Details on the DCR-DNHData

The State of Virginia, through the Department of Natural Heritage (DNH), has conducted research on naturally-occurring ecological communities for decades. Biologists have documented at least 4,736 plots across the state and determined that they fall within over 300 community types.

The natural communities commonly found in Northern Virginia are many fewer than 300. Outside of community types found only in the Potomac Gorge, most natural areas will fall into one of 15-20 Natural Community types; and of those, 6-8 will be predominant.

Details on the Natural Heritage Program Data

Over decades, the Natural Heritage Program of the Division of Conservation and Recreation has examined the remnant, most natural, and least disturbed areas across the state[1]. At over 4,700 specific locations, state and local vegetation ecologists created specific plots (usually 400 meters square) to sample these natural areas. In each, they identified all plants present and calculated a variety of coverage and density statistics, in addition to the underlying characteristics of the site (e.g. soils, aspect, hydrology, etc.). The amount of data collected and the vision necessary to have completed this magnitude of work is astonishing.

The research effort used cluster analysis and other techniques to identify the plots that most resembled each other, which created groups of plots. The groups were manipulated and tested to make sure each grouping was distinct and unique. Each unique grouping was compared across similar research conducted in the mid-Atlantic and at the federal level. When all was done, the research identified over 300 distinct Natural Communities. These Natural Communities are the closest evidence we have as to naturally occurring assemblies of plant species that exist across various regions, soil types, hydrological regimes, etc. There is no other data set like it to help understand what a specific natural community looks like in a mature form in Virginia.

The State published comprehensive data about each community, including verbal descriptions of the most common and diagnostic species and other identifying characteristics. The State also published the underlying data. This data set includes the specific plots that attach to each community type, along with a huge amount of data about each plot, such as the complete list of plants found there along with density and coverage and underlying biotic and abiotic characteristics[2].

It is appropriate to mention the sampling technique that created the Natural Communities. Because the plots are samples and are not comprehensive, it is likely that there are many species which simply did not show up in any plot but which exist in that Community in reality. The fewer the number of plots which contribute to the definition of a Community, the bigger this “missing species” issue becomes. Similarly, sampling error could also push some species higher or lower in prevalence across plots. Thus, the Natural Communities work is not perfect. But it is still the most informative science we have. The more plots sampled per community, the better the information is likely to be.

[1] The research published in 2017 builds on previous research and is called the Third Approximation. The complete description of the Virginia work can be found at: Fleming, G.P., K.D. Patterson, and K. Taverna. 2018. The Natural Communities of Virginia: a Classification of Ecological Community Groups and Community Types. Third approximation. Version 3.1. Virginia Department of Conservation and Recreation, Division of Natural Heritage, Richmond, VA.

[2] For an explanation of the process to create the communities, see Vegetation Ecology of the Potomac Gorge, which also includes an assessment of the actual communities present in the Gorge.