Age Structure, Species Composition and Succession in a Loess Hills Woodland.

The Loess Hills of central North America is a unique geologic region that historically was dominated by prairie with scattered oak trees in an environment of frequent fires. Following European settlement, woody-plant cover expanded throughout the Loess Hills so that today, existing prairies occur as remnants surrounded by an oak-dominated woodland. Large-, intermediate-, and small-sized trees were sampled along three adjacent lowland-to-upland belt-transects in western Iowa to document more accurately the dynamics of this woody expansion. Trees were cored or cross-sectioned and dated using standard dendrochronological techniques. Results characterize the pre-1850 woody canopy as consisting of a few widely scattered oak trees on both upper and lower slopes. The rapid development of an even-aged stand of bur oak (Quercus macrocarpa Michx) by the 1870s and 1880s suggests release of the species perhaps from its historic suppression by fire. Other species subsequently colonizing included hackberry (Celtis occidentalis L.) and green ash (Fraxinus pennsylvanica Marsh), both arriving principally in the 1920s and 1930s; American linden (Tilia Americana L.), arriving principally in the 1940s and 1950s; and ironwood (Oystra virginiana Mill.), arriving principally in the 1950s. Patterns of encroachment varied from uniform establishment across all elevational segments (e.g. bur oak and ironwood) to invasion from either upper slopes (e.g. green ash) or lower slopes (e.g. hackberry). The absence of small- and intermediate-sized trees suggest that bur oak will not remain a dominant species in future Loess Hills woodlands. These results quantify both our understanding of the dynamics of the Loess Hills woody-plant invasion and our knowledge of the historic plant community of the region, both important considerations as we move to better manage this unique resourc