Rock Creek Park Natural Communities and More

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Walking the trails of Rock Creek Park, it is easy to be taken with its wooded beauty. Whether or not you know anything about its plant communities, it is gorgeous greenery. But, as with all things in nature, if you look more closely, you will see layer upon layer of fascinating patterns.

What accounts for these patterns in Rock Creek Park? What can we learn about how things in nature interact with each other? What clues can we piece together that will narrate part of the story of Rock Creek Park’s natural communities?

Patterns

There are both subtle and bold patterns among the forests through which the park trails curve and climb. If you are familiar with any species of the trees, shrubs, or other plants around you, you may notice the abundance of some of those plants on certain parts of the landscape and their conspicuous absence on others. If you observe closely, you can find groups of plants that occur together regularly when certain environmental conditions are met.

Those groups of plants, in patterns that repeat themselves across the landscape of a region, are called “natural communities.” Ecologists study and name these communities—the Chestnut Oak / Mountain Laurel Forest, or the Red Maple Seepage Swamp, for instance. These natural communities function as essential habitat for wildlife species. Some animals may be found in most natural communities, some in only one or a few.

Some natural communities share broad characteristics and can be grouped together in what’s called an ecological system. An example is a River Floodplain Ecological System consisting of natural communities found on floodplains.

The more we learn about natural communities, the more fascinating they are—and the more precious, as we discover the roles they play in nature.

Explore these patterns:

Natural Communities

Ecologists recognize eight distinct natural communities that you can find along the trails at Rock Creek Park—some in the hills, some in the valleys. What parts of the landscape does each inhabit, and why? Explore each one below.

Sometimes the vegetation you see in Rock Creek Park will not look like any of the natural communities described above. Perhaps you are in a semi-natural community, a not-so-natural community, or a transition between communities. Find out more!

Ecobit: Natural Community Misfits

Natural Community Misfits (within a park)

(or, Places That Don't Fit the Natural Community Descriptions)

Sometimes what you're seeing along a trail doesn't look like any of the natural communities mapped locally. Why?

• Transitions (Ecotones). Usually, there’s not a sharp and obvious boundary between one natural community and another, even though the vegetation map makes it look like there is. The area of transition between two natural communities is called an ecotone, and it’s an area that may contain a combination of features from more than one natural community. Don't be surprised to find yourself in one of these transitional areas quite often as you hike up and down hills on a trail.
• Semi-natural communities. Some areas are still growing back from historic land disturbances (say, farming, intense fire, or unusually heavy storm damage). Read about the different semi-natural communities in this park, and see if one fits.
• Mapping limits. Rules limit how detailed maps are. Most maps don't show tiny patches of unique vegetation (say, under 1 acre or 1/2 hectare).
• Undiscovered natural communities. Occasionally, new finds lead ecologists to define a new natural community for a park.

So if what you're seeing doesn't fit one of the natural communities described, why do YOU suppose that is?

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Semi-Natural Communities

Rock Creek park has many acres of semi-natural communities, which are plant communities growing in areas where the soil and/or vegetation was substantially disturbed in the past by a severe natural disturbance, or human activities such as plowing, logging, or quarrying.

Because semi-natural communities are an integral component of the Rock Creek Park landscape, they are included in the Rock Creek Park Map, and are briefly described here. The vegetation in these areas is relatively young and is giving way (or succeeding) to other, more natural communities. As vegetation in a successional community changes over time, there is a turnover in the wildlife that use the area. For instance, grassland birds in an abandoned field eventually give way to woodland birds as the field succeeds to forest.

Ecobit: Forest Succession—Clues to the Past

Forest Succession—Clues to the Past (Mid-Atlantic)

Successional forests can be ideal places to read some of nature’s stories, since this kind of forest can reveal clues to what kind or degree of disturbance the land underwent in the past.

Where a forest was logged with minimal ground disturbance, a blend of various trees, including many of the original hardwood species, may quickly return from stump sprouts or from saplings that were left behind. Non-woody vegetation on the forest floor may recover as well. Trees within a formerly logged forest may vary somewhat in age, especially if the forest was only selectively logged.

Telltale signs of a logged forest include old stumps or multi-trunked trees (multiple sprouts regrown from a single stump). You may also find fairly deep holes in the ground left by rotted stumps, sometimes outlined at the surface by neighboring tree roots that once encircled the old trunk.

