W. Henry McNab, Southern Research Station, and Steven A. Simon, National Forests in North Carolina. (Prepared for the Chattooga Ecological Classification Guidebook)

The following discussion was excerpted from a document produced by the USDA Forest Service in October 1995 entitled "Ecological Classifications Mapping and Inventory for the Chattooga River Watershed." This study was funded through the Chattooga River Basin Ecosystem Management Demonstration Project, which was implemented by the Forest Service as a direct result of a proposal written by the Chattooga River Watershed Coalition (CRWC). The CRWC's proposal included a request that the Forest Service conduct scientific studies which could be used to guide management of the watershed's national forests based on principles of landscape ecology and conservation biology.

Climate, the statistical expression of daily weather events or the expected weather (Bradley 1985), is one of the most important environmental factors affecting biological relationships throughout the Chattooga River Basin. Historical climate provides insight into the broad-scale distribution of flora and fauna present today.

The current climate provides a basis for understanding the ecological patterns and processes that must be considered in making decisions for resource management. The following discussion presents an overview of past and present climatic relationships in the Chattooga River Basin. This information provides a basis for understanding the relationships expressed in the classification of ecological units.

Climatic History

The earth's climate has changed continually since the land surface formed, about 4.6 million years before present (BP). However, the Quaternary Period, from about 2 million years ago BP to the present, was a time of particularly significant environmental change in the Chattooga Basin. During the Pleistocene Epoch, from 2 million to 10,000 years BP and referred to as the Ice Age, at least four major glacial episodes in the northern latitudes had a profound effect on vegetation to the south. Following the Wisconsin glaciation, which ended about 6,000 years ago, vegetation continued to respond to gradual climatic change. Examination of climatic conditions and vegetation distribution during the recent Quaternary Period provides insight into the current vegetation patterns. The study of climate prior to instrumental measurements must be done indirectly by examination of ice cores, marine sediments and glacial deposits, tree rings and pollen analysis, and historical written records.

One of the most complete studies of Quaternary climate in the Southeastern US was obtained by Delcourt (1979) from an analysis of pollen stratigraphy [the arrangement of rocks in layers or strata] from sediment cores. This site was on the eastern Highland Rim, near Nashville, Tennessee, at an elevation of 1,000 feet. Although this pollen record was recovered about 160 miles northwest of the Chattooga area, the general climate and flora should be similar. The predominant arboreal species by time period are listed below, with an interpretation of the paleoclimate [ancient climate]:

25,000 +/- 3,000 yrs. BP: Cool but not severely cold climate, with sufficient soil moisture to sustain growth; jack pine, spruce, fir, mixture of deciduous trees.

19,000 to 16,300 yrs. BP: Full effects of Late Wisconsin continental glaciation; cool, long winters and short growing season; boreal conifers (spruce, fir, jack pine), some temperate deciduous taxa.

16,300 to 12,500 yrs. BP: Severity of winters diminished, growing season lengthened, progressive warming trend; spruce, fir, oak, ash, ironwood, hickory, birch and elm; sugar maple and beech present about 13,000 yrs. BP.

12,500 to 9,500 yrs. BP: Warm-temperate weather conditions; transition from coniferous to deciduous forest;rapid expansion of mixed mesophytic forest species including oaks, ash, ironwood, hickory, birch, walnut, elm,beech, sugar maple, basswood, hemlock.

9,500 to 5,000 yrs. BP: Warming and drying trend during mid-Holocene; predominance of warm temperature species including oaks, sweetgum, black gum, chestnut.

5,000 to 200 yrs. BP: Increased precipitation; species same as mid-Holocene.

200 yrs. BP to present: Similar climate; ragweed and red maple increased, along with sedimentation from wide spread land clearing following European settlement.

