Evidences of Vigor

Vigor of yellow-poplar trees is exhibited by the bark. High-vigor stems—those growing on high quality sites—have diamond-shaped, corky, and shallow-fissured, light-colored ash-gray bark with light-colored inner bark. As vigor diminishes, the bark becomes thicker and the ridges more pronounced. Some inner bark remains visible in the fissures. Then with further loss in vigor, the thickening bark turns darker and the deep fissures more pronounced, but the inner-bark is not visible.20 Rather demanding of the site, the species exhibits foliar symptoms of nitrogen, phosphorus, and potassium deficiencies.21

Vigor is also evidenced by crown form. High-vigor stems have many small ascending leaders, a sharp-pointed crown, and abundant lustrous foliage. With loss of vigor, the tree becomes more

Figure 4.12 Yellow-poplar in a Blue Ridge Mountain cove. Seeded-in in dense stands, the botanist's "tuliptree" exhibits many of the silvical characteristics of the principal southern pines. Browse plants able to endure the shade and the competition of the trees, cover the ground. (USDA Forest Service photo)

Figure 4.12 Yellow-poplar in a Blue Ridge Mountain cove. Seeded-in in dense stands, the botanist's "tuliptree" exhibits many of the silvical characteristics of the principal southern pines. Browse plants able to endure the shade and the competition of the trees, cover the ground. (USDA Forest Service photo)

thinly foliated, branches spread horizontally rather than ascend vertically, and the point of the crown appears blunter.

Seeds and Seedlings

Although yellow-poplar seed crops are produced annually, the heavy layer of litter that forms a matted humus discourages seed germination. If germination occurs, dense canopies impede seedling establishment. Low light intensity in crowded stands prevents seedlings from growing beyond the succulent tissue stage.

Good seed germination on bare mineral soil—encountered following fire and clearcutting—may be associated with chilling, a condition necessary for seeds of many species to break dormancy. While abundant organic matter in the soil of undisturbed sites serves to reduce the maximum and raise the minimum temperatures of the soil, such changes in surface soil temperature under these conditions are not sufficiently great for effectively serving as a pre-germination seed treatment.

Yearly production of seeds frequently exceeds more than 5 million per acre. Distribution in the autumn peaks during periods of high temperature and low rainfall. Strong winds dislodge them from the cup-shaped ring of basal scales, sending the terminally winged seeds a distance of four to five times the height at which the fruit clings to the tree. Some seeds drift from the "cones" of fruit clusters, each cone holding about 80 seeds, in the spring of the year following seed maturity. Some seeds lie viable in the soil a year before natural stratification (an essential pre-germination treatment) enables cotyledon leaves to break through the coat. Young seedlings freeze in the cold of winter or smother in the shade of heavy leaf fall in autumn.

The only shade under which the intolerant species survives is that of grass and the low bushes of old fields. Sumac, sassafras, and black locust trees form stands with canopies too dense for yellow-poplar seedlings to penetrate, even if seed germination is adequate.

A tree that occurs naturally through a thousand miles of mountains and plains may be expected to vary by provenance. However, the geographically racial distinctions of yellow-poplar are nearly imperceptible apart from the earlier height-growth initiation for seedlings in warmer areas. Survival of seeds depends on seed source, but not on the latitude of that source. Seeds from southerly climes when planted farther north break dormancy earlier in the spring, late frosts then taking their toll. Any growth advantage of a warm-climate seed source is canceled out by the early inception of tree dormancy in the colder northern zone. As the beginning of dormancy is also correlated with the date of the first killing frost at the point of parental origin, planting northern provenance stock to the south appears to have no value.

Yellow-poplar is sensitive to the length of daylight. Trees of this species grow well in a greenhouse under long-day conditions. Growth ceases and seedlings become dormant when day-length is shortened. Interrupting darkness with brief periods of artificial light increases height growth.

