Quantitative Analyses of Perennial Atriplex-Dominated Vegetation of Southeastern Utah
An Intensive study of the applicability of mathematical methodology to the ordination and classification of desert vegetation was undertaken during 1962, 1963 and 1964. The study area, forming part of the shad-scale zone vegetation and covering 13.5 square miles, is situated near Cisco in Grand Coun...
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| Format: | Others |
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DigitalCommons@USU
1967
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| Online Access: | https://digitalcommons.usu.edu/etd/3473 https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=4484&context=etd |
| Summary: | An Intensive study of the applicability of mathematical methodology to the ordination and classification of desert vegetation was undertaken during 1962, 1963 and 1964. The study area, forming part of the shad-scale zone vegetation and covering 13.5 square miles, is situated near Cisco in Grand County, southeastern Utah. Broad pediments originating from the nearby Book Cliffs are the main landform.
Geologically, the area was subjected to many cycles of erosion. The pediment and the quaternary remnants thin out with increasing distance from the Book Cliffs. The alluvial fan deposits are readily recognizable at three distinct levels. Mancos shale, a lead-gray Cretaceous shale of marine origin, forms the lower part of the Book Cliffs. The alluvial fan deposits are readily recognizable at three distinct levels. Mancos shale, a lead-gray Cretaceous shale of marine origin, forms the lower part of the Book Cliffs and of the pediments originating from it. The vegetation consists of widely-spaced species in which the dominant shrub species belong to the genus Atriplex. The soils have characteristics of Sierozem zonal soils (Aridisols), are often heterogeneous even within short distances and edaphic influences are strong.
In absence of any single over-riding factor, the erosion gradient provided the primary basis for the ordination of vegetation. The gradient is readily noticeable and is accompanied by edaphic and other micro-environmental changes. Four sub-divisions or segments can be easily distinguished. Each segment incorporates a degree of microenvironmental homogeneity and a distinct expression of vegetation in which the transition from the one to the next is usually abrupt. The dominant Atriplex species for each segment, I to IV, are Atriplex confertifolia, A. nuttallii gardneri, and A. Corrugata.
The data on canopy over and frequency were collected for each species. The analyses attempted embraced a wide range of quantitative techniques, namely grouping of species on peak CF (sum of relative canopy cover and relative frequency) value; analysis based on frequency x constancy index; association analysis (among species) using coefficient of interspecific association, chi-square, and their combination; derivation of homogeneous group of vegetation based on presence of single species showing positive association; association analysis and group study based on the use of correlation coefficient; multivariable approaches to the ordination of vegetation employing factor analysis preceded by partition of the sparse data matrix and the Q- and R-techniques of cluster analysis. Prevalent and modal species were also determined for each segment.
The study provided an excellent opportunity to compare and test the validity of results obtained from various analyses and also those that could be easily differentiated from inspection alone. The number and composition of groups derived showed considerable agreement in most cases, though slight variations were introduced inadvertently through subjective, and sometime inevitable, choice of qualitative and quantitative measures employed in particular analyses.
The quantitative approach, with an obvious advantage over the reconnaissance methods, provided a judgement on the significance of similarities and dissimilarities. It also made it possible to detect small differences which were more indicative of pattern, rather than a type, within vegetation area studies. The multivariate techniques of cluster analysis (Q- and R-analyses) illustrated superiority over other methods in that the cluster relations among various entities were readily discernable at all levels of affinity from the hierarchical dendrograms. On the other hand, the techniques utilizing statistical tests of significance necessitate preparing a new dendrogram every time a change is desired in the choice of probability level for testing hypotheses.
The analyses based on prevalent and modal species, and also those using peak values of CF and constancy x frequency indices, provided a quantitative measure of the habitat preference of component species. The quantitative approaches used in the study proved their usefulness and applicability, on the whole, to delimit accurate groups in the shadscale zone vegetation of Southeastern Utah. They also displayed a degree of flexibility, and sophistication, that may be needed in individual studies. |
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