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Upland Depression Wetlands in Arkansas

Arkansas Natural Heritage Commission - Friday, November 16, 2018
by Theo Witsell

Ancient glaciers that once covered parts of North America never made it as far south as Arkansas. This important fact is largely responsible for the impressive variety of native plant life we have here in The Natural State today. There are exceptions, but as a rule, botanical diversity is correlated to the age of a land surface, or how long it has been available to be colonized by plants. In simplest terms, the older a site, or the longer it has been exposed, the more kinds of plants you can expect to find there. That’s part of the reason why the Southeastern U.S. is so much richer, botanically speaking, than the Upper Midwest, which was mostly covered by a vast sheet of ice as recently as 12,000 years ago.

Our lack of glaciers is also responsible for the scarcity of natural (old) lakes and ponds in Arkansas, especially when compared to the formerly glaciated landscapes to our north. As glaciers move across a landscape they carve out pits and grooves and fill them with water as they melt and retreat. In the younger, lowland regions of the state, Arkansas has a fair number of oxbow lakes, which are our largest natural ponds. However, these are relatively young features, formed as large rivers (the Mississippi, Arkansas, White, Saline, Ouachita, Red, etc.) meandered through their floodplains over time and left abandoned channels. Most of these are still connected with the rivers that formed them and they fill with water when those rivers flood.


Image above — This ancient channel scar includes several types of wetlands and supports a number of species rare in the mountains and typically found in the floodplains of large rivers. This topographic map shows the Hanging Oxbow, an ancient river channel west of (and 60 feet above) the present-day channel of the South Fourche LaFave River in the Ouachita Mountains of Perry County. Photo by Theo Witsell.

It is in the Interior Highlands of Arkansas (the Ozark Plateaus, Ouachita Mountains, and upland parts of the Arkansas Valley) that we find our oldest ground. In these regions, natural ponds are rare, much older, and significantly more interesting botanically. Here we find three main types of natural upland ponds or depression wetlands: 1) mountain channel scar ponds, 2) karst sinkhole ponds, and 3) upland sandstone depression wetlands. All of these can be ancient, and all of them receive their water today primarily from rainfall, which is collected from a (usually) small local watershed. Consequently, water levels in these wetlands can fluctuate dramatically with local weather and climate cycles, and this is important for a variety of plant and animal life found in them. For one thing, because most of them dry out periodically, they are fishless, making them important and relatively safe breeding sites for amphibians like frogs and salamanders.

Here is a look at each of the three main types of upland depression wetlands in the highlands of Arkansas:

Mountain channel scar ponds can be thought of as the rare highland cousins of the lowland oxbow lakes, but they differ in several key ways. They are typically much smaller and occur on elevated, ancient stream terraces in depressions left by long abandoned stream channels. They are also much, much older than their lowland cousins. As streams carve their way down into the landscape over time, they abandon older, higher floodplains, which eventually become unconnected from the streams, even during the major flood events. The depressions left by these ancient channels however, remain. These can be open (marshes) or forested and while most in Arkansas are small, ranging in size from a fraction of an acre to a few acres, we do have a couple of large examples.

The largest and most impressive of these mountain channel scar ponds in Arkansas occur in the Ouachita Mountains. Richardson Bottoms, along the Irons Fork of the Ouachita River at the Garland/Montgomery County line, is an ancient, abandoned meander of that stream that is cut-off from the modern day river, which is lower in the landscape. It includes a variety of wetland habitats including wet oak flatwoods, a forested slough, and an extensive open marsh. The Hanging Oxbow along the South Fourche LaFave River near Hollis in Perry County is a similar abandoned meander that today is stranded on a hillside about 60 feet above the modern day river channel. We don’t know for certain how old these wetlands are but both are fascinating botanically and support a number of plants that are rare in the mountains and disjunct from their main ranges in big river bottomlands in the Mississippi Alluvial Plain and Gulf Coastal Plain. For example, they support native stands of overcup oak (Quercus lyrata), pin oak (Q. palustris), willow oak (Q. phellos), and water oak (Q. nigra) and the South Fourche site supports an old growth stand of loblolly pine (Pinus taeda), which is very rare in the mountains (except where planted). They also support a number of shrubs and smaller plants that are rare and/or far from home.


Photo above — An out-of-place overcup oak-willow oak forest in the Hanging Oxbow, a large channel scar wetland complex along the South Fourche LaFave River in Perry County. Photo by Theo Witsell.

Mountain channel scar wetlands are also home to a number of species of conservation concern. Examples include creeping St. John’s wort (Hypericum adpressum), giant sedge (Carex gigantea), cypress-knee sedge (C. decomposita), water purslane (Didiplis diandra), and southern rein orchid (Plantanthera flava).

Karst sinkhole ponds are found in Arkansas only in the Ozark Plateaus where they occur in natural sinkholes formed where underlying limestone has dissolved away and led to the collapse of the land surface. Occasionally these sinkholes get plugged up and will hold water. Like mountain channel scar ponds, karst sinkhole ponds support a number of out-of-range and uncommon wetland plants. Rare species known in Arkansas karst sinkhole ponds include lowland yellow loosestrife (Lysimachia hybrida), broad-winged sedge (Carex alata), northern water-plantain (Alisma triviale), and floating manna grass (Glyceria acutiflora).


Upland sandstone depression wetlands
are found primarily in the Boston Mountains and upland portions of the Arkansas Valley and occur in poorly drained flat places on acidic substrates derived from sandstone. The formation of these depressions is poorly understood but they occur primarily on ridgetops, in saddles, and on benches and oddly enough are often associated with glades (one of our driest habitats). Like karst sinkhole ponds, upland sandstone depressions support a number of rare species including cypress-knee sedge (Carex decomposita), false hop sedge (Carex lupuliformis), and small-headed pipewort (Eriocaulon koernickianum).

Photo above left — Small-headed pipewort (Eriocaulon koernickianum).


Other types of depression wetlands occur in the Mississippi Alluvial Plain between ancient sand dunes or on ancient elevated ridges. These sand ponds, and similar depression wetlands, are important habitat for several uncommon to rare woody plants including corkwood (Leitneria floridana), swamp cottonwood (Populus heterophylla), and pondberry (Lindera melissifolia). All of these depression wetlands may be open or forested, depending on the depth and duration of their flooding. 

Photo at left — Pondberry (Lindera melissifolia) growing in the Mississippi Alluvial Plain of Arkansas. Photo by Brent Baker.


It may seem improbable that such small, isolated ponds could have so many unusual and out of place species, but this can be explained by their old age. Even a small pond, if it has existed long enough, can accumulate an interesting assemblage of species.

 

 

 

 

 

 

 

 

 



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