By contrast, where intensive fire, plowing, or earth moving has removed not only trees, but also stumps and seed banks (seeds in the soil ready to germinate), an entirely new forest is created.

The new forest is populated by pioneer species whose seeds germinate quickly when they reach bare mineral soil.** These tree seedlings quickly spread their large, juvenile leaves to the sun, casting shade that reduces the ability of some other plants to germinate or thrive on the forest floor.

In the Mid-Atlantic, these forests tend to regrow as even-aged stands of pine and/or tuliptree (or sweetgum in the Coastal Plain). Little by little, as mature trees die, an even-aged successional forest of this type may eventually give way to an uneven-aged suite of trees, shrubs, and herbaceous plants typical of a natural community found in the same environment. This process can take centuries.

Oaks can be clues when piecing together the past. White oak and many other species of oak are not typically found in the canopy of a young forest with a history of severely disturbed soil. Why? The acorns of many oak species are slow to grow, and oak saplings usually end up in the shade of pioneer species, lagging far behind until sunlight reaches them through a canopy gap, sometimes years or decades later.

~~~~~~~~

**Examples of pioneer species in the Mid-Atlantic, quick to germinate on bare mineral soil: pine, tuliptree, sweetgum, black locust, red maple, or sassafras trees.

As vegetation in a successional community changes over time, there is a turnover in the wildlife that use the area. For instance, grassland birds in an abandoned field eventually give way to woodland birds as the field succeeds to forest.

The first three semi-natural communities on the list below have official names and codes because they are familiar patterns across the Mid-Atlantic and are part of an official classification called the USNVC. The last four help describe what is found at Rock Creek Park, but are not regionally recognized semi-natural communities.

Explore semi-natural communities:

• Successional Tuliptree Forest (Rich Type)
• Successional Tuliptree – Oak Forest
• Successional Virginia Pine Forest
• Successional Mixed Deciduous Forest
• Successional Vine-Shrubland
• Planted Evergreen Forest
• Historic Quarry

New leaves of canopy tuliptree (Liriodendron tulipifera)  unfurling in spring. (Foreground: northern spicebush (Lindera benzoin) leaves.) Sam Sheline / CC BY 2.0

Successional Tuliptree Forest (Rich Type)

Although it may look like an intact natural community with its magnificently towering, straight tuliptrees, the Successional Tuliptree Forest (Rich Type) is a semi-natural community because it likely grew up in response to a human-induced disturbance. How do we know that? Tuliptree (also called tulip poplar or yellow poplar) doesn’t produce many seedlings in an established forest. Instead, this species germinates and grows best where disturbance has exposed the mineral soil and produced a large sunlit opening in the forest canopy.

After a forest has been cleared, if tuliptree seeds are present in the seed bank or are blown in by the wind from nearby trees, they are quick to germinate. Tuliptree is often among the first trees to come back on moist soil, often shading out slower-germinating competitors for sunlight. An even-aged stand of pure tuliptree, therefore, is evidence of forest regrowth from some past major disturbance.

At Rock Creek Park, the Successional Tuliptree Forest (Rich Type) grows in areas that may have been cleared and farmed long ago. Upwards of a century has passed since the soil disturbance that created most examples of this semi-natural forest and the regrown forest appears quite majestic in some areas today.

This tuliptree-dominated community can be found occasionally throughout the park on somewhat moist, well-developed soils. The understory is often lush, containing northern spicebush and various herbaceous plants. In some sites, groundwater may supply extra base minerals. These forests tend to be heavily invaded by non-native plant species.

As older tuliptrees in stands of the Successional Tuliptree Forest (Rich Type) die out, they might be replaced by American beech and oak trees. These replacement species can germinate on the shady forest floor, where they stand poised to take the place of tuliptrees in the forest canopy when given the sunlight and space to do so. However, since some tuliptrees have reportedly lived to be 300+ years old, it could be a long time before these stands of pure tuliptree give way to a mix of tree species, unless storms or other disturbances bring down mature trees.1

Abbreviated Common Name: Successional Tuliptree Forest (Rich Type)

Scientific Name: Liriodendron tulipifera / (Cercis canadensis) / (Lindera benzoin) Forest

Scientific Name Translated: Tuliptree / (Eastern Redbud) / (Northern Spicebush) Forest

Classification Code: CEGL007220

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Successional Tuliptree – Oak Forest

