Ozark-Ouachita Streamside Forest

This system is found along streams and small rivers within the Ozark and Ouachita regions. In contrast to larger floodplain systems, this system has little to no floodplain development and often contains cobble bars and steep banks. It is higher gradient than larger floodplains and experiences periodic, strong flooding. It is often characterized by a cobble bar with forest immediately adjacent with little to no marsh development. Canopy cover can vary within examples of this system, but typical tree species include Liquidambar styraciflua, Platanus occidentalis, Betula nigra, Acer spp., and Quercus spp. The richness of the herbaceous layer can vary significantly, ranging from species-rich to species-poor. Likewise, the shrub layer can vary considerably, but typical species may include Lindera benzoin, Alnus serrulata, and Hamamelis vernalis. Small seeps and fens can often be found within this system, especially at the headwaters and terraces of streams. These areas are typically dominated by primarily wetland obligate species of sedges (Carex spp.), ferns (Osmunda spp.), and other herbaceous species such as Impatiens capensis. Flooding and scouring strongly influence this system and prevent the floodplain development found on larger rivers.

Typical tree species in examples of this system include Liquidambar styraciflua, Platanus occidentalis, Betula nigra, maples (Acer spp.), and oaks (Quercus spp.). The richness of the herbaceous layer can vary significantly, ranging from species-rich to species-poor. Likewise, the shrub layer can vary considerably, but typical species may include Lindera benzoin, Alnus serrulata, and Hamamelis vernalis.

This system has little to no floodplain development and often contains cobble bars and steep banks. It is often characterized by a cobble bar with forest immediately adjacent with little to no marsh development. Because these habitats are moister than adjacent uplands, the streamside zones have much higher plant and animal diversity. Orchids and many other species of mesic habitats can be found here. At the larger end of the size continuum, these streams can have gravel and even sand bottoms that support a range of species, including Salix spp., Justicia americana, and others. Pools provide refugia for invertebrate and vertebrate species that can then rapidly recolonize the stream during high water.

Flooding and scouring strongly influence this system and prevent the floodplain development found on larger rivers. It is traditionally higher gradient than larger floodplains and experiences periodic strong flooding. The distinctive dynamics of stream flooding are presumably the primary reason for the distinctive vegetation of this system, though not all of the factors are well known. Small rivers and streams, with small watersheds, have more variable flooding regimes that larger rivers. Floods tend to be of short duration and unpredictably variable as to season and depth. In addition to disturbance, floods bring nutrient input, deposit sediment, and disperse plant seeds. Fire does not appear to be a dominant factor, and most floodplain vegetation is not very flammable. Historical references to canebrakes dominated by Arundinaria gigantea suggest that fire may have once been more possible and more important in at least some portions.

Flooding is the major process affecting the vegetation, with the substrate more rapidly drained than in flat floodplain areas. The higher gradients of most of these streams and rivers limit floods to fairly short duration. Flooding is most common in the winter, but may occur in other seasons particularly in association with hurricanes, tornados, or microbursts from thunderstorms. Flood waters may have significant energy in higher gradient systems, but scouring and reworking of sediment are important in maintaining the small non-forested patches of the bar and bank communities. Flooding can act as a replacement disturbance in areas where beavers impounded a channel or in rare years with severe prolonged flood events. There are two general types of floods: occasional catastrophic, prolonged floods (due to beaver activity or other severe event); and more frequent repeated minor flooding (i.e., several minor floods within a 10-year period).

The wind disturbance associated with flooding is very significant along small streams because of wet and less dense soils and shallow-rooted trees. Canopy tree mortality from more common windstorms would have resulted in tree-by-tree or small group replacement. Windthrow is the primary cause of mortality in bottomlands. Major storms or hurricanes occurring at approximately 20 year intervals would have impacted whole stands.

In this system, the fire-return interval varies greatly. Except in canebrakes, most fires were very light surface fires, creeping in hardwood or pine litter with some thin, patchy cover of bottomland grasses. Flame lengths were mostly 15-30 cm (6-12 inches). Fire-scarred trees can be found in most small stream sites except in the wettest microsites. Stand-replacement fires are almost unknown in this type. Except where Native American burning was involved, fires likely occurred primarily during drought conditions and then often only when fire spread into bottomlands from more pyrophytic uplands. Trees may be partially girdled by fire in duff, followed by bark sloughing. While fire rarely killed the tree, this allowed entry of rot, which, in the moist environment, often resulted in hollow trees, providing nesting and denning habitat for many species of birds and animals. Surface fires occurred on a frequency ranging from about 3-8 years in streamside canebrake, streamside hardwood/canebrake, or pine, to 25 years or more in hardwood litter. Low areas having a long hydroperiod, islands, and areas protected from fire by back swamps and oxbows were virtually fire-free. Fire effects were largely limited to top-kill of shrubs and tree saplings less than 5 cm (2 inches) diameter, and formation of hollow trees.

Undoubtedly, the greatest historic stressors have been the conversion to intensive agriculture in the 1850-1950 period (with subsequent abandonment and re-establishment of forest vegetation) and the construction of dams for mills, hydropower, and water supply during the same period. The threat of development is exacerbated by the current surge in population in northwestern and north-central Arkansas. Urban and exurban sprawl into previously forested lands outside the major communities is expected to continue to increase (Arkansas Forestry Commission 2010). This will lead to the conversion of sites to human-created land uses. In addition, invasive exotic species, including Microstegium vimineum and Ailanthus altissima, can become dominant in the ground and shrub layers following canopy disturbance. For hardwood forests containing Fraxinus species, emerald ash borer (which as of October 2013 has been reported from southeastern Missouri) may also be (or become) a significant stressor.

This system is found within the Ozarks and the Ouachita Mountains of Missouri, Arkansas and Oklahoma.