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A Multivariate Analysis of the Distribution of Lichens on Populus tremuloides in West-Central Canada

1974; American Bryological and Lichenological Society; Volume: 77; Issue: 4 Linguagem: Inglês

10.2307/3241799

1938-4378

J. W. Sheard, M. Evelyn Jonescu,

Ecology and Vegetation Dynamics Studies

Frequency data on the occurrence of lichens on the trunks of aspen trees at three localities in the parkland region of the Canadian Great Plains have been subjected to agglomerative classification and ordination by principal components analysis in an attempt to define community and species relations. The results suggest that the lichen species are most strongly influenced by moisture stress but are also significantly influenced by nutrient status and light regime of the habitat. Moisture availability is primarily related to habitat factors, the Cypress Hills and Biggar forest sites being relatively mesic, compared to the more exposed parkland sites. Trees at the Waterton forest site have a very smooth bark and hence provide a more xeric substrate despite the higher rainfall at that locality. The effect of aspect is of secondary importance generally but was critical at the parkland and Waterton forest sites where moisture was limiting. The higher nutrient status of the Cypress Hills forest site is due to an adjacent road, and a similar but less strong effect at the Cypress Hills and Waterton parkland sites to cattle. The high density of trees at the Waterton and Cypress Hills parkland sites is probably responsible for the apparent low light intensities at these sites; Biggar parkland has the highest light intensity. The environmental preferences of the lichen species are discussed, partly by reference to the better known ecology of some of the species in Europe, and tabulated. Jonescu (1970) reported the results of a study, the purposes of which to determine what species of lichens occur on trembling aspen in western Canada, to consider the behaviour of those species according to aspect in three separate geographic locations in the Aspen Parkland of the Canadian Great Plains, and to test the similarity of the lichen-aspen community among the three locations. The analysis was based upon data (table 2 in Jonescu, 1970) aggregated to show the performance of species in each of the 24 stand-quadrants studied which are defined by four aspects on the trunk, two habitats (forest and parkland), and three localities (Waterton, Cypress Hills, and Biggar). These data are presented as Table 1 in a rearranged form. The results of the study indicated that the structure of the Cypress Hills lichen-aspen community showed more similarity to that of Central Saskatchewan 1 The authors gratefully acknowledge the assistance of Mr. J. Waddington in the preparation of the figures. The study was supported by an N.R.C. operating grant to the senior author. 2 Department of Biology, University of Saskatchewan, Saskatoon. 3 Faculty of Education, University of Regina, Regina. This content downloaded from 157.55.39.157 on Tue, 17 May 2016 04:49:18 UTC All use subject to http://about.jstor.org/terms 1974] SHEARD & JONESCU: DISTRIBUTION OF LICHENS ON POPULUS 515 (Biggar) than to the foothills area of the Rocky Mountains (Waterton). This was apparently related to the relative smoothness of the aspen bark in the latter locality and consequent low moisture status of the habitat despite the relatively high precipitation at Waterton. It was also found that aspen-lichen communities of the eastand westfacing quadrants were very similar and that the northand south-facing quadrants were most dissimilar, again reflecting moisture stress. Species abundance similarly reflects moisture availability, and the relationship with aspect, locality, and habitat is conveniently summarized in Tables 2 and 3, assembled from the bottom row of Table 1. North quadrants have the highest frequency and south quadrants the lowest in both parkland and forest habitats. Waterton has the lowest abundance while the Cypress and Biggar localities are comparable. Unlike the latter two localities, however, the Waterton parkland has a slightly greater abundance of lichens than the forest habitat. While these results are interesting and valuable, their further interpretation is inhibited by the fact that the analysis of the aspen-lichen communities was carried out by one-dimensional ordinations of the six sites (2 habitats per locality) in terms of aspects, and the aspects in terms of sites (fig. 10 in Jonescu, 1970). This aggregation of stand-quadrant data is likely to obscure much of the structure in the original data matrix which could potentially be extracted by multior even two-dimensional ordination. Furthermore, this aggregation makes it very difficult to establish causality in terms of species performance for the structure that it proved possible to extract. Thus detailed discussion of species/site and aspect relationships was confined to the seven most common species which had total frequencies of greater than 20% in terms of the maximum species frequency attained. However, abundance is not necessarily of much help in explaining community relationships, particularly if weedy species which have a wide range of ecological tolerance are involved. The object of the present study is, therefore, to attempt to define more rigorously the stand-quadrant and species relationships with the aid of standard multivariate classification and ordination methods now frequently employed by phanerogam ecologists and phytosociologists. In so doing, it is hoped that the ecological parameters controlling these relationships will also become apparent. Although cryptogamists were amongst the first to use them (Beals, 1965a; Yarranton, 1967a,b), such methods are still regarded as computer-wizardry rather than tools of ecological research by many bryologists and lichenologists. An effort is therefore made in the following section to explain the methods used in an intuitive rather than a rigid statistical manner. It is hoped that a better understanding of the techniques will lead to their increased use by cryptogamists in the future. These remarks apply equally to taxonomists and ecologists since the methods used in both areas of endeavour are essentially the same. For information regarding the geography, geology, and climate of the aspen parkland, readers are referred to the original work of Jonescu (1970) where information concerning the density, size, and trunk morphology of Populus tremuloides at the six sites will also be found, together with the sampling methods employed in the study.