the flight distance of the pollinator in order for fertile seeds to be produced. The specific pollinator species is not known. However the structure of the flowers is known to biologists as an “open design”, meaning that an insect with specialized feeding structures is not needed to effect pollination. It is likely any number of flying insects of suitable size may serve as pollinators.
It is known that N. dioica is specialized to occur along small and midsized streams that experience periodic spring flooding. However, the precise mechanisms by which N. dioica survives, and even thrives, under such a disturbance regime are not known.
Conservation / Management
Although recent surveys have located additional occupied sites in the southeastern part of the state, N. dioica is still considered quite rare and vulnerable, 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. Management of known sites should prioritize maintenance of existing native vegetation and control of non-native invasive species that may degrade the habitat. One of the most pernicious invaders of stream-side habitat is reed canary grass (Phalaris arundinacea). Because N. dioica is a large robust plant, it can compete with most native species that occur in its habitat, but reed canary grass will inhibit establishment of seedlings. Unimpeded flow of streams or rivers and the accompanying natural processes of flooding and bank scouring may also be necessary for seedling establishment (NatureServe 2021).
Best Time to Search
The best time to search for Napaea dioica is from June through mid-September, when important diagnostic features (flowers or fruiting bodies) are fully developed.
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.
Welby R. Smith (MNDNR), 2023
(Note: all content ©MNDNR)
References and Additional Information
Iltis, H. H. 1963. Napaea dioica (Malvaceae): whence came the type? American Midland Naturalist 70:90-109.
NatureServe. 2021. NatureServe Explorer [web applica]. NatureServe, Arlington, Virginia. (https://explorer.natureserve.org/). Accessed 21 June 2021.
Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis. 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.
Utech, F. H. 1970. Preliminary reports on the flora of Wisconsin. No. 60 Tiliaceae and Malvaceae - Basswood and Mallow families. Pages 301-332 in W. F. Peterson, editor. Transactions of the Wisconsin Academy of Sciences, Arts and Letters. Volume LVIII, Wisconsin Academy of Sciences, Arts, and Letters, Madison.