Density of Yellow Pine and Hemlock

SPB and HWA are two very different forest-damaging insects that inhabit host tree species, which exploit opposite ends of the moisture gradient found in the Southern Appalachian Mountains (Figure on the right), although they occasionally occur together at either end of their natural range. We choose these insects to illustrate the utility of LANDIS in investigating forest insect threats because they represent the extreme cases of an indigenous pest that has the potential to cause great damage (SPB) and an invasive pest that has the potential to remove an entire host plant species from eastern forests (HWA).

In the Southern Appalachian Mountains, xeric slopes and ridges have historically been dominated by yellow pines (Pinus spp.). Because altered disturbance regimes have begun to change the appearance of the landscapes, understanding the dynamics of these systems is important to forest managers in implementing management strategies on public lands. On these landscapes, fire and SPB are the two most influential natural disturbance agents. SPB has caused extensive damage to pine forests throughout the Southeastern United States (Coulson 1980, Coulson and others 2004). On Southern Appalachian xeric ridges, SPB colonizes a variety of pine species including pitch pine (Pinus rigida Mill.), Virginia pine (Pinus virginiana P. Mill.), Table Mountain pine (Pinus pungens Lamb.), and occasionally eastern white pine (Pinus strobus L.) (Payne 1980). Interactions between available soil moisture and resin flow, the primary tree defense against SPB (Tisdale and others 2003, among others), have long been noted (Hodges and Lorio 1975, Hodges and others 1979 ) and are likely affected by such landscape characteristics.

Fire and SPB are thought to drive the regeneration of yellow pine forests on xeric ridges in the Southern Appalachians (Harmon 1980, Harrod and others 1998, Williams 1998). Williams (1998) conjectures that SPB and other nonfire disturbances in xeric pine-oak forests will lead towards hardwood domination in the absence of fire. It has further been hypothesized that these communities are maintained in a drought-beetle-fire cycle (Barden and Woods 1976, Smith 1991, White 1987, Williams 1998). Understanding the relationship between fire, SPB, and mesoscale forest dynamics can provide direction for forest planners and managers in maintaining and restoring this unique environment.

Eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana) appear in mesic flats, draws, ravines, coves, and canyons of the Southern Appalachian Mountains (Whittaker 1956). Although once more abundant in the forest, hemlock populations declined dramatically approximately 5,500 years ago due to climatic shift resulting in summer droughts that weakened the hemlocks and left them vulnerable to a subsequent widespread insect outbreak (Allison and others 1986, Davis 1981, Haas and McAndrews 2000). In its northern range, canopy gaps were filled by Acer, Betula, Fagus, Pinus, Quercus, and Ulmus (Fuller 1998). Although hemlock did re-establish itself, its recovery may have taken up to 2,000 years and, in many sites, is still not as prominent as it was before the decline (Fuller 1998, Haas and McAndrews 2000). Now, hemlocks are at risk from the invasive exotic insect pest HWA .

In its native Japan, HWA populations are maintained at low densities on hemlocks (Tsuga diversifolia and T. sieboldii) by a combination of host resistance and natural enemies (McClure 1992, 1995a, b; McClure and others 2000). The first report of HWA in North America was in the Pacific Northwest in the 1920s; however, western hemlocks were resistant to the adelgid. In the Eastern United States, the first reports of HWA were in 1951 in Richmond, Virginia (Gouger 1971; McClure 1989, 1991). With no natural resistance or natural predators, HWA slowly made its way northeast and has subsequently been moving southwest along the eastern side of the Appalachian Mountains. Little is known about stand-level characteristics that influence HWA susceptibility in the Southeastern United States. However, studies on HWA infestation levels in the northeastern range of this insect noted only latitudinal effects on infestation severity (Orwig and Foster 1998, Orwig and others 2002). This would seem to suggest that all hemlock stands have the potential of being infested and killed, regardless of site and stand factors.