Global climate change and fragmentation of native brook trout distribution in the southern Appalachian Mountains

Current distributions of native brook trout (Salvelinus fonfinalis) in the southern Appalachians are restricted to upper elevations by multiple factors, including habitat requirements, introduced rainbow (Oncorhynchus mykiss) and brown (Salmo fnrtia) trout, and other human activities. Present-day distribution of brook trout habitat is already fragmented. Increased temperatures predicted by various global warming models are likely to further limit suitable brook trout habitat. Predicted changes in hydrologic cycles may exacerbate temperature effects, and hydrologic effects on trout may differ across the region. Models of present-day trout guild distribution were used in a Geographic Information System (GIS) to examine the changes in trout distribution that might occur with temperature increase. Both suitable area and stream length for trout decrease as suitable habitat is increasingly restricted to mountaintops. Furthermore, the remaining trout habitat is likely to be even more fragmented than at present. If trout habitat becomes more fragmented under warming trends, common local extinctions may become irreversible as avenues for recolonization are eliminated. CURRENT TROUT DISTRIBUTION The southern Appalachians represent the southern margin of trout in eastern North America. For this discussion, the southern Appalachians consists of the mountain areas of Georgia, South Carolina, North Carolina, Tennessee, and Virginia. Originally, only native brook trout (Salvelinusfontinal) were found in this area. During the late 19* and early 20* centuries, rainbow (Oncorhynchus mykiss) and brown (salmo PZMU) trout were introduced into the region. The guild of these three species now occupies streams in about 40,700 km2 of these states (Fig. 1). Current distribution of native brook trout in the southern Appalachians is restricted to upper elevations by multiple factors, including habitat requirements, introduced rainbow and brown trout, and other human activities. Stream ‘Research Eco/o@, U.S. Forest Setice, Southern Research Station, Blacksburg, VA 24061-0321; [email protected] temperature is a basic limiting factor that defines suitable habitat for all sahnonids, which require relatively low temperatures. Brook trout are found at slightly lower temperatures in field settings than are rainbow and brown trout (Eaton et al. 1995), and stream temperature generally increases with decreasing elevation in mountains. Other habitat conditions no doubt contribute to the current distribution patterns. In the early years of the 20ti century, logging and conversion to homesteads, fires, overfishing, and stocking all contributed to loss of brook trout habitat as European settlement moved upward. Historically, introduction of rainbow and brown trout certainly restricted the distribution of brook trout; many streams that now have introduced trout are known to have had brook trout at one time. But, the extent to which this replacement process continues today is tmknown and some think that relative distributions of the three species have Figure l.-Presentday suitable trout habitat in the southern Appalachians largely stabilized in the last few decades. In general, the introduced trouts have been more successful at lower elevations and latitudes, and brook trout tend to occupy higher elevations and latitudes (Larson and Moore 1985, Flebbe 1994). But brook trout remain most common and abundant in North Carolina and Virginia (Flebbe 1994). Sympatry is common, and within the region, these distribution patterns vary (Flebbe 1994). Presentday distribution of brook trout habitat is already ft-agmented. Some fragmentation is due to geomorphology, drainage patterns, and the fact that brook trout are at higher elevations in the southern Appalachian Mountains. Brook trout in the southern Appalachians are in major drainages of the Potomac (Shenandoah), Rappahannock, James, Roanoke, Yadkin, Catawba, Savannah, Alabama, Tennessee, and Ohio rivers. Even witbin these major drainages, brook trout are often restricted to headwater watersheds. Presence of unsuitable land uses also fragments brook trout distribution within watersheds. With few exceptions, developed land surrounding a stream makes that stream habitat unsuitable. Most agricultural land use also renders stream habitat unsuitable. Conversion of forest land to developed or agricultural land has tended to proceed from lower elevations to higher ones. Jfthis is the present-day situation for brook trout in the southern Appalachians, what do predictions of global climate change hold for brook trout here? GLOBAL CLIMATE CHANGE THREATS Global average air temperature has probably increased about 0S”C over the last century and, due to increasing levels of greenhouse gases, primarily carbon dioxide, may further increase by 1.0 to 3.5 “C during the next 100 years or so (Karl et al. 1997). The amount of temperature increase is not uniform over the planet and various models predict different magnitude of temperature increase. Jn the Southeast, models project increases of about 3 to 4 “C (or more) with a doubling of atmospheric CO2 (Mulholland et al. 1997). Effects of increased air temperature on water temperature will vary from site to site, depending on such factors as degree of groundwater influence, amount of shading by watershed and riparian vegetation, watershed aspect, etc. Possible changes to the hydrologic cycle are even more complex and uncertain than temperature change. Along with increased air and stream temperature, precipitation in the Southeast is also expected to increase, especially in the summer (Mulholland et al. 1997). Models indicate that summer convection storms will become more intense and frequent, with longer dry periods between them -a clustering e&ct (Mulholland et al. 1997). As a result, some small mountain streams may be more likely to dry out between storms and more intense storms may cause flash flooding and damage to streams. Evapotranspiration may or may not increase with increased carbon dioxide and warming, adding to the difficulty of predicting hydrologic changes. Experts do not agree on effects of climate change on hurricanes because processes are complex (Karl et al. 1997). Furthermore, effects of concomitant increased demand for water by humans add another source of uncertainty to predictions for hydrologic changes. CONSEQUENCES FOR TROUT IN THE SOUTHERN APPALACHIANS Increased temperatures and hydrologic changes predicted by various global warming models are likely to further limit suitable brook trout habitat. Many other, indirect effects of climate change on the stream environment of trout are possible, but will not be considered in this paper. For example, riparian zone vegetation may change, which in turn can alter inputs of allochthonous material and large woody debris. Changes in macroinvertebrate community structure and metabolism in response to all these changes represent changes in trout food availability. To date, consequences of hydrologic changes have not been proposed for trout in the southern Appalachians,