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The concepts of diagnostic species and fidelity have been used frequently in European phytosociology but rarely in North American vegetation classification. We developed a classification of the vegetation of a mountainous area of northern Utah and compared the diagnostic species approach with the indicator-species approach of habitat type classification sensu Daubenmire prevailing in the U.S. Interior West. A total of 157 forest and nonforested plots were described by vascular plants and basic environmental factors. Clustering with RandomForest classification and ordination reduced the original number of plots to 26 meaningful vegetation units. Of these 26 units, 22 were strong, having four or more faithful species. Four units were weak, having less than three faithful species. We identified species diagnostic of particular vegetation units that are potentially useful for recognition of these units in the field. We proposed vegetation types at the level of vegetation alliances and associations, and correlated them with environmental factors. We found our vegetation units to be more strongly associated with the underlying environment than major habitat types sensu Daubenmire. Our approach to classification has the potential to directly link vegetation with the physical environment and could be the basis for a substantial improvement of vegetation classification in the central Rocky Mountains. [ABSTRACT FROM AUTHOR]