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An ecological community is a group of species that are typically found together, because they have similar requirements in terms of climate, soil, and other aspects of their habitat. Communities are a fuzzy concept, as every species’ requirements are slightly different and habitats exist along a spectrum rather than in tidy bins. Still, if you’ve glimpsed a shale barren in the Ridge and Valley, an oak-hickory forest in the Piedmont, or a cypress swamp on the Coastal Plain, you’ll agree that certain groupings are distinctive and predictable enough to deserve a name. Defining and mapping the communities of an area can be a highly rewarding exercise, even if it’s never an exact science.
Such an exercise generally uses plant species in its community definitions, as these are the organisms that are most closely tied to environmental factors and therefore—often, but not always—the most predictable in their distribution. Still, it’s good to keep in mind that a distinctive plant community usually hosts a distinctive assemblage of animals as well. Sometimes an animal species is strictly limited to a particular community, such as the Virginia northern flying squirrel in spruce forest.
Here are some of the key environmental factors that control the development of natural communities. For the most part, each of the communities defined in our area can be found on sites with a unique combination of these factors.
Soil moisture. Availability of water in the soil is one of the most fundamental influences on plant growth. A mesic habitat has a well-balanced supply of soil moisture throughout the growing season, while in a xeric (dry) habitat plants are often stressed by lack of water. Hydric habitats, generally better known as wetlands, are at least occasionally saturated or inundated with water (more on these below). Some communities are associated with more complex moisture regimes, such as the hardpan forests that develop where impermeable clay layers limit water access at times and pool water at others.
Elevation is the most obvious factor impacting moisture availability, insofar as it reflects the distance from the surface to the groundwater table. On a typical quartzite ridge near my home in the inner Piedmont, for instance, communities follow a straightforward gradient of elevation and moisture: mesic mixed hardwood forest on the lower slopes, acidic oak-hickory forest (submesic to subxeric) on the middle slopes, and oak-heath forest (xeric) on the ridgetop. However, other factors like soil texture can have a significant impact as well. Even sites near sea level can be classified as xeric, if they have sandy soils that drain water very freely.
Soil/rock chemistry. Calcium, magnesium, and other bases are vital nutrients for plant growth. Different types of rock can translate into different degrees of availability of these nutrients in the soil. Rocks and soils with ample quantities of bases are often called basic, although this isn’t always reflected in soil pH in a straightforward way. These often support plant communities that are more lush and diverse, especially in the herbaceous layer (wildflowers, grasses, etc.).
In the Ridge and Valley region the most common basic rocks are the limestone and dolostone found in the valleys, although calcareous shales and sandstones also occur; all of these sedimentary rocks are rich in calcium from marine life in the Paleozoic Era. In the Blue Ridge and inner Piedmont, basic rocks are mostly igneous or metaigneous; these so-called mafic rocks like metabasalt and amphibolite were originally formed from magma originating deep in the Earth’s crust. On the Coastal Plain, basic soils are found where ravines expose shell deposits from the Tertiary Period.
Acidic soils are formed from sedimentary and metasedimentary rocks like sandstone, shale, quartzite, and slate; igneous and metaigneous rocks formed from shallower magmas, like many granites and gneisses; and unconsolidated sands and clays of the Coastal Plain. These soils are generally less fertile and support a less diverse flora. More unusual chemistry can be found in some exceptional settings, such as the highly acidic, organic peat soils formed from decomposing vegetation in the Great Dismal Swamp, or the magnesium-rich soils formed over soapstone and other ultramafic rocks in the Piedmont. These distinctive habitats develop equally distinctive plant communities.
Soil depth. Some sites have soils that are shallow enough to constrain root growth (especially for trees) and limit the availability of water and nutrients. These are often found on steep slopes, but also occur on flatrock outcrops, on hardpan soils, and under other special conditions. On the modern landscape these are the sites that tend to develop woodland, barren, and stunted forest communities.
Elevation. Elevation has an obvious relationship with moisture availability, as discussed above, but it also has a relationship with climate. Over most of our area the elevation range is too small for this to be a major factor. At the highest elevations (above about 3,200’), however–especially on exposed, north-facing sites–the climate can be cold enough to significantly impact the survival of plant species. Some of these sites serve as refuges for plants more characteristic of the far north, which were effectively “stranded” here when the Ice Age glaciers receded and the climate warmed.
