Harpers Ferry National Historical Park (NHP) provides a multitude of habitats for plants and animals—rocky heights, sheltered ravines, fertile floodplains, islands in the rivers, and more. These different settings are all created by interactions among abiotic (non-living) factors including climate, soils, geology, topography, and watersheds. (Plants also affect the physical setting. See Natural Processes for more.)
Precipitation, wind, and temperature—the agents of climate—help sculpt the landscape. Rain and wind wear away soil and rock and transport them elsewhere—downhill or downwind. Water seeps into cracks in bedrock, allowing plants to take root. In the winter, the water expands when it freezes, enlarging the cracks.
In Harpers Ferry NHP, winters are cold and snowy, and summers are hot and humid. The average low in winter is 27 degrees F, and snowfall averages 20-25 inches. The average high in summer is 82, and thunderstorms are common. Precipitation averages 38 inches a year.
Extreme weather events can lead to life-threatening floods in and around Harpers Ferry. When floods occur, the steep topography in this area gives floodwaters little space to spread out. Instead the waters can rise quickly and dramatically, on occasion damaging or destroying bridges, roads, buildings, and historical sites, and inundating natural communities that are well beyond the reach of typical flooding. In 1996, two floods of more than 29 feet occurred in one year for the first time in the history of the town of Harpers Ferry. 1 One was caused by a blizzard in January, and the other by Hurricane Fran in September.
Soils provide plants a place to sink their roots, a way to hold moisture, and a source of nutrients. In Harpers Ferry NHP, as everywhere, different types of soils support different types of plants.
Most of Harpers Ferry NHP’s natural communities grow in mineral soils weathered from bedrock. Much of the bedrock is acidic in the sense that it contains minerals with few available base elements like calcium or magnesium that act as plant nutrients.
Because most soils are acidic to some degree, ecologists often reserve the term “acidic” to describe very acidic soils, and “basic” to describe soils that are relatively less acidic. Truly basic soils, with a pH greater than 7.5, are nearly impossible for many plants to grow in.
In general, strongly acidic soils are considered infertile (containing little that plants can use), and less acidic or nearly neutral soils are considered fertile.
Two of the natural communities in Harpers Ferry NHP that thrive in these less acidic soils are the Limestone Oak – Hickory Forest and Red-cedar Limestone Glade. As their names imply, these communities are found in areas where the bedrock is limestone, which is full of calcium. As limestone weathers, it creates a nearly neutral soil that is rich and fertile.
What does geology have to do with natural communities? More than you might think. In Harpers Ferry NHP, there are many different kinds of bedrock. Some rock types are resistant to erosion and tend to form steep ridges like Maryland Heights. Other types wear away more easily and form valleys. Some bedrock contributes lots of nutrients, useful to plants, to soil, while other bedrock doesn’t. These variations in elevations and soil fertility influence where particular natural communities can be found.
The topography in Harpers Ferry NHP is dramatic. The town of Harpers Ferry itself sits on a low point of land at the confluence of the Shenandoah and the Potomac. Practically anywhere you look from town, you’ll see steep slopes or cliffs. The variety in the topography creates patches of landscape that are especially shady and cool. . . or sunny and hot . . . or exposed and dry. . . or sheltered and moist. These microclimates can help explain why specific natural communities are located where they are.
A watershed, or river basin, is all of the land whose runoff flows into a particular stream or river. Watersheds of different sizes support different kinds of natural communities. Rivers in large watersheds tend to have bigger floods than creeks in small watersheds. Some natural communities can tolerate bigger floods, while others do better with smaller floods.
Harpers Ferry NHP is in the Potomac River watershed.2The precipitation that falls in the park joins the Potomac River and eventually empties into the Chesapeake Bay near Washington, D.C. (This is not counting the precipitation that is soaked up by plants, lapped up by animals, that filters down to become groundwater, or evaporates before reaching the river!)
2.National Capital Region Inventory and Monitoring Program, University of Maryland Center for Environmental Science.
2006. Harpers Ferry National Historical Park.
