The parks of the National Capital Region Network (NCRN) are a fascinating collection of natural areas that fall within the immense Eastern Deciduous Forest ecosystem.
North America's Eastern Deciduous Forest ecosystem stretches over 26 states from Florida up to New England and southern Canada, and extends as far west as Texas and Minnesota. These forests are dominated by broad-leafed trees that shed their leaves annually (deciduous), with evergreen cone-bearing seed plants (conifers) such as pines and hemlocks common in some areas.
The Eastern Deciduous Forest once occupied about 2,560,000 km2 (Delcourt and Delcourt 2000). For thousands of years its unbroken expanse was dominated by oaks, chestnuts, and hickories that provided valuable food and shelter for wildlife. The 18th and 19th centuries brought unprecedented changes as forest was cleared for agriculture, timber, fuelwood, and urban expansion.
Despite these and many other challenges, parts of the Eastern Deciduous Forest in the southern and Mid-Atlantic U.S. are recognized as high priorities for worldwide conservation. Two parts of the Eastern Deciduous Forest in the NCRN, the Southern Coniferous and Broadleaf Forests and the Appalachian and Mixed Mesophytic Forests, were chosen as part of the "Global 200" list of areas important for global conservation. These areas were selected for recognition for their high biodiversity and their large area compared to similar forests worldwide.
The Eastern Deciduous Forest is a type of "temperate deciduous forest." These forests occur across the world in the mid-latitudes (between the tropics and the polar regions) in western Europe, eastern Asia, southwestern South America, and the eastern U.S. They are distinguished by warm and cold air masses that cause four distinct seasons a year. Trees change color and lose their leaves in fall as temperatures and precipitation levels drop. Winters have low precipitation levels and cold temperatures, and trees and other plants are inactive. In spring, temperature and precipitation levels rise causing plants to break dormancy and new growth and flowering to burst forth. In summer, plants of temperate deciduous forests grow the most, fueled by the warmest temperatures and highest precipitation levels of the entire year.
Geology and TopographyThe vast majority of NCRN lies within the Potomac River watershed. The Potomac is the second-largest tributary of the Chesapeake Bay—America's largest estuary. The parks of the NCRN have a diverse array of natural forest communities determined in part by a wide range of underlying soils, geology, and topography. In the eastern U.S., these foundational groupings typically occur as north-south bands parallel to the Atlantic coast and the Appalachian Mountains.
The easternmost portion of the NCRN, the Coastal Plain is the mostly flat and low lying area between the Atlantic Ocean and the foothills of the Appalachian Mountains. It is made from layers of sediment primarily eroded from the Appalachian Mountains and deposited by rivers as sediment over the Coastal Plain.
The Piedmont occupies the Appalachian foothills west of the Coastal Plain. The rock underlying the surface, or bedrock, changes from softer, mostly sedimentary rock in the east to harder, mostly metamorphic rock in the west. This transition occurs in a zone called the "Fall Line." It gets this name as it is marked by a series of waterfalls and rapids as water moves from higher elevation in the Piedmont to lower elevation in the Coastal Plain in the Potomac Gorge at Great Falls Park (Virginia) or C&O Canal National Historic Park (in Maryland).
NCRN's Prince William Forest Park is the largest National Park in the Piedmont region. Other parks within the Piedmont include George Washington Memorial Parkway, C&O Canal National Historical Park, Manassas National Battlefield Park, Monocacy National Battlefield, Rock Creek Park, and Wolf Trap National Park for the Performing Arts.
To the west of the Piedmont is the narrow Blue Ridge region. It encompasses the Blue Ridge Mountains, the easternmost part of the Appalachians. Its rocks are the oldest in the National Capital Region. They formed over millions of years as an ancient ocean floor was folded, smashed, and pushed up into the Appalachian Mountains, which at the time soared to heights resembling the modern Himalayan range. Erosion during the following 265 million years cut down the once towering mountains to their modern elevations, and exposed rocks that were once buried deep within the core of the mountain belt. The Blue Ridge is typified by steep terrain covered by thin, shallow soils.
The westernmost physiographic region in the NCRN is the Ridge and Valley, which makes up part of the Appalachian Mountains. Its parallel ridges are made of rocks that are more resistant to erosion than those of the valleys. Therefore, the ridges remained as high points on the landscape, while the valleys were eroded into gently sloping lows. Areas with carbonate bedrock formations (e.g. limestone) have "karst" topography that can include sinkholes, underground streams, and caves.
Forests Characteristics and Species
The Eastern Deciduous Forest canopy is dominated by oaks (Quercus), hickories (Carya), tulip poplar (Liriodendron tulipifera), American beech (Fagus grandifolia), "hard maples" (Acer spp.), and basswood (Tilia). Important conifers are eastern hemlock (Tsuga canadensis) and eastern white pine (Pinus strobus). All of these are species common in the NCRN except for eastern white pine and eastern hemlock, which is in pest- and disease-related decline. American chestnut (Castanea dentata), formerly dominant throughout the Eastern Deciduous Forest, was virtually eliminated by disease in the early twentieth century.
Based on the the four geologic and topographic areas described above, forests in each NCRN park have somewhat different types of forest and tree species. For example, American holly (Ilex opaca) is common on the Coastal Plain and Piedmont, but largely absent from the more western Blue Ridge and the Ridge and Valley. Chestnut oak (Quercus prinus) is found in many NCRN parks, but is most common in Catoctin Mountain Park and Harpers Ferry National Historical Park, in the Blue Ridge.
It is also possible to look at forests at an even finer scale of "natural communities." For example, Rock Creek Park is a Piedmont forest in a highly urbanized environment. Forests within Rock Creek have been mapped and described in great detail to the natural community level.
Land Use History and Protection
Most of the land in eastern U.S. has been logged or cultivated at some point in the recent past. It is estimated that only 0.1% of this area was undisturbed, 82.6% was human dominated, and the remainder were lands with some disturbance (logging, second–growth forests on abandoned farm fields, etc.). The forests that do remain are often found in smaller fragments. In the NCRN, the forests are typically surrounded by urban or agricultural areas. The surrounding lands have the potential to introduce invasive species or prevent the movement of native species.
Prior to the 1900s fires were common in Eastern Deciduous Forests. In the NCRN, fires were frequent, but low intensity, primarily affecting the understory. Currently, due to fire suppression, fires are less frequent, and this can have profound impacts on forest vegetation. In general, the lack of fire in eastern forests benefits plants that tolerate shade, but are vulnerable to burning. These include maples, American beech, mountain laurel, and many others. Fire suppression is detrimental to species that tolerate fires but not shade. These include oaks, hickories, and pines. Further complicating this situation is the impact of browsing by white-tailed deer (Odocoileus virginianus). Deer are more abundant now than they were historically. This high abundance of deer can change forest community composition, as deer preferentially browse on some tree species, such as oaks and hickories.
NCRN's long–term monitoring is designed to help us understand how our forests are changing. Today, forest fragmentation, overabundant deer populations, exotic species, pollution, fire suppression, acid rain and other changes in atmospheric chemistry, and human-induced climate change all threaten NCRN forests. The American chestnut has been nearly eliminated by an exotic pathogen and ashes are imminently threatened by an exotic borer beetle. Oaks and hickories are in decline while the shade-tolerant beech (Fagus grandifolia) and maples (Acer spp.), which hold less value for wildlife, are becoming dominant. Exotic species, deer, and fire suppression are likely driving these changes. Yet, NCRN parks are still refuge for a wide range of plant and animal life, home to rare natural communities, and offer visitors the opportunity to experience the natural environment in a relatively urbanized area.
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Last updated: December 29, 2017