Forest Regeneration on Abandoned Agricultural Fields in Western Tennessee

Southeastern Geographer Vol. 30, No. 1, May 1990, pp. 36-47 FOREST REGENERATION ON ABANDONED AGRICULTURAL FIELDS IN WESTERN TENNESSEE David Shankman INTRODUCTION. Plant succession on abandoned agricultural fields in the Southeastern United States has been the subject of intense interest since the 1930s. (I) Much of the land cleared for cultivation in this region has been abandoned and is now covered by young forest stands typically dominated by pines. Hardwood species occasionally establish with pines but usually are not a major part of the canopy until later successional development. Bormann investigated factors determining the role of pines and hardwoods in early old-field succession. (2) He found that sweetgum and other hardwoods were less common than pines because they were less tolerant of drought conditions on exposed fields. Later studies generally have been consistent with these findings. (3) Early succession cannot be accounted for entirely by timing of colonization but is also the result of differences in rates of growth and mortality . Species that are dominant at later successional stages have been found colonizing recently disturbed sites in a variety of habitats. (4) Early succession may be accounted for, in part, by some colonizing species outgrowing others and achieving dominance at a relatively young age. The slower-growing species initially are suppressed in the understory, but being longer-lived they may eventually dominate the overstory. Christensen and Peet found establishment of hardwoods to be common in young pine stands on the North Carolina Piedmont but noted that their presence in early succession did not necessarily indicate eventual canopy dominance. (5) There is substantial evidence to suggest that inhibition is an important mechanism in forest development. The early colonizers may inhibit establishment of later arrivals by usurping available resources; later establishment occurs only after disturbance or death makes more growing Thanks to Mark E. Ritke and Laurel G. Drake for their assistance in the field and Robert K. Peet and Thomas T. Veblen for reviewing an earlier draft of the manuscript. The figures were prepared by the Cartographic Research Lab at the University of Alabama. Dr. Shankman is Assistant Professor of Geography at the University ofAlabama in Tuscaloosa, AL 35487. Vol. XXX, No. 1 37 space available. Researchers have noted this limited colonization by new individuals after initial forest canopy development, referring to it as 'stem exclusion.' (6) Studies of small disturbances caused by the removal of a few trees in a stand at this stage of development showed that new stems do not emerge into the canopy; instead, preexisting individuals grow into and fill the available space. (7) Christensen and Peet suggested that initial rates of hardwood establishment in the North Carolina Piedmont depended partly on pine densities which influenced resource availability. (8) Both Bormann and Peet and Christensen noted that removal of pines or significant disturbance in early succession favored the establishment of hardwoods; moderate thinning of pines, however, caused accelerated hardwood growth but did not change the rate of hardwood colonization. (9) In contrast to this general pattern through much of the Southeastern United States in which pines dominate the initial sequence of old-field succession, the loess hills of western Mississippi and Tennessee are among the few areas in which pines are usually absent in early forest development. These loess areas were largely in cultivation as late as the 1940s, but the dense hardwood stands now occupying them indicate that the successional role of the early colonizing hardwoods is considerably different from that reported in pine-dominated areas. On loess, hardwoods are capable of rapid rates of colonization and growth when uninhibited by pines. The purpose of this investigation is to determine when hardwoods initially are established and to describe the population size structures of the dominant hardwood species at different stages of successional development on recently abandoned agricultural fields in western Tennessee. Age and size structure analysis is used to reconstruct stand history and to determine the successional status of the dominant species. Some of the questions addressed are 1) What is the initial period of hardwood colonization in the absence of pines? 2) How early does a hardwood canopy develop, and what is its composition? 3) What is the regeneration status of the dominant...