Fire. This is perhaps the hardest environmental factor for us modern observers to appreciate. At the time of European settlement,wildfire would have had a significant and complex influence on many of our plant communities. Some fires would have been started by lightning strikes; these were the primary shaper of the longleaf pine woodlands and savannas of the southeastern coastal plain, where vast areas of low relief experience frequent growing-season thunderstorms. Many other fires would have been set by Indigenous peoples, who used fire to manage and hunt game, maintain transportation routes, and clear areas for agriculture. This human activity helps to explain the large areas of prairie and woodland encountered by seventeenth-century explorers of the Piedmont. On a landscape featuring frequent wildfires with little to impede their spread, fire-intolerant plants like beech and tulip-tree would have been restricted to ravines and other sheltered sites, while fire-adapted species would have thrived in the more exposed uplands. This latter group ranges from thick-barked, fire-tolerant hardwoods like blackjack and chestnut oak to conifers that depend on fire for regeneration (e.g. table-mountain and pond pines with their serotinous cones that open under high heat). Some entire communities had features that made them more fire-prone and thus self-perpetuating, as with the thick layers of flammable duff that accumulated in pitch pine woodlands of Highland ridges and white-cedar swamps of the Dismal Swamp.
Flooding regime. Wetland communities may experience either saturation or inundation (flooding) by water. The former applies to communities like the seepage swamps and bogs that develop where groundwater emerges along slopes, as well as the wet flatwoods communities of low coastal plain sites where groundwater is very near the surface.
For wetlands that experience flooding, the depth, duration, and frequency of that flooding are among the principal factors determining community composition. Flooding may be seasonal, inundating wetlands in the winter and early spring; this is the case for many bottomland hardwood forests on river floodplains, as well as many sinkhole and depression ponds. It may be rarer and briefer, only occurring during exceptional floods, as with higher terrace or levee forests. It may be deep and semipermanent, as with some cypress-tupelo swamps, or fully permanent, as with the aquatic beds in lakes and rivers. Finally, many riverine wetlands below the Fall Line experience a tidal pulse of 2-3 feet that floods and then exposes them twice daily.
Salinity. Most larger streams in Tidewater Province, including the entire James River below Richmond, are technically part of the Chesapeake Bay estuary: an area where fresh water from rivers mingles with salt water pushed upstream by tides. Wetland communities here are subject to a gradient of salinity, with saltier sites supporting an increasingly limited and specialized set of plant species. The upper tidal James, lower Appomattox, and middle Chickahominy experience tidal freshwater conditions, with salt concentrations less than 0.5 parts per thousand. The middle tidal James (including Jamestown Island) and lower Chickahominy experience oligohaline (brackish) conditions, with salinity between 0.5 and 5 ppt. The lowermost James, along with the Nansemond and Elizabeth rivers, experience mesohaline and polyhaline conditions from 5 to about 21 ppt—which, while still considerably less salty than seawater, are enough to support communities known as “salt marshes.”
Even if you can’t identify a single plant, you’ll be able to pick up on some of the community differences that these factors produce. The most obvious are differences in physiognomy, or vegetation structure. A few useful terms:
- Forests are communities in which the tree canopy is mostly or fully closed (greater than 80% cover according to some definitions).
- Woodlands are communities with partial tree cover (30%-80% cover).
- Savannas are areas of mostly herbaceous (non-woody) vegetation with scattered trees (less than 30% cover).
- Shrublands are communities dominated by shrubs. Shrub is a term with multiple definitions, but in ecology it usually applies to woody plants under six feet in height with multiple stems.
- Grasslands, often also known as prairies, are communities lacking trees and dominated by herbaceous cover.
The term barrens is a loose one with no strict ecological definition, but in our area it usually applies to communities with patchy shrub or herbaceous cover interspersed with significant areas of exposed rock. - Among wetland communities, the informal term swamp is often used for those dominated by woody plants, while marsh is used for those dominated by herbaceous plants.
- A seep or seepage swamp is a wetland that is saturated by groundwater, often where a clay soil layer forces water to the surface along a slope.
- A couple of other terms are commonly used to distinguish between seepage wetlands. Although our area lacks bogs in the strictest sense (rainwater-fed peatlands), that term is used more loosely here to refer to flattish, open seeps where the soil is acidic and the vegetation features abundant peat moss. These communities can be found in all three provinces of Jamesland. Fen, by contrast, is used to denote saturated herbaceous or shrub wetlands over base-rich soils; these are restricted to limestone substrates of the Valley and Ridge region.
Human activity has had a profound impact on our landscape, and the vegetation in many areas today is dramatically different from how it looked at the time of European settlement. Although the past few hundred years have seen unprecedented types and degrees of change, it’s important to recognize that the notion of an unbroken “forest primeval” greeting the first colonists is a myth. The reality is that humans have been shaping their environment since they first arrived in North America some twenty thousand years ago—and that the environment was already then in a highly dynamic state as it adapted to the end of an ice age. So when we assess the ecological impacts of the modern era, we are not using some imaginary “natural” or “climax” state as our baseline, but simply reconstructing as best we can the patterns that preceded these. A good case can be made that those precolonial dynamics were more resilient and sustainable, but their product was not a pristine wilderness—it was something far more complex and interesting.
