Dry Oak – Pine Forest Ecological System
The Dry Oak – Pine Forest Ecological System in Rock Creek Park occurs on hilltops, ridges, and upper and middle convex slopes—areas where the shape of the land promotes runoff of rainwater, creating dry conditions.
Natural Communities in This Ecological System
Natural communities in this ecological system are adapted to dry, infertile soils and occasional forest fires. Relatively few plant species thrive in this challenging environment: look for dry-site oaks, heath-family shrubs, and a noticeable lack of American beech.
Tip: to see ecological systems on the interactive map of Rock Creek Park, use Layers On/Off to switch the main data layer to Ecological Systems.
Images of This Ecological System
The Dry Oak – Pine Forest Ecological System is found on the highest and driest sites in Rock Creek Park: hilltops, ridgetops, and upper slopes. These areas tend to be dry because of the elevation and because of exposure on all sides to sun and wind.
In some places, the parent material of the soil in this ecological system is bedrock and in other places, it is ancient river deposits. In Rock Creek Park, many hills are topped with rounded river cobbles and gravels that were deposited long ago by an ancient river. Ecobit: Ridgetop River Terrace Riddle
Where to See It in Rock Creek Park
Natural Processes That Maintain This System
Natural processes are responsible for maintaining the character of an ecological system. Important factors in the Dry Oak – Pine Forest Ecological System are extreme dryness and exposure. In addition, natural disturbances help to maintain the health of this system, including storms and forest fires.
Occasional fires in these dry to extremely dry forests have helped to maintain a characteristic combination of plant species by favoring fire-adapted species. Mature chestnut oak, for instance, has thick bark that protects it from most fires. The heath-family shrubs in this drought-prone ecological system have flammable foliage and stems, high in natural oils. They quickly burn to the ground in a fire, but sprout up again from the roots.
Historically, fires were started by lightning and by Native Americans. Higher elevation areas are more likely to get struck by lightning, and because fires move upslope, they are also likely to burn when a fire starts at lower elevations.
Studies suggest that Native Americans set occasional low-intensity fires in this ecological system for a number of reasons: to promote nut and fruit production by chestnut oak and American chestnut trees; to clear out the understory of shrubs for easier hunting; and to clear land for corn cultivation or for small settlements. Occasional fire kept fire-sensitive species such as American beech and red maple from becoming established, and provided patches of exposed soil and sunlit openings in the canopy.
Ecological Threats to This System
Now that fire is suppressed, American beech and red maple are occurring more often in the understory of the Dry Oak – Pine Forest Ecological System in Rock Creek Park.
Over-Browsing by Deer
Another threat to this ecological system is over-browsing by white-tailed deer, whose population has exploded in the park due to a decline in both natural predators and hunting by humans. It’s normal for animals to eat plants, but the presence of too many herbivores can cause problems. In general, deer don’t seem to like American beech seedlings as much as oak seedlings (with the exception of chestnut oak). This taste preference has the potential to change the composition of this ecological system, especially in combination with fire suppression. In the future, the canopy may contain American beech and fewer species of oak. Read more about the population dynamics of deer in Ecology Basics.
Non-Native Invasive Plants
The Dry Oak – Pine Forest Ecological System is relatively resistant to invasion by most non-native plants due to the dry, rocky, acidic soils, lack of sunlight on the forest floor, and scant rooting space because of colony-forming native heath-family shrubs. However, some non-native invasive plants can make inroads even on these drier hilltops, including
Defoliation of oaks by the non-native gypsy moth is another potential stressor to this dry oak ecological system. Read more about the gypsy moth in Ecology Basics.
It is likely that American chestnut was once a familiar canopy tree in the Dry Oak – Pine Forest Ecological System, but all mature trees have been eliminated by the chestnut blight fungus. (Read more about chestnut blight in Ecology Basics.) With the removal of American chestnut from the canopy of these forests, dry-site oaks appear to have increased.1 2
Finally, this system is also susceptible to erosion caused by foot, hoof and paw traffic in areas of heavy use, and especially near steep slopes.
Range Beyond Rock Creek Park
Primarily occurring in the low-elevation to mid-elevation Central Appalachians and Piedmont from New England south to the Roanoke River in southern Virginia, the Dry Oak – Pine Forest Ecological System is at the eastern edge of its range at Rock Creek Park, where it extends slightly into the Coastal Plain.
- 1. . 1973. Distribution of major forest species in southeastern Pennsylvania. Ecological Monographs. 43:303-327
- 2. . 1978. A study of forest communities in southern Chester County, Pennsylvania. Proceedings of the Pennsylvania Academy of Science 52:37-44.