Similar species composition and climatic inferences have been made from pollen records at other sample sites in Georgia, North Carolina, and in northern states. Watts (1980) suggests that low mountain crests in the Southern Appalachian Mountains were not forested during the Late Wisconsin (22,000 - 13,500 BP), and provides the following comparison of climate then and now (in parenthesis) for Columbia, South Carolina: January average temperature 14 degrees F (46 degrees F); July average 68 degrees (81 degrees F); number of frost-free days 114 (248); precipitation 41 inches (42 inches). Whitehead (1973) summarizes evidence from studies of pollen at 14 sites, and in reconstruction of full-glacial vegetation concludes that the Chattooga area consisted of a pine-dominated boreal forest with few deciduous species.

Relatively little is known of the paleofauna [ancient fauna] associated with the climate and flora in this area. Although no fossil discoveries are known in the Chattooga, discoveries in other areas (Arizona and coastal North Carolina) suggest that the American mastodon (Mammut americanum) was present as recently as 8,000 yrs. BP. The mastodon and other large fauna might have been important in extending the distribution of some tree species with large seeds, such as osage orange and Kentucky coffee tree, as climate warmed following the retreat of the glaciers (personal communication, Rodney Snedeker 8/95). Other large ungulates probably included the woodland bison (Bovidae sp.). In addition to climate influences on paleoflora, Native Americans are believed to have had a significant influence on the structure of vegetative communities through their use of fire, which indirectly affected distribution and composition of paleofauna.

Paleoclimatologists believe the earth's climate has gone through a number of similar climatic cycles during the past several million years. The distribution of species responded to each cycle, and continues to show long term changes in climate. The restricted distribution of cool climate communities, such as northern red oak and northern hardwoods, to a small area at the highest elevation in the Chattooga Basin indicates that adjustments in ranges are continuing.

Current Climate

Our knowledge of the contemporary climate of the Chattooga River Basin is based on a relatively short history of observations. Information is available from a thin network of recording sites, usually located at lower elevations in mountainous terrain, and occasional scientific articles that report unusual phenomena. Climate, however, is one of the most important factors affecting ecological relationships in the basin. Temperature influences length of growing season, evapotranspiration [the total water loss from the soil, including that by direct evaporation and that by transpiration from the surfaces of plants], and soil properties. Precipitation quantity and seasonal distribution affect soil moisture relations and stream flow. Many land management practices and concerns, such as stream sedimentation, prescribed burning and forest regeneration are affected by the prevailing climate. Description of the current climate of the Chattooga River Basin will be presented below, following the structure of the Forest Service's national hierarchy of terrestrial ecological units, beginning with subsections and extending through land-type phases.

Current Climate of the Southern Blue Ridge Subsection

The geographic location of the Chattooga Basin in the Southern Blue Ridge Subsection of the southeastern US allows climatic patterns to be controlled by three sources of influence. First and most widespread are subtropical cyclonic disturbances that originate west of the Mississippi River, or in the Gulf of Mexico, and move in a northeasterly direction across the Atlantic states. The second source is from warm, moisture-laden air currents from the Gulf of Mexico that produce high rainfall when they rise and cool along the Blue Ridge escarpment. Third, and most unpredictable, are remains of tropical cyclones known as hurricanes that strike the Atlantic or Gulf coasts and occasionally follow a path near the Chattooga River Basin. Precipitation along the escarpment above the Chattooga Basin is highest in the US east of the Cascade Mountains of Washington and Oregon.

The prevailing climate of the Southern Blue Ridge Subsection consists of cool, short winters, and long, warm, wet summers. Winter weather is largely controlled by continental influences, where cold fronts move from west to east, often bringing large amounts of precipitation and cool temperatures. Polar air masses are responsible for several short periods when temperatures will reach below 10 degrees F. Minimum daily mean temperatures occur during January. Precipitation occurring as snow during mid winter is usually uncommon, happening perhaps only 3 days annually and amounting to a small percentage of the annual total. During later winter, climatic patterns begin to change due to maritime influences as low pressure systems move north-easterly from the Gulf of Mexico and bring smaller amounts of precipitation. Heavier snowfall occurs occasionally as a result of a low pressure front from the Gulf meeting a cold air mass along the Blue Ridge escarpment.