Suppression of natural wildfire, and elimination of Indigenous prescribed fire practices, have been among the most impactful ways that modern humans have transformed Jamesland’s ecology. Seventeenth-century explorers encountered vast areas of woodland and savanna that were sufficiently clear of undergrowth that one could easily ride a horse through them for many miles. Fire suppression has helped to replace most of that landscape with a forest choked with dense thickets. It has extirpated or virtually extirpated entire community types from the nation, including the canebrakes and longleaf pine savannas of the coastal plain, the prairies and woodlands of the Piedmont, and the seepage bogs of both provinces. More subtly, it has diminished the importance of fire-adapted species like shortleaf pine, while clearing the way for plants more characteristic of mesic ravines and high floodplain terraces to invade the uplands.
In place of fire modern humans have substituted other forms of disturbance. Logging is one that is familiar to us all. Most of the forests of the Coastal Plain and Piedmont had been removed by around 1850, in a haphazard process of clearing, farming, and abandonment that left soils eroded and exhausted. Once railroads and other technologies had made it feasible, the mountain forests succumbed (largely to the timber, coal, and iron industries) in a more accelerated rush over the late nineteenth and early twentieth centuries. The forests that replaced them were not the same. Instead they were full of adaptable, fast-growing, light-loving “pioneer” trees like tulip-tree, sweetgum, red maple, red cedar, and loblolly and Virginia pines. In many of these second- or third-growth stands, longer-lived, more shade-tolerant species like oak, hickory, beech, and sugar maple never get the chance to mature.
The importation of exotic species, whether by accident or by misguided plan, has been another cause of modern ecological upheaval. These days many suburban woodlots, abandoned crop fields and pastures, and the like—some of the communities most frequently encountered by Jamesland residents—are novel, unstable assemblages dominated by a few non-native plants. Among the worst offenders in terms of invading native ecosystems and crowding out the locals are tree-of-heaven (Ailanthus altissima), Callery pear (Pyrus calleryana), Japanese honeysuckle (Lonicera japonica), common reed (Phragmites australis), English ivy (Hedera helix), and Japanese stiltgrass (Microstegium vimineum), but the list has grown long. Of the plant species that now maintain wild populations in Jamesland, the proportion of exotics is close to twenty percent.
Even more insidious has been the introduction of non-native fungi and insects that take aim at native plants. The horror stories have steadily piled up over the past century. An Asian fungus called the chestnut blight arrived in New York in 1904, and soon proved nightmarishly efficient at killing defenseless native chestnut trees. It stormed south to Virginia by 1914, and within forty years had brought down virtually all of the American chestnuts in the East. In areas like the Jamesland highlands where chestnut had been among the dominant canopy species, millions of saplings continue to sprout from stumps each year, only to be girdled and felled yet again by the blight before they can reach maturity.
Next in line was Dutch elm disease, a fungus spread by a bark beetle, which appeared on American shores in 1928 and followed a very similar trajectory to its predecessor. This plague wiped out some three-quarters of the elms on the continent over the next six decades, with intensive fungicide and insecticide regimes needed to save trees in places like the National Mall. For yet another invasive pest, the hemlock woolly adelgid, Jamesland boasts the dubious distinction of being the apparent port of entry. An aphid-like insect from East Asia that sucks sap from needles and can defoliate whole trees in the process, the species was first detected at an arboretum near Richmond in 1951. Our region’s highland forests, cool enough to support lush hemlock growth but not cold enough to kill off the pests, have suffered some of the most devastating impacts. An estimated 95 percent of the hemlocks of Shenandoah National Park have already succumbed, along with many of the mightiest specimens in old-growth refuges like Ramsey’s Draft Wilderness.
New menaces continued to rear their heads in the new millennium. The emerald ash borer, a shiny green beetle whose larvae tunnel beneath the bark of green, white, and pumpkin ashes, debuted in Virginia in 2008 and has since spread nearly throughout the state. In the National Park Service’s properties throughout the greater D.C. region, roughly four out of five mature ash trees have been killed. Meanwhile our beeches may be next on the chopping block, as not one but two southward-spreading epidemics affecting this species (beech bark disease and beech leaf disease) were spotted in northern Virginia within the past few years. Much work is being done to try to stem the tide of these plagues and bring back our beleaguered trees, including efforts to breed new resistant varieties, and to import the pests’ natural enemies as agents of biological control. But our forests are unlikely to regain their composition as it was before globalization unleashed ecological bedlam.
Jamesland boasts a wealth of identifiable communities, most of which have been intensively studied and described by ecologists working for organizations like Virginia’s Department of Conservation and Recreation. The list can be somewhat overwhelming, but it can be broken down in several ways. For starters, each of the nation’s three provinces (Highland, Piedmont, and Tidewater) offers a fairly distinct environment, and has a set of communities that overlaps little with those of the other provinces. From there, physiognomic (structural) and moisture-related descriptors are especially helpful in grouping communities into recognizable categories. We have used a combination of these criteria to organize our community list. Use the links below to jump straight to the corresponding sections:
- Drier Forests
- Mesic Forests
- Woodlands, Grasslands, and Barrens
- Forested Wetlands
- Herbaceous/Shrub Wetlands