An objective index to probable successional relation- 

 ship between the community types in the aspen-conifer 

 cover type series and recognized conifer habitat types 

 (Mauk and Henderson 1984; Youngblood and Mauk 

 1985) was arrived at by computing a similarity index 

 comparing species' presence. Our premise was that spe- 

 cies with high constancy in one type are more likely to 

 occur with higher constancy in a related than in a non- 

 related type. A constancy of 50 percent in either one of 

 the compared types was required for a species to be 

 included in the comparison. Sorensen's community coeffi- 

 cient (Mueller-Dombois and EUenberg 1974) was then 

 computed as an objective similarity index comparing the 

 similarity of species constancy values for the two types. 



Data on 460 plant species representing 210 genera 

 were collected and evaluated. Of these, we used 168 of 

 the more important species to prepare the tables in 

 appendix A, which summarize the constancy of occur- 

 rence of species within community types and their aver- 

 age canopy cover. These tables permit a ready compari- 

 son of species composition in the different aspen 

 community types. 



The basal area and site index of aspen are summarized 

 by community type in appendix B. We computed 80-year 

 site index for each of the intensively sampled stands 

 using Jones' (1967) site index curves for aspen. Appen- 

 dix C contains total tree basal area and the percent con- 

 sisting of conifers. 



Undergrowth production, composition by vegetational 

 classes, and forage suitability are summarized in appen- 

 dixes D and E. We derived an approximation of suitabil- 

 ity as Uvestock forage from species forage suitability 

 ratings developed for the Intermountain Region by 

 USDA Forest Service (1981), and then adjusted this to a 

 percentage composition based on canopy cover esti- 

 mates. We consider the proportion of canopy cover com- 

 posed of species in each of three suitability classes 

 (desirable, intermediate, least) to be an index to the 

 value of the undergrowth as livestock forage. This was 

 computed for each community type by summing the con- 

 stancy times the cover data (appendix A) for each spe- 

 cies within each suitability class, and dividing by the 

 summation of constancy times cover for all classes to 

 give a relative percentage within each class. 



Appendix F shows the proportion of the stands sam- 

 pled on each of Utah's six National Forests that were in 

 each community type. A field form (appendix G) is 

 provided to facilitate acquisition of the data needed to 

 classify Utah aspen lands. 



Other Considerations 



An understanding of the general concepts guiding 

 development of this classification may add perspective 

 to its potential use. We sought to develop a natural clas- 

 sification, yet one that would incorporate technical con- 

 siderations, particularly with respect to vegetational 

 structure and type nomenclature. Perceived usefulness 

 was an overriding consideration. Vegetation structure is 

 considered important because of its effect on under- 

 growth composition and production, its reflection of 

 environment, and its possible relevancy to management. 



The nomenclature of the community types is arranged to 

 reflect up to three distinct structural layers that com- 

 monly form aspen communities: the overstory tree layer, 

 the shrub layer (when present), and the herbaceous layer. 

 The type name consists of the name of the indicator spe- 

 cies within each layer separated by a slash. 



The classification is based upon existing rather than 

 potential vegetation. Thus, the units should be readily 

 recognizable by forest and range technicians. However, 

 because it is a classification of existing vegetation, the 

 units reflect the effects of both abiotic and biotic 

 influences. This problem is abated somewhat by our 

 attempt to indicate the probable successional status of 

 each type and its relationship to recognized habitat 

 types. In arriving at type separations, our approach was 

 to stress the "boundary" or difference between types 

 rather than the modal conditions of each type. Usually 

 we found it best to separate the wettest types first and 

 proceed to those occupying drier sites. Our concept was 

 that species with high moisture requirements have more 

 difficulty growing under drier conditions than species 

 with less stringent moisture requirements have in grow- 

 ing under wetter conditions. 



A word of explanation regarding our concept of plant 

 community organization. Seldom, if ever, do plant com- 

 munities fall into clearcut taxonomic units, because no 

 two plant communities are identical; species composition 

 differs from place to place because of environmental and 

 successional gradients and because of chance. Even 

 where environmental stability and lack of physical dis- 

 turbance have permitted the development of climax com- 

 munities, subtle differences in the microenvironment, 

 and the factor of chance, contribute to certain differ- 

 ences in the kinds and amounts of species making up 

 each community. Such differences in species composition 

 are even more pronounced in seraJ vegetation, which 

 reflects, in addition, the variable effects of past distur- 

 bance. Vegetation classification, therefore, becomes a 

 matter of arbitrarily deciding upon the amount of varia- 

 bility in composition tolerable in defining the taxonomic 

 units. The amount of variability acceptable should be 

 determined by the need for and projected use of the 

 resulting classification. 



No single flora satisfactorily described all plant species 

 encountered during the course of our study. We relied 

 heavily on Hitchcock and Cronquist (1973) for northern 

 Utah, and Harrington (1954) for general Utah coverage. 

 Welsh and Moore (1973), Martin and Hutchins (1980), 

 and Arnow and others (1977) were used to help clarify 

 identifications when the primary floras appeared 

 inadequate. 



Certain species nomenclature usage should be noted. 

 The difficulty of separating some species without flowers 

 or mature fruits was resolved by combining the species 

 in question. Thus, Osmorhiza chilensis and Osmorhiza 

 depauperata are treated as O. chilensis, and Rosa 

 woodsii and Rosa nutkana are under R. woodsii. Other 

 species, though separately identified in the field, were 

 combined in the community-type summaries for con- 

 venience because of their similarity. These are: 

 Sambucus racemosa and Sambucus cerulea as 

 S. racemosa; Fragaria vesca and Fragaria virginiana as 



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