During spring and early summer, weather patterns begin to shift toward control by low pressure fronts from the Gulf, which can bring moderate amounts of precipitation. Thunderstorms are more frequent as air temperature begins to peak; maximum daily temperatures occur during July. During mid to late summer, weather patterns are controlled by high pressure areas, sometimes referred to as the Bermuda high, which blocks warm fronts from the Gulf. Brief to extended intervals of drought can occur during mid to late summer. Occasionally, low pressure cells with high rainfall, but low winds, occur in late summer as a result of hurricanes from the Gulf. The complex topography of the Chattooga Basin, particularly the orographic [having to do with mountains] effect presented by the Blue Ridge escarpment, and proximity to weather pattern influences by the Gulf of Mexico combine to result in areas of the highest precipitation in the eastern United States [See Table 1].

The influence of hurricanes on the climate of the Chattooga basin, particularly precipitation, is less predictable than continental weather patterns. The main influence of hurricanes over oceans is from their high wind speeds, but winds subside quickly over land and the major climatic influence changes to precipitation. Because of their relatively slow movement, an area within the path of a tropical cyclone can receive several days of precipitation with amounts of several inches daily or more. Wind speeds decrease quickly as a hurricane travels inland, but intense, fast moving hurricanes may occasionally bring winds of over 50 mph to the Chattooga Basin. For example, this occurred in early October 1995 when hurricane Opal followed a path across the Florida panhandle then northward over Atlanta, bringing over 6 inches of rainfall and high winds resulting in considerable downed trees. The orographic effects of the Blue Ridge escarpment can also increase precipitation rates.

Based on records from 1871 to 1977, 847 tropical cyclones of various intensities were recorded, but only six were charted as passing directly over the basin after making landfall. However, because of the large areal extent of tropical cyclones, which range in diameter from 100 to 600 nautical miles, those that follow a path along the South Carolina coast (about 200 nautical miles eastward) could also affect weather in the Chattooga Basin. During this 107-year interval, 103 tropical cyclones passed within 200 nautical miles of the basin. On the average, we estimate the climate of the Chattooga Basin could be influenced by a tropical cyclone for 65 years out of 100. About 34 percent of tropical cyclones form during September, and about 22 percent each during August and October.

Subregional climatic relations of an area 100 miles square and centered over the Chattooga Basin were obtained from a forest atlas maintained by USDA Forest Service (1990). Based on the 34-year average (1951-1984) from standard NOAA weather stations, and interpolated at 0.5 degree intervals for monthly minimum and maximum temperature and precipitation, the Chattooga River Basin has an average annual temperature of about 57 degrees F. Average January temperature is about 40 degrees F, and for July is about 75 degrees F. Precipitation ranges from 52 inches at lower elevations, to over 70 inches at higher elevations along the Blue Ridge escarpment in North Carolina. Orographic effects of the escarpment, which rises about 1000 feet from the Appalachian piedmont, is largely responsible for the difference in precipitation.

Land-type Associations; General Climate

More specific climatic data, adequate to compute 30-year normals, are available for weather stations in three of the five land-type associations (TLAs) in the Chattooga Basin: Long Creek, South Carolina; Clayton, Georgia; and Highlands, North Carolina (NOAA 1990). Mean temperatures are highest and precipitation is lowest at Long Creek. These relationships are reversed for Highlands, and Clayton is intermediate. Precipitation is evenly distributed between the winter and summer seasons at the three locations. Temperatures among the stations are relatively uniform during the winter, and vary somewhat during the summer. The effect of altitude on temperature is evident by comparing values for Highlands with the other stations. Winter temperatures are slightly higher at Clayton than Long Creek, even though the former is at a higher elevation and is farther north. An explanation is unknown, but could result from the large intermountain basin where Clayton is situated. The overall climate of the mountain uplands has the same seasonal regime and pattern as that of the lowlands.

