Introduction and Background
The taxonomy/identity of the Abies species in the eastern United States and Canada has been confusing. Taxonomists have traditionally recognized two species as being native to eastern North America. Balsam fir (Abies balsamea (L.) Mill.) has an extensive and more or less continuous natural range through Canada and southward into Minnesota, Wisconsin, Michigan, New York, and northern Pennsylvania, with disjunct distribution through central Pennsylvania and northern West Virginia and Virginia, while Fraser fir (Abies fraseri (Pursh) Poir.) occurs only at higher elevations in the mountains of southwestern Virginia, eastern Tennessee, and western North Carolina (Figure 1).
The most prominent taxonomic feature used to distinguish between balsam and Fraser fir has been the relative length of the cone scales and bracts. For balsam fir, the bract is much shorter than and is fully enclosed within the cone scale, while in Fraser fir the bract is much longer than the cone scale and is exserted from the cone and reflexed downward. Attempts have also been made to differentiate between these two species on the basis of numbers of lines of stomata on the leaves and internal leaf anatomy, but individual variations make interpretations using those characteristics uncertain.
Some taxonomists have recognized two varieties of balsam fir, A. balsamea (L.) Mill. var. balsamea, the “typical” balsam fir and a “bracted” variety, A. balsamea (L.) Mill. var. phanerolepis Fern., which is distinguished from var. balsamea on the basis of the relative length of the bract and awn to length of the cone scale and by a slight variation in cone size. The range of var. phanerolepis has been identified as occurring within the range of var. balsamea at higher elevations in the mountains of the northeast, at lower elevations in Maine and the maritime provinces of Canada, as well as the small, isolated stands in the mountains of northern Virginia and West Virginia (Perry 1931, Fernald 1950, Little 1953).
Figure 1. Natural range of the balsam fir (Abies balsamea) complex in the eastern United States and Canada.
Classification of the small populations of fir at higher elevations in northern West Virginia and Virginia (Figure 1) has been particularly confusing. Trees from those populations have cones similar to balsam fir as well as trees with exserted and reflexed bracts characteristic of Fraser fir and intermediate-appearing forms (Figure 2). These populations have, at various times, been identified as A. balsamea (Millspaugh 1892, Core 1934, Core 1940), A. fraseri (Millspaugh 1913, Zon 1914, Brooks 1920, Fulling 1934, Wyman 1943), and A. balsamea var. phanerolepis (Perry 1931, Fosberg 1941, Fernald 1950, Little 1953, Strausbaugh and Core 1964), while Fulling (1936) and Core (1934) suggested that they might represent a separate species, A. intermedia, which was of hybrid origin between balsam and Fraser fir.
A number of studies have attempted to clarify the status of the Abies species in eastern North America. Oosting and Billings (1951) suggested that during the most recent glacial advance (Pleistocene), spruce-fir forests extended from Canada, south along the Appalachian Mountains to North Carolina and Tennessee, with a clinal pattern of phenotypic variation within that range. Since the glacial retreat, populations have become separated and have evolved to their present phenotypic expressions. Mark (1958) proposed that as the climate warmed, fir populations at lower elevations in the southern part of the range were replaced by other species, leaving only isolated stands at higher elevations. The gap between the A. balsamea and A. fraseri populations prevented gene flow from the northern populations, resulting in a reduction in the gene pool of A. fraseri during the recent xerothermic period, with genes responsible for phenotypes similar to A. balsamea being eliminated.
Figure 2. Cones from three trees of the Stone Coal Run collection, showing variations in bract length.
Myers and Bormann (1963) studied phenotypic variation in trees of A. balsamea var. balsamea and A. balsamea var. phanerolepis in response to altitudinal and geographic gradients in cone scale/bract ratios to measure intergradation between the two varieties. Their studies found a complete series of morphological forms connecting the two, with two clines within the A. balsamea population – one from lower to higher altitudes in the mountains of the northeastern United States and one at lower altitudes from coastal regions toward the interior of the continent. Based on their data, they questioned the taxonomic validity of separation of A. balsamea into two varieties and also suggested that A. balsamea and A. fraseri represent closely related and recently separated populations. Studies by Robinson and Thor (1969) and Thor and Barnett (1974) compared various characteristics of trees from the “intermediate” populations of fir growing in northern West Virginia and Virginia with those of trees of Fraser fir from Virginia, North Carolina, and Tennessee and balsam fir from Pennsylvania and New York. They concluded that the “intermediate” populations were not of hybrid origin but rather are relicts of a once continuous fir population having clinal variation along a north-south gradient.
Thor and Barnett (1974) also proposed that only one species of Abies be recognized in eastern North America, with three varieties: var. balsamea, var. phanerolepis (including the northern Virginia and West Virginia populations), and var. fraseri.
Studies by Clarkson and Fairbrothers (1970) using serological and electrophoretic investigations of seed protein of trees also concluded that A. balsamea var. balsamea and A. fraseri are closely related and recently separated taxa and that A. balsamea var. phanerolepis (from the mountains of northern West Virginia and Virginia) is more closely related to A. balsamea than to A. fraseri and is not of hybrid origin. Studies by Jacobs et al. (1983), using electrophoretic study of seed proteins, came to similar conclusions; their study also found that electrophoretic patterns for seed of “bracted” sources from Canaan Valley, West Virginia, and Mt. Desert Island, Maine, were identical.