A Comparison of Ordination Techniques for Interpreting Forest Composition in the Southern Appalachians

Southeastern Geographer Vol. 26, No. 1, May 1986, pp. 12-24 A COMPARISON OF ORDINATION TECHNIQUES FOR INTERPRETING FOREST COMPOSITION IN THE SOUTHERN APPALACHIANS Albert J. Parker and Kathleen C. Parker INTRODUCTION. The proliferation of multivariate statistical techniques and the parallel evolution of sophisticated computers have revolutionized the study of vegetation in the past several decades. Enormous quantities of field data may now be handled efficiently by computers and distilled into a few prominent ecological controls by ordination techniques. An important methodological theme over the past twenty years in vegetation science has considered the efficacy and appropriateness of using a variety of different ordination procedures for describing, analyzing, and interpreting vegetation patterns. (J) Generally speaking, contributions to this literature approach the issue on the grounds of statistical rigor, often with some appeal to biological reason. (2) Here, we depart from this approach. We propose to apply two of the most commonly used ordination techniques—principal components analysis and detrended correspondence analysis—to field data in order to compare the interpretive value of each technique. (3) Although we do not argue that our findings can be generalized beyond the present example, the results offer a commentary on the practical application of ordination methodologies to field data. The small data set being analyzed with the two different techniques consists of species lists compiled for eleven trial segments in both the highlands of northern Georgia and the Smoky Mountains of North Carolina and Tennessee. These segments were chosen to reflect an elevational gradient within the two regions and a topographically-controlled site moisture gradient within the lower elevation zones of each region. STUDY AREA AND METHODS. The vegetation of the southern Appalachians has been studied by several plant ecologists in the past half century, with noteworthy contributions by Cain, Braun, Oosting and Dr. A. J. Parker is Associate Professor of Geography and Dr. K. C. Parker is Assistant Professor of Geography at the University of Georgia in Athens, Georgia, 30602. Vol. XXVI, No. 1 13 Billings, Whittaker, and Golden. (4) We do not intend to add substantively to this literature. Indeed, we make use of vegetation patterns described by these authors to interpret our own data. Species lists and environmental descriptions were compiled for each of eleven trail segments, four located in the highlands of northeastern Georgia (within the Chattahoochee National Forest), and seven located in Great Smoky Mountains National Park, North Carolina/Tennessee (Table 1). In northern Georgia, the Dukes Creek site is from a southwest -facing slope at lower elevation (640 m), the Anna Ruby site is from a sheltered southward-draining ravine at lower elevation (640 m), the Tesnatee Gap site is an east-facing ridge/gap at intermediate elevation (1000 m) and the Brasstown Bald site is a south-facing slope at high elevation (1370 m). In the Smokies, five sites represent the lower elevation zone: the open southwest-facing slopes of the Piney Mountain site (940 m), the northwest-facing slopes of the Rocky Spur Branch site (910 m), the westward-draining ravine slopes of the Roaring Forks site (850 m), the open, northwest-facing slopes of the Maddron Creek site (790 m), and the sheltered, north-facing cove setting of the Albright's Grove site (980 m). At middle elevation is the southwest-facing Alum Cave Creek site (1220 m), and at the highest elevation is the Forney Ridge site (1830 m). Species lists were compiled by walking 0.1 to 1.0 km segments of each of the eleven trails and recording all tree species encountered. TABLE 1 SITE CHARACTERISTICS AND RELATIVIZED DETRENDED CORRESPONDENCE ANALYSIS AXIS SCORES Trail Segment Locale Elevation (m) Aspect Topographic Position DCA Axis Score Forney RidgeSmokies1830S upper slope100 Alum Cave CreekSmokies1220SWlower slope40 Brasstown BaldNo. Ga.1370S upper slope34 Roaring ForksSmokies850NWlower slope23 Albright's GroveSmokies980N middle slope20 Anna RubyNo. Ga.640S lower slope11 Tesnatee GapNo. Ga.1000SEupper slope10 Rocky Spur BranchSmokies910N middle slope8 Maddron CreekSmokies790NWmiddle slope8 Piney MountainSmokies940SWupper slope1 Dukes CreekNo. Ga640SWupper slope0 14Southeastern Geographer Efforts were made to select trail segments which provide representative floras and which remain within a single habitat type. Ordination techniques arrange samples on the basis of their compositional similarity. Although different ordination techniques make different assumptions about...