Impact of the balsam woolly adelgid (Adelges piceae ratz.) on an Abies fraseri (Pursh) Pior. Dominated stand near the summit of Mount LeConte, Tennessee

Castanea, Jun 2003 by Jenkins, Michael A

ABSTRACT

To assess changes in a spruce-fir stand resulting from overstory mortality caused by the balsam woolly adelgid (Adelges piceae), I resampled 5 permanent plots first established in 1979 near the summit of Mount LeConte, Tennessee. Overstory basal area and density of Abies fraseri decreased, but understory basal area and density increased. Understory density and basal area were highly variable for most species. Prunus pensylvanica and Sorbus americana appeared to increase in understory density as a response to overstory mortality, although these changes were not statistically significant (P 0.001), while that of Dryopteris campyloptera increased from 9% to 15% (P = 0.063). Although not statistically significant, the cover ofRubus canadensis increased from 2% to 30% (P = 0.159) but was highly variable between plots. The severity and extent of mortality caused by the adelgid has greatly altered composition and structure, and even after 22-years the stand is still in a state of structural and compositional reorganization.

INTRODUCTION

Spruce-fir forests in the southern Appalachians occur in several high mountain areas of Tennessee, North Carolina, and southwestern Virginia, typically ranging from 1,500 to 2,000 m in elevation, but extending down to 1,370 m on north slopes within the Great Smoky Mountains (Whittaker 1956, Busing et al. 1988). Picea rubens Sarg, and Abies fraseri (Pursh) Poir. are the dominant overstory species with A. fraseri typically dominating stands above 1,890 m elevation and P. rubens codominating between approximately 1,675 and 1,890 m elevation (Whittaker 1956; nomenclature follows Kartesz 1999). While the range of P. rubens extends to northeastern Canada, A. fraseri is endemic to the southern Appalachian Mountains (White 1984). Betula alleghaniensis Britt. is the dominant hardwood species, but Acer spicatum Lam., Prunus pensyluanica L. f., and Sorbus americana Marsh, also are common (Whittaker 1956).

The topographically and geographically isolated spruce-fir forests of the southern Appalachian Mountains are rich in rare and endemic plant and animal species. Eight plant species, 17% of the species characteristic of these forests, are endemic to high peaks of the southern Appalachians (White 1984). One of these species, Rugelia nudicaulis Shuttlw. Ex Chapman (Rugel's ragwort), is endemic to Great Smoky Mountains National Park (GSMNP). Southern Appalachian spruce-fir forest is listed as critical habitat for the endangered spruce-fir moss spider (Microhexura montivaga Crosby & Bishop) and provides important foraging habitat for two endangered southern Appalachian subspecies of northern flying squirrel (Glaucomys sabrinus fuscus Miller and G. s. coloratus Handley; Loeb et al. 2000).

Spruce-fir forests in the southern Appalachians have experienced serious degradation in the past century. As a result of commercial logging and slash fires, present-day southern Appalachian spruce-fir forest has been estimated to occupy 10% to 50% of its former extent (Korstian 1937, Saunders 1979). The reduction of spruce-fir forest in GSMNP has been less severe. According to PyIe (1984), the area that became GSMNP contained 17,910 ha of spruce-fir forest prior to logging. Following logging, this total was reduced to 13,370 ha, a 25% reduction. Dull et al. (1988) estimated that present-day spruce-fir forest in the southern Appalachians occupies 26,610 ha, of which 74% is found in the Great Smoky Mountains.

Over the past thirty years, these unique forests have been decimated by the balsam woolly adelgid (Adelges piceae), an aphid-like insect introduced from Europe. The insect was first detected in the southern Appalachians in 1957 (Speers 1958), reached Mount Sterling in GSMNP by 1963, and in subsequent years spread to Abies fraseri stands throughout the Park (Eagar 1984). The adelgid feeds in the bark fissures of A. fraseri, primarily on trees over 4 cm dbh (Eagar 1984). During feeding, the adelgid injects salivary compounds that cause the tree to produce abnormally short and heavily lignified tracheids (Puritch 1971). The resulting reduction in water conduction usually kills the tree in 2-7 years (Amman and Speers 1965). Most mature A. fraseri trees have died as a result of adelgid infestation (Pauley and Clebsch 1990).

Unlike many other spruce-fir areas in the southern Appalachians, forests near the summit of Mount LeConte were never commercially logged (Nichols 1977).

However, with the establishment of LeConte Lodge near the summit in 1926, trees were cut to provide fuel and construction materials (Nichols 1977). After creation of GSMNP in 1934, salvage cutting of down and damaged trees followed windthrow. When woodcutting was banned in 1976, approximately 3.6 ha near the summit had sustained severe windthrow and subsequent salvage cutting, while another 4.4 ha received partial windthrow and less intensive salvage cutting (Nichols 1977).

In june-July 1979, the staff of Uplands Research Laboratory in GSMNP established and sampled five 0.1 ha (20 m x 50 m) permanent vegetation plots within an intact spruce-fir stand near the summit of Mount LeConte. These plots were established to describe spruce-fir forest under attack by the balsam woolly adelgid (White and Busing 1993). According to Hay et al. (1978) the summit of Mount LeConte was encircled by isolated infestations in early 1977. By November of 1977, trees near the summit were found to support isolated, but frequent, adelgid infestations (Hay et al. 1978). Although these plots were established after infestation by balsam woolly adelgid (Hay et al. 1978), they pre-date the major fir mortality that occurred in 19821984 (Smith and Nicholas 2000). In July of 2001, these plots were resampled as part of GSMNP's ongoing long-term vegetation monitoring program. In this paper, I use data from these plots to assess how the structure and species composition of this stand have changed in the 22 years since adelgid infestation and resulting overstory mortality. While the scope and statistical power of this study are limited by its small sample size (n = 5) and narrow spatial distribution, these plots are among the oldest permanent plots on Mount LeConte and offer a rare look at how herbaceous species cover has changed in response to overstory mortality.