Fire Ecology in Shenan- Doah National Park
Total Page：16
File Type：pdf, Size：1020Kb
Fire Ecology in Shenan doah National Park GENE WILHELM1 Slippery Rock State College Slippery Rock,PA 16057 INTRODUCTION SHENANDOAH National Park was established by an Act of Congress in 1935 to preserve a representative segment of the scenic Central Appalachian Mountains for the future benefit and enjoyment of the American people. The park is unique among most in the National Park System because of its very origin. Prior to park creation the mountainous region was practically decimated of all natural vegetation and animal life. Likewise the area in the early 1930's was "home" for over 3,000 mountain folk. From almost total degradation then, parkscape protection over a 35 -year period has permitted natural and human scars to heal, so that today the park exists in a more or less natural state. The chief scientific interest in the park is the ecological changes that have taken place since its establishment. Shenandoah National Park presents a rare opportunity for ecologists to study the dynamic processes of natural succession from extensive clearings to dense forests. The primary objective of this report is to indicate the signifi cant role that fire has played in the ecological changes to the park scape in time and space. Field data were collected during nine summers of residence in Shenandoah National Park from 1963 through 1971. Total field time approximated 28 months. Additional information was acquired 1 Associate Professor of Geography and Coordinator of Environmental Sciences, School of Social & Behavioral Sciences, Slippery Rock State College, Slippery Rock, Pennsylvania 16057. 445 GENE WILHELM from park personnel, former park inhabitants, and Alderman Library at the University of Virginia, Charlottesville. Field techniques in cluded establishing study plots in clearings and forests, mapping and photographing plant communities, and recording daily phenology occurrences. THE STUDY AREA From Front Royal to Waynesboro, Virginia, the Blue Ridge Mountains consist primarily of a single high ridge with perpendicular spurs between two adjoining lowlands-the wavelike foothills of the Piedmont Plateau to the east and the Great Valley of Virginia to the west (Fig. 1). The range trends in a northeast to southwest direction and nowhere exceeds 15 miles in width (Connelly 1968). In its narrower pans, as between Afton and Waynesboro, it is a single ridge that stands about 2,000 feet above the Piedmont and 1,200 feet above the Great Valley (Wright 1927). VIRGINIA MAIN REGIONS o TIDEWATER @FALL LINE [[] PIEDMONT ~BLUE RIDGE o 50 100 t t t MILES FIG. 1. Main regions of Virginia. Adapted from J. Gottmann, Virginia at Mid Century (New York: Henry Holt & Co., 1955). 446 FIRE ECOLOGY I~ SHENANDOAH NATIO~AL PARK Overall,. the main ridge has an average altitude of 3,000 feet above mean sea level. Its mid-section forms a slight arch, the crest of which averages 3,400 feet. The highest peaks in the study area, Hawksbill Mountain (4,049 feet) and Stony Man Mountain (4,010 feet), are in this part of the range (Anderson 1935). The rounded Blue Ridge Mountains record more than a billion years of earth history. They consist basically of very old crystalline rocks (pre-Cambrian granites, granodiorites, and schists), which oc cur as uplands, and younger sedimentary and metamorphic rocks (Cambrian limestones, shales, and quartzites), which form low hills and valleys in a narrow zone on the Shenandoah Valley side. Catoc tin greenstone schists, formerly massive flows of metabasalt (Keith 1894), cap most of the summits and ridges of the Blue Ridge and are well exposed in gaps and highway- cuts between Rockfish Gap (Waynesboro) and Chester Gap (Front Royal). The essentially continuous exposure along the western side of the ridge has allowed weathering and erosion to break through to the underlying intrusives in the vicinity of Hughes River and Thornton Gaps. On the eastern side of the ridge outcrops are generally confined to the mountain summits. The main ridge forms a backbone divide between the headwaters of streams that follow an easterly course over the Piedmont and Coastal Plain to the Atlantic Ocean, and those that flow westward into the Shenandoah River to reach, eventually, the Atlantic via the Potomac River. The drainage descent is precipitous on both sides of the main ridge, resulting in the formation of short canyons, nu merous waterfalls, cascades, and rapids. In fact, topography is rugged throughout the region, with less than 10 percent of the surface area having a slope less than 10 percent. The Blue Ridge, with numerous streams, cascades, springs, water falls, and dense forests, superbly illustrates a humid mid-latitude landscape. The mountains