At Rock Creek Park, this community is characterized by the dominance of tuliptree with some similarly-aged oak including white oak and northern red oak in the canopy. Non-native invasive species are often dominant in the shrub layer and field layer. This forest commonly has a weedy appearance because it consists of relatively small trees growing close together, and contains an abundance of non-native species in the understory.

The Successional Tuliptree – Oak Forest occurs in small patches throughout the park on land where the vegetation was heavily disturbed—perhaps logged—but the soil was not plowed.

This community is distinguished from the Successional Tuliptree (Rich Type) by the presence of oak in the canopy, and by a low diversity of native shrubs and herbaceous plants. Northern spicebush is not common here. As this successional forest ages, its distinctiveness from the Mesic Mixed Hardwood Forest or other natural communities gradually disappears, except perhaps for its greater abundance of invasive species in the understory.

Abbreviated Common Name: Successional Tuliptree – Oak Forest

Scientific Name: Liriodendron tulipifera – Quercus spp. Forest

Scientific Name Translated: Tuliptree – Oak species Forest

Classification Code: CEGL007221

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Successional Virginia Pine Forest

Virginia pine, like tuliptree, is an early successional tree, but exposed mineral soil is essential for its seedlings to get established, and it can tolerate drier conditions than tuliptree. Through the 1970s, pine trees were frequent components of upland forest communities in Rock Creek Park2, where they may have become established in areas cleared for agriculture or Civil War forts and sight-lines. Many have since died, but some can still be seen near the Rock Creek Golf Course and Horse Center. As the park’s forests age and mature, the Successional Virginia Pine Forest is fast disappearing. When a pine tree dies in the interior of a forest, oak or American beech usually replaces it in the forest canopy.

Currently, Virginia pine trees are more often encountered as part of the Coastal Plain Oak Forest in the Fort Circle Parks, or as individual remnants within other natural communities such as the Mixed Oak / Heath Forest between Bingham Drive and Wise Road in the northern portion of the park core.

Abbreviated Common Name: Successional Virginia Pine Forest

Scientific Name: Pinus virginiana Successional Forest

Scientific Name Translated: Virginia Pine Successional Forest

Classification Code: CEGL002591

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Successional Mixed Deciduous Forest

Successional Mixed Deciduous Forests in Rock Creek Park are of variable composition and age. Most of these stands are a mix of native and non-native sun-loving, fast-growing “weedy” tree species that colonize disturbed ground. Typical native species include black locust trees, tuliptree, elms, box-elder, and poison ivy, and native cherry species. Non-native species include Norway maple, honeysuckle shrubs, multiflora rose shrubs, English ivy, and non-native cherry species.

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Successional Vine-Shrubland

The Successional Vine-Shrubland is dominated by non-native invasive vines and shrubs. Live native trees may be present but they may be sparse and choked by vines.

Ecobit: Escaped Art

Dumbarton Oaks Park was once part of the fabulously planned and meticulously maintained gardens of a splendid estate in north Georgetown, Washington, D.C. In 1940, the owners, Ambassador Robert and Mildred Woods Bliss deeded the formal gardens and hilltop mansion to Harvard University. The lower, "more naturalistic" 27 acres they generously gave to the U.S. government for a park for the American people.

Although the grounds that are now a unit of Rock Creek Park had been intended to maintain the look of a natural woodland, non-native vines such as Japanese honeysuckle and periwinkle had been planted among the natural features as part of the original grand design. Some of these plant species turned out to be aggressive invaders. Moreover, ornamental plantings from the estate’s formal gardens uphill have seeded themselves into this stream valley. By the beginning of the 21st century, meadows and woodlands were infested with non-native vegetation that threatened to keep spreading, including porcelain-berry, bush honeysuckle, and English ivy.1, 2

Volunteers, in partnership with the National Park Service and others, launched efforts to return Dumbarton Oaks Park to a truly naturalistic state. YOU can help with this sustainable restoration plan that began when Dumbarton Oaks Park Conservancy formed in 2010.

• 1. Joseph MD, Fanning K, Davison M. 2000.  Cultural landscape report: Dumbarton Oaks Park, Rock Creek Park.
• 2. THE CULTURAL LANDSCAPE FOUNDATION. 2006.  Dumbarton Oaks Park: Farrand’s garden in the valley.

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Planted Evergreen Forest

