Dynamics of Green Ash Woodlands
in Theodore Roosevelt National Park
Introduction
Woodlands dominated by green ash (Fraxinus pennsylvanica Marsh.) comprise 2% or less of the landscape in the Northern Great Plains (Girard et al. 1989, Wood et al. 1989, Hansen et al. 1995) but add structural diversity to the landscape and support a wide array of plant and animal species (Boldt et al. 1978, Hopkins 1983, Hansen et al. 1984, 1995, Girard et al. 1987, 1989). Settlement of the Northern Great Plains by Euro-Americans caused extensive changes in green ash communities (Severson 1981). Suppression of fire and elimination of bison (Bos bison) may have favored green ash communities (although many woody species in ash communities do resprout following burning [Wasser 1982]), but homestead construction, firewood cutting, farming, and the introduction of livestock have had negative impacts on green ash communities (Severson 1981, Girard et al. 1987, Hansen et al. 1995).
Livestock production is currently regarded as the major human-associated impact on green ash communities (Hansen et al. 1995). Livestock, especially cattle (Bos taurus), are attracted to green ash stands because the stands provide forage, shade, protection from wind and insects, and are associated with water sources (Uresk and Boldt 1986, Girard et al. 1987, Hansen et al. 1995). Continuous heavy use of these stands by cattle leads to loss of the tree canopy and replacement of stands by shrub communities (Butler 1983, Hansen et al. 1995). When the tree canopy disappears, biological diversity is reduced as species dependent on trees or the shade from trees disappear, and livestock lose shade and shelter.
Development of grazing systems for livestock that are compatible with green ash communities has been hampered by a shortage of sites managed under low intensity grazing regimes. Studies have shown that excessive grazing by domestic livestock and severe droughts degrade green ash communities (Uresk and Boldt 1986, Girard et al. 1987, Hansen et al. 1995, W. F. Jensen, North Dakota State Game and Fish Department, Bismarck, Unpubl. data), but we do not know the normal range of year-to-year variability in plant cover and species composition in green ash stands used only by native ungulates. Without this information, evaluation of the efficacy of different livestock management strategies will be difficult.
We measured plant density and canopy coverage in 12 green ash stands in the South Unit of Theodore Roosevelt National Park (TRNP) in southwestern North Dakota at one to two year intervals from 1985 to 1996. These data allowed us to assess changes in plant coverage, species composition, and stand structure between wet and dry periods in stands protected from domestic livestock grazing for 30 to 47 years. Green ash stands we measured were accessible to low to moderate densities of mule deer (Odocoileus hemionus), white-tailed deer (O. virginianus), bison, pronghorn antelope (Antilocapra americana), elk (Cervus elaphus), and feral horses (Equus caballus).
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