When eastern hemlock occurs with white pine, the two species compose a type occupying many old-field sites in the Southern Appalachians. As the less-tolerant white pine is not perpetuated without fire, harvest, or tree-topping storms, the species passes from the stand, leaving a pure hemlock forest. Encroachment by shade-tolerant hardwoods, principally oaks and hickories, may preclude establishment of a pure stand. In old fields, hemlock often seeds-in after white pine is well established.
In appraising a situation in North Georgia, an understanding of this relationship was essential to defuse a politically critical matter. A government agency was being accused by a big-city newspaper of destroying a virgin forest by marking the stand for a selection harvest. Paint appeared on the chosen trees and the sale was ready to be bid. The trees in the aesthetically beautiful cove were indeed large, many exceeding three feet in diameter.
With a spade, one could readily locate the old plow sole about 12 inches below the present level of the soil in the cove. Ordinarily the plow sole, a distinctive line separating the plow layer from the horizon beneath, is but 6 or 7 inches below the surface. (Occurrence of a plow sole is attributed to the frequent turning of the soil by a farmer's tilling many times a year for many years. Oxidation of the minerals in the soil by exposure to the air causes a change in color and an alteration in the density of the soil at that stratum.)
Above the plowed zone was an overburden of topsoil that, during cultivation and since the time of land abandonment, had washed down the slope from the surrounding hillsides and into the cove. This intruded material accounted for the greater than usual depth to the plow sole.
Further research at the county courthouse revealed when this land had been purchased by the government. It also showed when an agricultural depression had occurred that forced farmers from their small patches of arable lands in the mountain coves. Soil washed down on that freshly abandoned land. And on this deep and exposed mineral soil, free from organic duff or perennial plants, winged white pine seeds borne by the wind had come to rest. Here they germinated to form a dense stand of white pine, the high quality of the site encouraging tree growth that exceeded 2 or 3 feet in height each year and annual radial growth greater than one-half inch.
Shortly, the shade-tolerant eastern hemlock trees, their winged seeds also blown by the wind, seeded in under these pines. Seeds of hemlock need not have mineral soil for germination, but may sprout on moist organic matter, the fine roots then working their way through the pine needle duff to adequate sources of water and nutrients for sustained growth. The germinated hemlock seedlings extended shallow, wide-spreading root systems in the fertile soil of the alluvial overburden.
These intruding hemlocks, like the pines, grew well. As some pines died, hemlocks took their places in the canopy of the forest. In time, many of the hemlocks were the largest trees in the woods. Together with the pines, these conifers made a dense dark stand of large boles whose crowns were intermingled in the canopy.
It was reasonable for newsmen and lawyers to consider this a virgin forest, a remnant of the past, and too beautiful to harvest. Indeed it was too beautiful to harvest, but not because of its supposed virgin condition. Rather, the excellent tree growth on the site—caused by the farmers' earlier control of competing vegetation with cultivation of the land and the fertile loam that had washed in following abandonment—presented the appearance of a primeval forest, a woodland in which man had never raised an ax. But, it was the relative tolerance of the two species that gave the matter away; the pines had to precede the hemlock in ecological succession.
The shade-intolerant white pines produce even-aged stands. A crop of their own seeds is not likely to germinate under the canopy of trees established even a few years earlier. Thus, increment core borings of small pine stems recorded the ages, by growth-ring counting, of neighboring stems too large for the use of the increment borer, for all stems of this species in the stand began life together. Not so with hemlock, for the continual, perhaps annual, introduction of new seedlings within the forest and under the canopy of older ones makes increment boring useful only for the particular tree bored. Trees too large in radius for the borer tool's length leave their dates of birth unknown. The ecological relationship of the two species, however, required the largest of the hemlocks to be younger than the smallest of the pines: shade-tolerant hemlocks had to follow in ecological succession the establishment of the pines in the mixed stand.
Was this article helpful?