Yellow-Poplar Associations and Variation

Yellow-poplar is a component of 16 forest cover types (USDA Forest Service 1965). It is a major species in 4, and a minor species in the other 12:

At lower elevations in the Appalachian mountains, yellow-poplar is found with black locust; eastern white pine; eastern hemlock; hickories; white oak and other oaks; black walnut; shortleaf, pitch, and Virginia pines; flowering dogwood; sourwood; sweet birch; blackgum; basswood; and Carolina silverbell. At higher elevations, associated species include northern red oak, white ash, black cherry, cucumbertree, yellow buck-eye, American beech, sugar maple, and yellow birch. In the southern Appalachians, the percentage of yellow-poplar increases noticeably with increasing quality of the site. Where yellow-poplar occurs in pure or nearly pure stands on sites of medium or lower quality, it likely originated on abandoned old-field sites.

On bottomlands and better drained soils of the Coastal Plain, yellow-poplar occurs in mixture with tupelos, bald-cypress, oaks, red maple, sweetgum, and loblolly pine. In the Piedmont, associated species include oaks, sweetgum, blackgum, red maple, loblolly pine, shortleaf pine, Virginia pine, hickories, flowering dogwood, sourwood, and red cedar. Trees associated with yellow-poplar in nonmountainous areas of the North and Midwest include white oak, black oak, northern red oak, white ash, beech, sugar maple, blackgum, flowering dogwood, and hickories. Pure stands of yellow-poplar occupy only a small percentage of the area within the natural range of the species, but pure stands are usually on the kind of productive sites that include some of the most valuable timber-producing forests in eastern North America (Olson 1969).

Natural Variation

Many traits of yellow-poplar vary significantly among individual trees, among stands, and between geographic sources. These variations are of interest to forest managers and users of wood products, because they can be exploited through silvicultural practices and through selection and breeding programs (Dorman 1966, McKnight and Bonner 1961).

Varying degrees of genetic control have been demonstrated for wood and tree properties such as specific gravity and fiber length; straightness; branch angle; natural pruning ability; leaf, fruit, and seed characteristics; disease resistance; growth of seedlings; and length of growing season (Kellison 1967, Kellison 1968, Taylor 1964, Thor 1965, Thor 1975, Thor 1976, Thorbjornsen 1961, Wilcox and Taft 1969). For other important traits, such as the tendency to produce epicormic sprouts, evidence exists that the traits are strongly inherited, but the evidence is not conclusive (Della-Bianca 1972, Wahlenberg 1950).

Vaartaja (1961) has demonstrated the existence of photoperiodic ecotypes of yellow-poplar. Under very long days (3-hour dark periods) the northernmost source (Michigan) grew best, and the southernmost source (Georgia) grew least. With 6- to 12-hour dark periods, the converse resulted. At all photoperiods, the Indiana source was intermediate.

The most consistent difference among geographic seed sources has appeared in dormancy relationships (Farmer and others 1967, Funk 1958, Limstrom 1955, Sluder 1960, Webb 1970). In an experiment near Asheville, North Carolina, growth initiation of seedlings from 16 geographic sources ranging from Mississippi to Michigan and New York was highly correlated with conditions at the site of the mother tree; height-growth cessation was correlated with date of first killing frost of the source (Sluder 1960). In general, the more northern sources began to grow later and ceased earlier than the more southern sources. Noting a similar dormancy pattern, Funk (1958) found that damage from spring frost in a 3-year-old plantation in Ohio was generally related to latitude of seed source and planting site. Trees from the four southernmost locations suffered more extensive dieback than those from the three more northern sources. Funk (1958) emphasized the desirability of using locally produced yellow-poplar seed in forest planting. In a study of four geographic sources ranging from central Mississippi to Sewanee, Tennessee, trees from more southerly sources foliated earlier than those from northern areas (Farmer and others 1967).

Few studies are old enough to permit good comparisons of volume differences for different seed sources, but significant differences in early height growth have been reported (Farmer and others 1967, Kellison 1968, Limstrom and Finn 1956, Sluder 1960, Thor 1975, Thor 1976).

While most geographic differences are associated with latitude of source, environmental differences associated with altitude are also important. In North Carolina, Kellison (1967) demonstrated a clinal pattern of variation from coast to mountain for several seed and leaf characteristics. There was also a clinal pattern of variation in height growth for seedlings in an upper coastal plain nursery; height growth decreased from coast to mountains for seedlings from those locations (Kellison 1968). Thor (1975, 1976) found differences in height and diameter after 15 years for different attitudinal sources from Tennessee and adjoining states when outplanted in Tennessee. In general, the high-altitude sources performed most poorly. However, the test was limited in scope, so generalizations about altitudinal effects must be weighed with caution.

At least one distinct ecotype of yellow-poplar has been confirmed in North Carolina. The first evidence came from a plantation near Charleston, South Carolina; trees from a coastal plain source in eastern North Carolina were twice as tall 3 years after outplanting as those from a mountain source in western North Carolina (Lotti 1955). Later, Kellison (1968) found that trees from the Coastal Plain source of North Carolina performed poorly in comparison to upland sources when planted at a piedmont location, but were far superior to upland sources when planted on organic soils of the coastal plain where a pH of 4 is seldom exceeded. The Coastal Plain yellow-poplar ecotype has a distinctive leaf pattern and color- rounded lobes and copperish-red leaves. That ecotype is apparently adapted to highly acidic, water-saturated organic soils of the Coastal Plain, and can withstand periodic inundation without harm. Schultz and Kormanik (1975) showed that coastal plain yellow-poplar is very sensitive to soil moisture stress in comparison to upland sources. The Coastal Plain ecotype is apparently not restricted to North Carolina. Sources with the distinctive leaf characteristics have been found as far south as Florida (Kellison 1968).

No single seed source has been found that is adapted to geographical extremes, but neither have local sources always produced best growth. Distant seed sources and individual trees have performed well in some cases; but local sources have proved themselves to be in phase with their environment and have efficiently used the growing season available to them (Webb 1970). Current recommendations for seed collection would include selecting seed from a source where climatic and edaphic factors are similar to the proposed outplanting site, with altitude as well as latitude considered. Kellison (1968) recommended that North Carolina be divided into four zones for collection of yellow-poplar seed: 1) lower coastal plain (organic soils), 2) upper coastal plain (mineral soils), 3) piedmont, and 4) mountains.