Epiphyte Communities of the Swan Valley, Montana

Epiphyte communities in low-elevation conifer forests of the Swan Valley of western Montana appear to respond to complex gradients of canopy structure and density, stand age, and moisture. Epiphytes "read" the environment differently than do vascular plants. Specifically, stand age and moisture gradients that are reflected in the vascular vegetation are differently expressed by epiphytes. Epiphytes tend to equate young stands with dry stands and old stands with wet stands in the Swan Valley, more so than vascular plants. A strategy of investigation using both ordination and classification methods was used to clarify patterns in composition and environment. The ecology of epiphyte communities can be studied at various spatial scales, ranging from fine-grained patterns of competition or microclimate to global distribution patterns. Depending on the size of our observational window, different factors are important in controlling the distribution and abundance of epiphytes. Progress has been made in recent years toward understanding the relative importance of the various environmental factors operating at each scale. Yet at each level there are many questions which remain unanswered. The objectives of this paper are: (1) to give a descriptive account of epiphytes in the Swan Valley, (2) to suggest factors that may be important in controlling the distribution and abundance of epiphytes, and (3) to relate compositional patterns of epiphytes to those of the vascular species at the stand level. To evaluate these and other questions, all macrophytes were quantitatively sampled in 42 forest stands in the Swan Valley of western Montana. Average cover values are used to represent each species in each stand, an area that is more or less homogeneous in vegetation, environment, and history at the level of the largest species present. The approach of treating epiphyte communities at the stand level has been criticized for its failure to account for host specificity (Adams & Risser 1971): ". . . association values for lichen pairs in the forest are generally not useful since they are composites of the association values on the two tree species. The two tree species are not equally 007-2745/82/1-12$1.35/0 This content downloaded from 157.55.39.58 on Sun, 31 Jul 2016 05:37:17 UTC All use subject to http://about.jstor.org/terms 2 THE BRYOLOGIST [Volume 85 inhabited by all lichens; hence associations between lichens vary-sometimes radicallyfrom one tree to another." This is a problem in all community studies, as averages at one scale poorly express variation in smaller scales. For instance, a study of host specificity could be criticized for failing to account for differences in species associations on different aspects of the same tree species. Practical limitations usually confine studies to one or a few spatial scales. We feel that useful insights may be found at a variety of levels. Here we approach problems of epiphyte distribution and abundance at the "stand" scale.