Detailed historical climatic data for precipitation from 1893 to 1957 is available for Rock House, South Carolina in the upper part of the Chattooga Gorge LTA, as reported by Dumond (1970). This station was located 1.7 miles southwest and 4.2 miles east-southeast of Highlands, North Carolina at an altitude of 3,100 feet. Precipitation during the 64-year period averaged 82 inches, and ranged from 46 to 114 inches. Annual mean temperature averaged 54.9 degrees F, with a range of about two degrees. Although no longer active, this station provides an important record of annual, long-term variation of climate that is especially important to document periodic drought in the Chattooga Basin. The Rock House data records 1925 as a year of historic low precipitation and high maximum temperature.

Land-type

Because local topographic relief has a strong influence on climate, estimates of climate conditions on land-types requires computer programs that use appropriate relationships to extrapolate conditions from nearby stations. Specific climatic relationships are not available for land-types in the Chattooga Basin.

Land-type Phase

Land-type phases are the smallest ecological unit in the hierarchy. Characterization of climate for LTPs will require use of on-site instrumentation.

Extreme Weather Conditions

Hursh and Haasis (1931) documented effects of the 1925 drought on arborescent [treelike in shape or growth; branching] species in the Southern Appalachians near Asheville, North Carolina at an altitude of 2,100 to 2,600 feet. Summer precipitation (June-August) averaged 12.5 inches for the period 1903-1929, but only 3.0 inches was recorded in 1925. Reduced radial growth and high mortality was observed four years following the drought in black oak (Quercus velutina), scarlet oak (Q. coccinea) and red oak (Q. borealis). Chestnut oak, (Q. montana), hickories (Carya spp.) and pines (Pinus echinata and P. rigida) showed little effects. Mortality was highest on areas with shallow soils of 18 to 20 inches in depth. They concluded that drought is a significant factor affecting composition and distribution of tree species in the Southern Appalachians.

Neary and Swift (1987) reported on effects of intense and heavy rainfall on land disturbance caused by debris avalanching in the Southern Appalachian Mountains near Asheville. A storm system from the Gulf of Mexico, with an estimated return interval of about 100 years, occurred during early November 1977. During a 3-day period, some areas reported over 10 inches of rainfall. The heavy rainfall associated with this storm caused a number of debris avalanches, particularly on areas with steep (70%) slopes and shallow residual soils of less than 36 inches in depth. High rainfall is a climatic factor associated more with shallow soil erosion and water quality, than with the distribution of vegetation.
Other Climatic Relationships

In an early study of orographic effects on precipitation throughout the Southern Appalachian region, Donley and Mitchell (1939) found considerable variation within uniform geographic zones that was not associated with altitude, but which appeared to be related to local topographic effects that could not be quantified. Dickson (1959) studied the effect of altitude on climatic variables in the Southern Appalachian Mountains, and presented regression models for estimating temperature, length of growing season, precipitation, and evapotranspiration at un-instrumented sites. The relationship of rainfall with altitude was poorer than for temperature. Dickson mentions a "spillover" effect on narrow ridges where updrafts carry precipitation over the crest to the leeward slope, which results in less than average rainfall on some ridgetops. Billings and Anderson (1966) measured soil moisture on an exposed, narrow, pine-dominated ridge, and reported that "the ridge is a local area of regular microclimatic drought," even in a region where annual precipitation exceeds 100 inches.

In parts of the Chattooga Basin, Helvey and others (1972) investigated soil moisture in relation to slope position and soil depth during the growing season. They found that a simple sine function accurately describes annual soil moisture trends, because growing and dormant seasons are almost equal in length and rainfall is evenly distributed during the year. During a 20-day summer drought, soil moisture losses on all slope positions are about three times greater than for non-drought periods; however, ridges lose about 25 percent more soil moisture than coves.

In a nearby area of higher elevations (1610 to 1855 meters) outside the Chattooga Basin, Smathers (1982) evaluated the contribution of fog condensing on vegetation to annual precipitation. He reported that fog increased precipitation from about 50 to 90 percent in mixed hardwood-conifer and heath balds, respectively. These results suggest that even at lower elevations where fog may be less common, precipitation increase from condensation could be a factor that influences soil moisture gradients.

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