have a variable climate due to their length, relief variance, and daily to monthly changes in atmospheric condi tions. Seldom does the entire range experience the same daily at mospheric phenomena. Further, seasons contrast considerably more in the amounts and kinds of precipitation, in the number of sunny 447 GENE WILHELM and cloudy days, and in the extent and intensity of heat and cold. Weather stations at Luray (5 miles east of town at Shenandoah National Park Headquarters, elevation 1,000 feet) and at Big Mead ows (10 miles southeast of Luray, elevation 3,500 feet) give some indications of general climatic conditions in the study area. Data collected at these stations are available for a 35-year period (1935- 1969) and have been analyzed in Table 1. Soil is one of the most obvious factors which determines the extent of plants and animals in the Blue Ridge, yet little scientific attention has been given to it. Perhaps the reason for this is that most of the study area is under National Park Service control and is primarily considered to be a recreational zone with few permanent inhabitants. The result, nonetheless, is the lack of a detailed soil survey and soil map of the study area. Basically there are two soil orders or subdivisions in the Blue Ridge Mountains (Marschner 1959): 1) zonal soils, which formed under stable conditions through the prolonged action of climate and vegeta tion, and 2) azonal soils, which have no or poorly developed charac teristics, either because they have had insufficient time to develop, or because they are on slopes too steep to allow profile development (Strahler 1960). Blue Ridge zonal soils are the gray-brown podzols of the great soil groups. They are best represented in the study area by three types: 1) Porters Loam, 2) Porters Stony Loam, and 3) Mixed Silty, Sandy, and Stony Loams. Together these three soil types constitute about 90 percent of the local soils and have de veloped under forest cover mainly of hardwoods, although condi tions were not favorable for the accumulation of vegetable matter. Two azonal soils, alluvium and stony colluvium, make up the re mainder of area soils and have formed in the hollows and coves. Under normal conditions these soils are well-drained and higher in fertility and productivity than stony loam soils. NATURAL VEGETATION The term "natural vegetation" is sometimes thought to refer to the original vegetative types, which in the Blue Ridge Mountains belong to pre-Columbian times. However, very little of the present 448 TABLE 1. CLIMATIC DATA FOR Two WEATHER STATIONS IN THE BU:E RIDGE: 1935-1969 1\lonthly Means Station Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Annual Totals "l Big Meadows ~ t%j Temp. 28.8 30.0 36.6 48.9 56.8 63.4 66.7 65.4 59.8 51.4 39.6 30.0 48.1° F. 8 Prec. 3.37 3.15 3.72 4.14 4.18 4.36 4.58 5.63 4.86 4.68 4.09 3.34 49.05 in. § Luray ><: .... Temp. 34.4 36.3 43.3 53.8 60.9 70.0 73.2 72.2 65.7 56.3 45.6 35.4 56.7° F. Z Prec. 2.65 2.40 3.19 3.11 3.66 3.46 3.86 4.59 3.37 3.36 2.89 2.55 38.72 in. en ::t: t%j SoVRCE: U. S. Department of Commerce, Weather Bureau, Climatological Data on Virginia, Annual and Monthly Sum- Z maries (Washington, D. c.: Government Printing Office, 1935-1969). ~ ~ ~ S r<~ ~ ~ ~ '" ~ GENE WILHELM mountain vegetative cover (about 1 percent) belongs to these types (Berg and Moore 1941). Profound changes in most of the original plant associations were caused by natural events, such as lightning caused fires and flash floods, and by aboriginal and European activi ties, such as firing the land, cutting of trees, plowing, and grazing. Natural vegetation as used in this study, therefore, implies all vege tative types that grow without cultivation and include the few rem nants of the native vegetation still existing in the Blue Ridge Mountains. The key to an understanding of the original Blue Ridge vege tation as a whole, and the spatial relations of its integral parts, is obviously to be sought in those areas where ancient vegetative types have survived, despite dire natural and human events in the past (Braun 1964). In some parts of the Blue Ridge, like Shenandoah National Park, landscapes have made a complete cycle from native vegetation, throul!h almost total denudation, to natural environ mental renewal (Wilhelm 1968). It is important to examine in this report each one of these stages of change. Emphasis is placed on the higher forms of vegetation, the forests, because of their ecological dominance in the landscape. THE NATIVE BLUE RIDGE FOREST The Blue Ridge Mountains have been occupied by the deciduous or summer green forest formation certainly for thousands of years (Braun 1964). There have been repeated changes during its long period of existence in land relief, climate, water supply, and soils.