Variation in Rhododendron calendulaceum
A Return to the Nantahala
Frank F. Willingham, Jr.
Dept. of Biology, Wake Forest University, Winston-Salem, N.C.
During the past year or so I have received a number of inquiries about the labels, bearing my name and address and the admonition "Experimental Plant - Do Not Remove," found on scattered plants of Flame Azalea throughout the Nantahala and Chattahoochee National Forests and along portions of the Blue Ridge Parkway. They seem to have been especially noticeable along the road leading to Wayah Bald, N. C. Accordingly, I thought it might be of some interest to ARS members in this area to know what this is all about.
Rhododendron calendulaceum (Michaux) Torrey, as anyone who has enjoyed spring in the southern Appalachians knows, is a quite variable species, especially with regard to flower color. In our most widely used regional flora (Radford, Ahles, and Bell, 1968) no special significance is attached to this apparently broad variation, but in some literature the notion exists of distinct early-blooming and late-blooming forms (Lee, Coe, Morrison, et. al., 1952: Skinner, 1961). It has been further suggested that the late-blooming form may have originated by introgressive hybridization of the early-blooming form with R. bakeri. The Nantahala National Forest region of North Carolina has long been suspected of being a center of this promiscuous azalea activity because of the great diversity of bloom types there, which have been interpreted as backcrossed recombinations. Wayah Bald, N.C. and Soco Gap, N.C. are usually considered to be the best locations at which to find typical late blooming R. calendulaceum.
With this information in mind, I began a study in the spring of 1971 to answer the following questions:
- What is the nature and pattern of variation in the Flame Azaleas of the Nantahala region?
- Are there sufficient discontinuities to justify the recognition of biologically distinct bloom forms?
- To what extent does R. bakeri occur in the Nantahala region? It is not now recognized as a part of the flora of North Carolina (Radford, Ahles, and Bell, 1968).
- Is there sufficient evidence to support the hypothesis of introgressive hybridization in the azaleas of this area?
Finding answers to these questions began with continuous field studies covering the period from spring, 1971 to mid-winter, 1973. Approximately 600 plants were tagged (hence the labels) throughout the study area and ecological and morphological data collected for each plant. A portion of each plant was permanently preserved (with permission!) for later use in biochemical, cytological, and microscopic studies. At present most of the data has been assembled and is being statistically analyzed with the aid of a computer. Although the final results are not yet in, some trends seem to be emerging. I am now under the impression that R. calendulaceum is somewhat better reproductively isolated from other azalea species than has been previously thought, and that hybridization between it and neighboring diploid species is a rare event. Also, some previously used taxonomic characters, such as flower color, appear to me to be profoundly influenced by environment and therefore should be used with caution, especially as markers of sub-specific taxonomic categories. A series of transplant studies is now being planned to fully document this.
Finally, R. bakeri does not appear to be present in the study area to any great extent, a fact which puts some stress on the introgressive hybridization hypothesis, but I welcome challenges on this point from anyone with conflicting information. I emphasize again the premature nature of these conclusions and request the privilege of altering them if the complete data analysis should warrant it. My purpose at this point is simply to explain those labels that keep turning up in the forest recesses. They will all be removed at the end of the study in the interest of good ecology.
Lee, F., F. O. Coe, B. Y. Morrison, et. al. 1952, The Azalea Handbook. Am. Hort. Soc., Washington, D. C. 148 ppg.
Radford, A. E., Harry E. Ahles, and C. Ritchie Bell, 1968, Manual of the Vascular Flora of the Carolinas. Univ. of N. C. Press, Chapel Hill, N. C., 1183 ppg.
Skinner, Henry T., 1961, Classification of the native American azaleas, Proc. Int. Rhododendron Conference, Portland, Ore., The American Rhododendron Society.