Xerophytic shrubs such as sagebrush, are a major component of rangelands throughout the western United States. Nevada rangelands are no exception. Several species are widely distributed across the state, and are often dominant members of rangeland plant communities. Because of the almost ubiquitous presence of sagebrush species on Nevada rangelands, federal agencies are mandated to include them in emergency fire stabilization and rehabilitation plans. They are often included in seed mixes on wildlife improvement projects, ecosystem restoration activities, and mined-land reclamation.
Large fires and poor success rates have re-emphasized the importance of research efforts to identify new shrub establishment technologies, particularly for sagebrush. Because of its wide amplitude of distribution, one or more species are often the primary shrub involved in restoration efforts. Because its prominent position in rangeland communities of Nevada, sagebrush establishment and ecology have been and continue to be important areas of research.
Although sagebrush is well adapted to the region when mature, it has long been known that establishing sagebrush from seed is often difficult (Blaisdell 1949, Medin and Ferguson 1972, McDonough and Harniss 1974, Harniss and McDonough 1976, Young and Evans 1991, Young et al. 1991, and Meyer and Monsen 1992). To date most of the research efforts surrounding sagebrush have been directed only at the discovery of cultural practices that may enhance initial establishment and early survival (Stevens et al. 1981, Coenenberg 1982, Monsen and Meyer 1990, Skilbred 1990, Cockrell et al. 1995, and Schuman et al. 1997). Simply ensuring initial germination and establishment is no guarantee of survival through or after the first critical growing season. Maier et al. (2001) and Perryman et al. (2000) described the episodic nature of big sagebrush recruitment and the climatic controls responsible in Wyoming. Climatic controls are different for different species and subspecies and vary on a regional basis. Nothing is currently known about recruitment pulses and their controls in Nevada.
Annual growth rings which are formed in the wood of many temperate plants have long been used as a means to assess plant age, past growth rates and to infer climate (Fritts 1976). The use of annual growth rings to describe population dynamics has received little attention. However, notable works include studies on age structure and mortality patterns of eastern North American trees (Hett 1971 and Hett and Loucks 1976), and on age structure and reproductive success of shrub populations in western North America (Roughton 1972). Baker has studied climatic control on both riparian woodland species (1990) and fire effects on the age structure of Pinus aristida (1991) using tree ring data. Related to this proposal, Cawker (1980) demonstrated evidence of climatic control on the age structure of big sagebrush in southern British Columbia, and estimated that 40-50% of the age structure variance could be attributed to climate. West et al. (1979) discovered that shrub recruitment in semi-arid plant populations occur in pulses with semi-logarithmic models best describing their survivorship curves. West et al. (1979) also concluded that climate controls plant cover and that years of drought create low cover values in the herbaceous community associated with sagebrush. Low herbaceous cover values in the previous year were correlated significantly to sagebrush population pulses the following year if precipitation was not limiting in the following year.
Ferguson (1964) established the reliability of dendrochronology methods as applied to big sagebrush. Growth rings are reliable and have been utilized to determine sagebrush plant ages for many years, (Diettert 1938, Cottam and Stewart 1940, Booth 1948, Blaisdell 1953, Quinnild and Cosby 1958, and Ferguson 1964). False and missing rings are almost non-existent in big sagebrush, and annual rings are generated very consistently and generally very easily identified (Ferguson 1964). Preliminary field studies for this project found similar characteristics. Crossdating can also be used as a control for accurately determining the establishment year for this species.
Sagebrush root growth occurs primarily in spring, a month before initiation of vegetative growth, when soils are still cold and wet (Fernandez and Caldwell 1975 and De Puit and Caldwell 1973). For seedlings that have poorly developed root systems, the "normal" climatic regime in Nevada may preclude root growth after initial germination. Poorly developed root systems may not be able to sustain the young plant through the usual summer drought period. This study will identify the temperature and precipitation regime necessary for overcoming the typical conditions and allow survival of natural sagebrush populations. Once these climatic controls are identified and understood, restoration personnel will be able to reproduce those conditions by utilizing existing or creating similar climatic conditions on both macro and microsites, by various means, in areas designated for sagebrush colonization.
Climatic variables may influence seed production in native sagebrush stands as well. Cawker (1980) concluded that drought or heavy rain in autumn (the time of seed production) strongly affects recruitment failure. Climate (drought) also reduces competition through the reduction of cover. Antecedent drought could weaken herbaceous competition enough for cohort establishment with a less than optimum amount of precipitation. Drought effects are similar to the effects of herbivory. Vale (1975) states “As brush was abundant in times prior to livestock grazing, its dominance of the vegetation today cannot be considered evidence of over-grazing.” Grazing may influence sagebrush density, but does not affect presence or absence.
Research has shown that the species range limit reflects climatic control, and that demography patterns of seedling establishment and survival reflect local and regional climatic fluctuations. That is, a record of these fluctuations is be preserved within the population age structure as variations in age class frequency. Since no work has been conducted in Nevada, we hypothesize that regardless of the influence of edaphic factors and competition, if signature optimum moisture conditions (climatic control) occur in any given year or group of years, there will be establishment and survival of a sagebrush cohort.