Planted Evergreen Forests are areas at Rock Creek Park where evergreen trees appear to have been planted in the past. Evidence of planting may be more or less obscured in today’s landscape depending on the length of time since planting occurred, and how long ago the stand was last thinned of other kinds of trees and shrubs.

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Historic Quarry

The so-called Historic Quarry semi-natural community at Rock Creek Park is dominated by hardy and fast-growing non-native invasive trees, vines, shrubs, and small plants. Not too many native plants can be found growing in these areas that were formerly disturbed by quarrying.

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Not-So-Natural Communities

Lawns, golf courses, and maintained meadows are neither natural communities nor semi-natural communities. We'll call them not-so-natural communities. Sometimes called “cultural vegetation,” these areas are managed and maintained by human intervention. Rock Creek Park has several types of these non-natural vegetation communities.

Maintained Meadows

The National Park Service maintains about a dozen or more small areas as meadows for wildlife diversity. Many creatures, such as field mice and their predators, butterflies, pollinator insects, and grassland birds benefit from openings such as these. These open areas are maintained by occasional mowing. In the past, it’s possible that fires caused by lightning and Native Americans may have created infrequent prairie-like openings in what is now Rock Creek Park.

Picnic Areas

Rock Creek Park has several picnic areas near the creek. While the trees remain typical of floodplain communities, the understory is maintained as lawn by regular mowing.

Golf Course and Other Landscaped Areas

Another category of not-so-natural communities is landscaped areas, such as the Rock Creek Golf Course and any other regularly mowed areas, and plantings around buildings.

Stormwater Retention Ponds

Wetland swales dominated by cattails were created to slow down and filter urban runoff before it enters streams. These human-made wetlands occur northeast of the core park, in the Fort Circle Parks area.

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Ecological Systems

Natural communities don’t exist in isolation, but rather occur in a larger context, called ecological systems. Ecological systems consist of groups of natural communities that tend to occur together and that share certain natural processes, such as flooding or fire. Rock Creek Park’s natural communities belong to four ecological systems.

Ecobit: Natural Processes are Bigger Than Natural Communities

Natural Processes are Bigger Than Natural Communities

Some natural processes affect whole clusters of natural communities.

For example, think of a long high ridge. One natural community on the ridgetop may consist almost entirely of chestnut oak, while another might have a wider variety of oaks and shrubs, but all share some things in common. They all grow in relatively dry soil and are exposed to the elements on all sides—sun, wind, storms—as well as to fires that are the result of lightning strikes. The natural processes at work on this ridge help explain why certain natural communities, and not others, tend to cluster here. We call these clusters ecological systems.

Or think of a large river. There may be many different types of natural communities along the river—a forest of American sycamore and tuliptree, a shrubby willow community, a mucky alder swamp—but what unifies them? They all occur in areas prone to flooding by this river, which exposes them to a whole set of related natural processes. They are all part of one large river ecological system.

Ecological systems—and the natural processes that partly define them—provide a helpful context in which to understand natural communities and help predict what types of plants and animals might thrive there: You wouldn’t expect to find a water-loving, flood-adapted natural community on a dry ridgetop, nor would you expect to find a fire-adapted natural community along the banks of a river.

Ecological systems, with their natural processes, provide a helpful context in which to understand natural communities and help predict what types of plants and animals might thrive there: You wouldn’t expect to find a water-loving, flood-adapted natural community on a dry ridgetop, nor would you expect to find a fire-adapted natural community along the banks of a river.

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• Fleming GP. 2007. Personal Communication re: vegetation communities at Rock Creek Park. Vegetation Ecologist.
• Anderson RR, Mcfaden DM, Kramer RJ, Dee JC, Jones GC. 1977. Rock Creek Park and Rock Creek and Potomac Parkway: vegetation community structure and automated classification of vegetation communities. 1

Part of a series of articles titled
Rock Creek Park Ecology.

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