Damaging Weather and Other Agents

Glaze destroys many stems of this deciduous tree throughout its range. Ice forms as cold rain falls on branches when temperatures drops below freezing. Strong winds further lower the chill factor. The ice weighs down branches and boles on the windward side. When frozen, the brittle branches easily crack. Broken tops cause permanent crooks as lateral branches assume phototropic dominance. Injuries also expose wood and inner bark to attack by insects and fungi. Branches stripped of leaf-bearing twigs by ice reduce growth for several years. Thus, dense stands in areas where glaze frequently occurs are less subject to injury than are open, or thinned, woodlands. The weight of snow also breaks tops from low-vigor yellow-poplar trees, enabling introduction of rot-causing fungi, greatest decay occurring where heartwood is exposed.

The sun's rays on southerly facing sides of trees growing on slopes of that aspect penetrate the thin bark to kill new cambial cells and to raise blisters on the boles. Sunscald is then apparent. At the other weather extreme, frosts cause similar injury, again the cold penetrating the thin bark with poor insulating quality.

Birds of many kinds in their search for food peck on the surface of yellow-poplar boles, leaving hundreds of black spots in tangential lines when viewed from the end of a felled and bucked log. Some stems appear to be immune; others repeatedly attacked. The holes made by the birds' beaks in time fill with callus tissue formed with the bark.

Microorganisms, including fungi and nematodes, are consistently more abundant in soils supporting yellow-poplar than in those supporting pines. The heartwood-rotting fungus most destructive of this species is a Collybia velutipes infecting 25% of the trees in a stand. The organism's mycelia grows as much as a foot a year, upward or downward, from the point of entry into the tree. Pocket rot, caused by the shoestring-forming fungus Armillaria mellea, appears to dissolve wood in the region of the medulary rays, those ribbon-shaped strands of tissue extending radially from the pith across the grain. Irregular cavities then develop. Sometimes roots are infected.

Fungi that cause other heartrots, honeycomb pocket rots, butt rots, and cankers also attack yellow-poplar. Boles with a target-like growth on the trunk, by which the Nectria canker disease is readily identified, seem not to be greatly damaged as long as annual diameter growth is at least one-tenth inch. Growth then is sufficiently rapid to heal the canker wounds.

Of great current concern is a dieback caused by either a fungus (Myxosporium) or a bacterium (Xanthomonos). Symptoms are chlorotic atrophy of foliage, a sparse crown, and dying twigs. As epicormic branches die, lateral cracks are left in the bark, giving the appearance of frost-shake. Death of stems occurs within a few years.

Fire wounds on the thin-barked tree are entry points for fungi, the spores coming to rest on exposed wood. Almost all yellow-poplar trees with wood exposed from fire injuries contain decay, the infection growing beneath the bark of healed-over scars. While the thicker, more fire-resistant bark of larger, older trees provides some insulation, the heat of hot fires penetrates it.

Figure 4.13 Clumps of mesquite tree, the legume originating from seed in cattle pats in Texas and Oklahoma. Larger stems provide charcoal material and, because of the rich reddish color of the wood, are milled into ornamental pieces, furniture and gunstocks. Attempts by ranchers to eradicate the tropical tree are futile. Also known as the screwbean, as the thick, linear pod twists 12 to 20 turns into a narrow, straight spiral.

Figure 4.13 Clumps of mesquite tree, the legume originating from seed in cattle pats in Texas and Oklahoma. Larger stems provide charcoal material and, because of the rich reddish color of the wood, are milled into ornamental pieces, furniture and gunstocks. Attempts by ranchers to eradicate the tropical tree are futile. Also known as the screwbean, as the thick, linear pod twists 12 to 20 turns into a narrow, straight spiral.

Insects also take their toll of yellow-poplar trees in the southern forest. An ambrosia beetle, called the Colombian timber beetle, is probably the most injurious. The tuliptree softscale, found in great clusters on branch bark, kills only limbs. Leaf-feeding aphids, maggots, and the caterpillar of a moth also feed on these trees, but do little damage.

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