Invasion of western juniper into vegetation dominated by mountain big sagebrush and perennial bunchgrass on the Owyhee Plateau of southwest Idaho appears to be directly related to cessation of periodic fires. Evidence from adjacent climax juniper stands indicates that fires were frequent for at least several hundred years preceding white settlement. Fires have been much less frequent during the past century due to active fire control, development of roads and other fire barriers, and reduced fuel because of heavy grazing and a shift towards decreased precipitation. Physical and biotic factors affecting the establishment of juniper, seed dispersal mechanisms, and the fire history of the study area were investigated. Results indicated that range condition as such had a negligible effect on juniper establishment. Juniper seedlings became established most readily on areas supporting well-developed herbaceous and shrubby vegetation. Seed dispersal was primarily localized, and accomplished by gravity and disturbance by animal trampling. Abundant evidence of fire in the form of charred stumps and fire scars on living trees was found throughout the study area. Old juniper stands are confined to rocky ridges where understory vegetation is sparse and fires less intense. Juniper was apparently kept out of the denser vegetation of deeper soils by more intense fires. Most herbaceous and shrubby species survived this treatment due to greater tolerance to fire, or rapid reproduction from seed.
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The climate cycles of the 2 million years of the Quaternary were a major force in the evolution of plant response to change. Quaternary climate has been primarily glacial with interglncials such as the current Holocene a minor component. Plant species responded individually to climate changes and, consequently, species composition has continually changed. The legacy of Quaternary climate change is that plant communities are far less stable than they appear to be from our perspective. They are unique at each location, difficult to define, and communities that are relics from a previous environment can be sensitive to small or transient environmental changes. Plant communities are variable both in space and time. Many ecological principles and concepts, and ecosystem pardigms derived from them, require revision to incorporate this variation. The concepts of habitat type and condition and trend, for example, do not reflect dynamic vegetation response to changes in climate. Our knowledge is presently insufficient to adequately Authors wish to thank Bill Laycock, Neil West, and the anonymous reviewers for their comments on the manuscript. Manuscript accepted 13 Mar. 1993. describe interactions between ecosystems and changing climate, but the patterns of vegetation response to environmental changes of the past may provide important information on vegetation response to present and future climate change. The concepts of thresholds, multiple steady states, and multiple successional pathways are helpful in understanding the dynamic interrelationships between vegetation and environmental changes.
Shrub crown characteristics useful in regression equations for predicting two biomass components (annual production and fine fuels) were identified for six shrubs common to the Great Basin. Shrub characteristics most useful in these equations were maximum and minimum crown diameter, and crown denseness and depth. Prediction equations were developed for each species or subspecies included in this study. Additionally, biomass equations were developed for combined species or subspecies of morphological similarity within the Artemisiu genus. As early as 1958, Evans and Jones addressed the practical importance of a method for determining forage production in which clipping or mowing was not necessary. Since then, much attention has been directed toward developing methods for predicting shrub production from easily measured crown dimensions. Volumetric or crown area relationships based on crown height and diameters have been described for serviceberry (Amelanchier alnifolia) by Lyon (1968), for eight Chihuahuan desert shrubs by Ludwig et al. (1973, and for big sagebrush (Artemisia tridentata wyomingensis) by Rittenhouse and Sneva (1977). Brown and Marsden (1976) and Alexander (1978) developed equations for predicting fuel loadings from height and percent crown cover measurements. Various stem diameter measurements were used as biomass predictors by Ohmann et al. (1976), Brown (1976), and Grigal and Ohmann (1977). Davis et al. (1972) related forage production to ring widths for several salt desert shrub species. Thus, the value and practicality of such methods has been well documented. The sagebrush taxon, Artemisia, constitutes the most abundant and widespread shrub component of ecosystems in the Great Basin and provides a source of food and cover for livestock and wildlife. Taxonomic difficulties exist within this genus at the species, subspecies, and variety levels. Although ecological differences have been described (Winward and Tisdale 1977), field identification of sagebrush species is often difficuh to determine. This is especially true between low and black sagebrush (A. arbuscula and A. nova) and the subspecies differentiations for big sagebrush (A. tridentata ssp. tridentata, A. tridentata spp. vaseyana, and A. tridentata spp. wyomingensis). A reliable dimension analysis technique for determining canopy biomass components in which species differences were not necessary would expedite field data collection and data analysis procedures. The primary objective of this study was to identify shrub crown characteristics which could be used to predict two shrub biomass components (annual production and fine fuels) and to formulate reliable prediction equations from these characteristics.
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