Inventory of the Brownwater Floodplain Habitats
of the Lower Roanoke River
By Stephen Hall and Harry LeGrand, N.C. Natural Heritage Program
January 24, 2014
A Natural Areas Inventory
The North Carolina Natural Heritage Program has just completed an inventory of the Lower Roanoke River floodplain, conducted by Harry LeGrand and Steve Hall, with funding from the Albemarle-Pamlico National Estuary Partnership and the Natural Heritage Trust Fund. A natural areas inventory is a systematic search for the best examples of natural habitats and locations of rare species within a defined region. Collectively, the reports and maps produced by these surveys identify the most important natural areas in the state and serve as an important guide for conserving the state’s biological diversity.
The Coastal Plain portion of the Roanoke River floodplain – located between Weldon, NC on the Fall-line and the mouth of the river at the Albemarle Sound – has long been considered a hotspot for natural diversity, both for its aquatic ecosystems and its floodplain forests. The N.C. Natural Heritage Program (NCNHP) first targeted the Lower Roanoke for inventory in the 1970s, culminating in an initial conservation plan prepared by former NHP biologist Merrill Lynch in 1981. Since then, a total of 171 square miles have been protected, with a primary focus on management of the floodplain’s ecosystems and species.
The 2012 inventory covered much of the same area as studied previously, but also included several new areas. Floodplain ecosystems are by nature highly dynamic, with species adapted to major flood events and changing river courses over time. Floodplains are also susceptible to a number of less natural changes which have the potential to permanently reduce the diversity of native species and natural communities. These include changes in the flood regime and sediment deposition, invasions by exotic species, changes in land use, and the potential effects of global climate change. The effects of all of these changes, natural or otherwise, need to be monitored, with conservation plans periodically reviewed and revised.
Some of the areas most affected by all of these changes also contain several of the most distinctive habitats along the lower Roanoke: the levees, bottomlands, and slopes covered with nutrient-rich sediments carried down by the river from its sources in the Piedmont and Mountains. While the survey documented the presence of several previously unreported species for this area, it also revealed the decline of several species of plants and animals.
Plant Survey Results
Rare plant populations associated with rich soil habitats were reconfirmed at many of the sites during this survey. Several new species were discovered --including Veined Skullcap and Limestone Wild Petunia -- that have a high affinity for nutrient-rich sites and were previously known from just a couple of occurrences in North Carolina. Many of these plant populations appear to be barely hanging on, however, with many of the richest areas within the floodplain now completely overgrown by Common Chickweed and other invasive exotic plants. Several previously documented populations could not be relocated at all.
Animal Survey Results
Results for the animal survey were also a mix of positive and negative findings. A large number of insects and other invertebrates were recorded in the Coastal Plain for the first time during this survey, including several that have been recorded at only a couple of other locations in the entire state. Populations of some animals appear to be increasing along the Roanoke, including Swainson’s Warbler, Mississippi Kite, Bald Eagles and Anhingas, all of which were previously considered rare within the floodplain. Several southern species also appear to be extending their range northward into the Roanoke Basin, possibly as a result of climate change. The Cocoa Clubtail Dragonfly, for example, had never been previously recorded north of the Neuse River. On the other hand, the population of Cerulean Warblers – one of the rarest birds in the state and long considered a hallmark of the Lower Roanoke – appears to be strongly declining, with none heard at several sites where they had previously been recorded. Wood Thrushes – which are also declining in the state – were also heard at only a few sites.
Most of the areas surveyed during this inventory possess high quality natural communities or populations of rare or declining species, thus qualifying for status as Significant Natural Heritage Areas (SNHAs). An SNHA is a site (terrestrial or aquatic) of special biodiversity significance. An area’s significance may be due to the presence of rare species, exemplary or unique natural communities, important animal assemblages, or other important ecological features. One new SNHA was added as the result of this inventory, one previously delisted SNHA was resurrected, and the boundaries of several more were modified. Currently, twelve of these SNHAs are not under any form of conservation ownership or management and are consequently high priorities for protection.
All together, the SNHAs only comprise about 43% of the natural forests of the Lower Roanoke. The remainders are either younger stands, lack populations of imperiled species, or have yet to be inventoried. Almost all are located on private lands and are managed for multiple uses, particularly timber production. Nonetheless, these lands may play an important role in maintaining the floodplain’s biodiversity. Particularly in disturbance-maintained landscapes, species need to be widely enough distributed so that not all of their populations are affected by any one disturbance event. Their habitats also need to be sufficiently connected that populations lost from one area can be eventually recovered through recolonization from areas that were not as drastically affected. For this system to continue to work, the entire landscape needs to have a high degree of integrity.
In addition to identifying SNHAs, the 2012 inventory focused specifically on these wider aspects of landscape and ecosystem integrity. Surveys were conducted for species – all animals – that we believe are sensitive to fragmentation of specific types of habitat; the more such species we found within an area of that type of habitat, the higher the estimated landscape integrity for that habitat. Species belonging to twelve habitat indicator groups were documented during the survey, with enough species present in each of them to indicate “good” to “excellent” landscape integrity. Combined, the total area of natural habitats occupied by these species encompasses approximately 300 square miles, with most habitats showing strong connectivity all the way from Weldon to the mouth of the river. Movement bottlenecks were identified at a few sites, however, where only narrow strands of habitat provide the connections or where there are significant gaps that have no natural habitats at all. Along with the SNHAs, these bottlenecks were identified as high priorities for restoration and conservation protection.
In all, this inventory provides a renewed look at a well-studied ecosystem, providing data on population trends and needs, which can be used to revise conservation priorities within this region. As with the past inventories that it supplements, this survey does not provide the final word but only a snap-shot of current conditions. Like the floodplain of the river itself, the job of conserving the state’s biological diversity is continually shifting and needs to be regularly re-assessed.