Host Specificity of Epiphytic Bryophytes near Mountain Lake, Virginia

Abstract

Fifty-four species of mosses and eighteen species of liverworts were present on the bases (0-1.8 m) of 120 trees belonging to six host species in a cove forest. Of the epiphytic species occurring at a frequency of 20% or more, only three were restricted to just one host species but 21 species showed strong singlehost preferences. Epiphytic communities named after their host species were ordered using frequency-based coefficients of similarity as follows (with the most dissimilar communities at the two poles): red oak-red maple-beech-yellow birch-black birch-hemlock. This arrangement also represented a gradient of decreasing species richness of epiphytic communities. The ordering of epiphytic communities was correlated with a gradient of decreasing bark pH and in part with the water absorption capacities of the barks. Epiphytic bryophytes are important in the ecology of forest communities. Not only do they serve as sites for accumulation of minerals (Witcamp 1970; Rasmussen & Johnsen 1976; Grubb et al. 1969) and radionuclides (Odum et al. 1970) but they also provide food, moisture, shelter and camouflage for certain species of arthropods (Gerson 1969). Epiphytic bryophytes are also significant as ecological indicators. The composition of epiphytic communities of bryophytes varies predictably with levels of pollutants such as sulphur dioxide (Gilbert 1970; LeBlanc et al. 1972). Species composition also varies with climate. Impoverishment of epiphytic communities in some cities may be attributed partly to aridity (Stringer & Stringer 1974). Changes in species composition at different heights on a tree corresponds with progressive changes in microclimate (Hosokawa et al. 1964). In view of the ability of epiphytes to indicate levels of particular chemical and physical factors, understanding the reasons for the host specificity of many epiphytic bryophytes is an interesting challenge. When an epiphytic bryophyte prefers a given tree as a host, it has chosen a complex hierarchy of interacting biotic, chemical and physical factors of substrate and habitat (Barkman 1958). The problem is made all the more interesting by the fact that host specificity is rarely absolute. Most species of epiphytic bryophytes in Europe and eastern North America are evidently facultative; in different geographic regions epiphytic species may switch preferred host species or even switch to terrestrial substrates (Barkman 1958; Slack 1976). It would seem logical to investigate the problem of host specificity in an environment where bryophytes display luxuriant growth and high species richness. Therefore a cove forest identified by Patterson (1940) as having an unusually rich bryoflora was chosen as the study site. For the same reason, the focus was on tree bases where bryophytes are more abundant than further up the trunk (Culberson 1955; Hale 1955). This study had two major objectives. Firstly, the host specificity of bryophyte communities on six species of trees (red oak, red maple, beech, yellow birch, black birch and hemlock) was investigated. 007-2745/82/37-50$1.55/0 This content downloaded from 157.55.39.35 on Thu, 01 Sep 2016 04:53:23 UTC All use subject to http://about.jstor.org/terms 38 THE BRYOLOGIST [Volume 85 Secondly, an attempt was made to correlate differences in bryophyte communities on different species with bark characteristics.