Sweetgum

Second-growth sweetgum in alluvial soils of the South usually occurs in fully stocked, relatively pure, even-aged stands. If seeds are available, stands develop within 10 years of clearcutting. Trees of this species exhibit vigor: those of high vigor exceed 3 inches dbh growth in 10 years, those of medium vigor between 2 and 3 inches, while low vigor stems have less than a 2-inch increment during the period. These vigor classes, however, apply only to trees 16 inches dbh or greater. Vigor can also be ascertained by bark color and fissures and by crown form.

The sweetgum blight of unknown origin was first noted in 1950, a drought year. The first visible indication appears in late summer when leaves on some branches prematurely develop fall coloration. Crowns then thin gradually from the top down. Some buds do not open; others produce small chlorotic foliage, the tree dying a year or more after symptoms first appear. Sometimes the dieback is arrested, and trees appear healthy except for the dead top. As much as 90% of the fine feeder roots in the surface layer of the soil die on blighted trees, while larger roots remain healthy in appearance. Cutting into the wood of diseased branches reveals irregular tan or dark brown streaks in the white sapwood. The blight is more severe on slack-water soils and less on natural levees.37 Loose soils which drain readily and retain less water seem most susceptible to the blight.

Sweetgum is also subject to another malady of unknown origin and as yet without a name. Although trees are not killed, lumber is degraded as the bark becomes encased within the stems, and bumps and ridges of callus tissue form over the lesions. Young stands seem most vulnerable.38

Figure 4.25 Sweetgum blight, a dieback of unknown cause in southern river bottoms. Both crowns and roots appear to be infected. An environmental insult, rather than a fungus, is suspected. (authors' collection)

The cambium of sweetgum can be heated to a lethal temperature of 140°F, much more rapidly than for the southern pines and baldcypress. This points up the necessity for fire exclusion in stands of this species.

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