Moisture Effects

Growth of yellow-poplar is sensitive to spring rainfall on most sites. Other than in the moist coves, where soil moisture is seldom limiting, growth depends on precipitation at the onset of earlywood growth and when buds break dormancy. Abundant soil moisture at the time cells divide in both the cambium layer beneath the bark and in the terminal buds encourages greater growth. Growth, however, seems unrelated to the previous growing season's temperature and precipitation. While not atypical for hardwoods, this contrasts to the response expected for conifers: for needleleaf trees, the previous year's rainfall often controls growth in the present year.

Old stands of mixed hardwoods nearby yellow-poplar forests exhibit lower ambient temperatures during the growing season than do the yellow-poplar woodlands. Lower evaporation and transpiration losses reflect this modifying influence of yellow-poplar upon microclimate. The effect extends beyond the edge of the mixed hardwood stand, those trees utilizing available soil moisture from under the adjacent pure yellow-poplar stand. Hence, water in the upper six inches of soil under the mixed hardwoods stands containing the moisture-absorbing lateral yellow-poplar roots, is higher than would be the case if the yellow-poplar trees were not present."

The soil landscape often appears as a mosaic of profiles reflecting occurrence and chemical characteristics of the ground cover vegetation and of individuals of the various tree species present. Thus, under individual crowns of yellow-poplar, in contrast to eastern hemlock, the soil has a higher pH, and more calcium, magnesium, and potassium than in the open. Mineralizable nitrogen also scales higher. Where certain other plants, such as rhododendron, occur in the understory, the differences in soil characteristics fade.18

That a tree so demanding of moist sites should also be killed by flooding indicates its sensitivity to a narrow range of moisture conditions. Short-term dormant-season inundation apparently does no damage, but a single overflow of a river bottom during the growing season virtually eliminates a stand.

The heartwood of most species is the darker, richer wood used for furniture and paneling. For yellow-poplar, the color and the amount of heartwood are controlled by soil moisture. The famous early-day forester W. W. Ashe noted that stems of this species found in fertile coves have a large core of heartwood. For those growing in rich limestone soils, the heart is dark brown, while trees on dry sites have a smaller zone of a lighter-colored interior core. As heartwood is desirable, and the darker the better for furniture and paneling, moist coves provide the best habitat for this tree. (Manufacturers now stain sapwood to appear as heartwood: some, however, prefer to retain the sapwood appearance for the variation in color of the surface of a furniture face19).

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