The principal enemies of spruce and fir trees in the South are fire, wind, the woolly aphid, and rot-causing fungi. Apart from the cool, moist climate in which these species occur, natural fire protection is lacking, and once the woods are ignited, dense interwoven crowns that hug the ground, resinous foliage and wood, and thin bark are the ingredients for a holocaust. On the ridges, severe soil surface destruction by fire removes as much as two feet of surface moss and peat. Fire exposes entire root systems, which are then unable to maintain soil stability. With the destruction of the organic component go the seeds, stored in the humus for a future forest. Rapid drying of the soil follows. Trees such as fire cherry, sweet birch, and pin cherry; shrubs such as blackberry bushes; and ferns quickly capture the site.
Complete rehabilitation of these forests may require hundreds of years if seed-bearing trees have been destroyed. Twenty years after a fire, few conifers likely have seeded-in, as ecological succession proceeds from pioneer brush to these climax conifers.7
Wind topples Fraser fir at high elevations where soils are shallow. Extensive tracts have been felled in a single storm. Virtually impermeable layers of subsoil 12 to 18 inches below the surface,
into which tree roots cannot penetrate to gain a firm grip, encourage windthrow. The shade-tolerant characteristic of the species and the uneven-aged character of these stands, however, encourage reproduction establishment. Young trees then have an advantage over shrubs that otherwise invade freshly opened sites, and can develop rapidly into even-aged sapling and pole stands.
Periodically, the balsam woolly aphid kills Fraser fir at an accelerated rate compared with mortality caused by endemic populations of the insect. Attacks made on living tissue beneath the surface of the bark on terminal and lateral branches below 15 feet are most damaging to trees on low-lying waterlogged soils. Aphid attacks are readily identified by the white wax "wool" on twigs. Other evidence is the compression-like wood of the growth rings, greatly narrower on one side than on the other of the cross-section of the tree. Predators introduced from Germany and Australia to biologically combat the menace did not provide adequate control.
The misnamed spruce budworm, destructively feeding mostly on fir trees, generally confines its activity to the northeastern and western United States and hence is no problem in the South. However, three fungi-caused rots—top, brown, and white-string—do decay much of the wood in standing trees.
In these woodlands deer browse both species, though preferring the fir. Black bear also consume the trees, reducing vigor and limiting height growth. Browsing is not sufficiently severe to obliterate trees of all ages.8 It is in these high-elevation forests that deposition of sulfuric and nitric acids are expected to be most injurious, in contrast to woodlands of southern pines and hardwoods. Ozone also takes its toll in reducing growth. Even if emissions from factories and vehicular exhausts should remain constant, growth rates will likely decline.9
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