Long-term temperature and precipitation trends at the Coweeta Hydrologic Laboratory, Otto, North Carolina, USA

Coweeta Hydrologic Laboratory, located in western North Carolina, USA, is a 2,185 ha basin wherein forest climate monitoring and watershed experimentation began in the early 1930s. An extensive climate and hydrologic network has facilitated research for over 75 years. Our objectives in this paper were to describe the monitoring network, present long-term air temperature and precipitation data, and analyze the temporal variation in the long-term temperature and precipitation record. We found that over the period of record: (1) air temperatures have been increasing significantly since the late 1970s, (2) drought severity and frequency have increased with time, and (3) the precipitation distribution has become more extreme over time. We discuss the implications of these trends within the context of regional and global climate change and forest health.

Drought Impacts on Tree Growth and Mortality of Southern Appalachian Forests

The Coweeta LTER Program represents the eastern deciduous forests of the southern Appalachian Mountains in the United States. Coweeta Hydrologic Laboratory was established in 1934 and hence has a long record of climate measurement and vegetation response to both natural and human disturbance (Swank and Crossley 1988). The general climate of the area is classified as marine humid temperate because of high moisture and mild temperatures (Critchfield 1966; Swift et al. 1988). These conditions have favored the evolution of high species diversity in organisms in the southern Appalachians at many levels. In recent years, however, Coweeta has experienced several droughts that have caused significant tree growth reduction and increased mortality rates (Swift et al. 1990; Clinton et al. 1993; Vose and Swank 1994; McNulty and Swank 1995). In this chapter, we describe the general climate and features of Coweeta as well as the impact of droughts on tree growth and mortality. The timescale of this climate variability is annual, with the potential for preexisting soil moisture conditions either providing a buffer or further exacerbating the drought conditions. Mean annual precipitation at Coweeta Hydrologic Laboratory (latitude 35º14' N, longitude 83º26' W) varies from 1798 mm at the base climate station (686 m) to 2373 mm at the high-elevation Mooney Gap climate station (1364 m). Mean annual growing season precipitation, defined as May to October, is 782 mm at the base climate station (figure 3.1). Mean annual streamflow from watershed 18, a low-elevation reference watershed, is 1011 mm or 56% of precipitation (figure 3.1). Short-duration thundershowers at Coweeta are typical for midsummer and fall with occurrences of large rainfalls stimulated by tropical disturbances near the Atlantic or Gulf coasts. Forty-nine percent of the 133 storms each year have a total precipitation amount less than 5 mm, and 69% of the annual precipitation falls with an intensity less than 10 mm per hour. Snow is a minor part of the annual precipitation, averaging 2–5% depending on elevation. Snow cover rarely lasts for more than 3 or 4 days, even on the upper slopes.