Other Problems

The more serious insects affecting bottomland trees are forest tent caterpillars. These defoliate tupelo, blackgum, sweetgum, willow oak, overcup oak, and river birch. Larvae spin cocoons in early May; moths are in flight two weeks later; defoliated trees may put out new, but smaller and lesser, foliage by June. An unknown fungus, a virus, or fly parasites may stop the outbreaks.31

Fire running through bottomlands may promote the spread of buckvine and honeysuckle. The problem becomes severe where stands are sufficiently sparse to permit light to penetrate the canopy.

The increase in soil moisture that follows harvests or natural death of stands of trees increases the danger of windthrow. The rise in ground water levels coverts flats and sloughs to brushy swamps. Inundation also affect to some degree water absorption and nutrient adsorption, transpiration, water movement within the trees and soil, the amount and kind of chemicals and organic matter in the water, microbes, algae, and chemical and physical properties of the soil. These, in turn, influence the tolerance of trees to submergence. And this, in its turn, influences the amount of free oxygen available to plants. It is this oxygen deficiency or its corresponding carbon dioxide toxicity which detrimentally influences seed germination, seedling survival, and tree growth.3

Figure 4.19 Cottonwood trees on new land. As new land formed in this river bottom, cottonwood and willow trees promptly seeded-in, holding the sandy soil in place. In time, other more-demanding species encroach as the short-lived pioneer vegetation passes from the site.

Free oxygen is considered the limiting factor in the germination of bottomland hardwood seeds in flooded sites. Except for siltation, which covers seeds too deeply, flooded bottomlands for up to one month do not appear to reduce germinative capacity.32 Even if bottomland tree seeds germinate while immersed in water, establishment of seedlings is unlikely unless the immersing water recedes before the seedlings perished from other causes.

The amount of light reaching the forest floor also influences germination. It is more favorable for river birch, sycamore, and American elm under conditions of full sunlight than under crown canopies. In contrast, red maple, winged elm, and alder germinate best in deep litter and under low light intensity, accounting for these less-desirable species outnumbering favorable ones where litter is deep and ground vegetation dense. New land that is regenerated with cottonwoods and subsequently invaded by weed trees is an exception to this generality.

In ponded areas exposed to sunlight after a rain, the rising temperature increases respiration and the activity of microorganisms, resulting in oxygen deficiency and carbon dioxide toxicity. Root-growth ceases when oxygen levels diminish to 0.5% in the gas around roots, but top growth then continues, possibly accompanied by toxic accumulations of iron. Flooding might also cause cessation of downward movement of carbohydrates and auxins. The accumulation of carbohydrates and hormones at the water line possibly accounting for adventitious rooting of flooded trees. Sprouting from root collars indicates that death of roots has not preceded top death and, therefore, root-kill by flooding was not responsible for tree necrosis. 33

In the bottomlands of Louisiana and Texas, Chinese tallowtree has become a heavy invader, especially following stand-destroying storms. Infusion of this exotic (imported as a potential source of paint dryer) has dramatically reduced species diversity.

Figure 4.20 Pure black willow grove of older trees on new land formed when sand washed from one pointbar to another. The site had been foraged by cattle and browsed by deer, keeping the undergrowth checked. (USDA Forest Service photo)
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