Sagebrush Demography

 

Background
The INL Site boundaries encompass approximately 2,300 km2 of the Upper Snake River Plain. The plant communities within the INL Site are characteristic of cold desert ecosystems, and many of them are dominated by sagebrush (Artemisia) species. The historical range of sagebrush steppe generally included much of the western United States; however, the extent and condition of sagebrush steppe plant communities have been declining rapidly in recent decades. Declines in sagebrush steppe have been followed in turn by declines in populations of sagebrush-obligate plant and animal species, which often result in consideration for regulatory protection for those species.

Although many of the sagebrush-dominated plant communities on the INL are still in good ecological condition compared to rangelands across the West, they have not been immune to the threats and stressors that have caused declines elsewhere. Results from the most recent analysis of the long-term vegetation transects indicate that the abundance of non-native species is increasing across the INL Site and that the abundance of big sagebrush (Artemisia tridentata) is decreasing. Over the past three decades, big sagebrush cover has declined to about half of the mid-1970s values.

At-risk populations, such as those of several sagebrush-obligate species, cannot be readily conserved through direct population manipulation. Instead, successful conservation strategies often involve habitat conservation or improvement or both. At the INL Site, developing an effective habitat management strategy for sagebrush-obligate species will certainly require defining best management practices for big sagebrush populations and associated plant communities. Adaptive management approaches for big sagebrush requires an understanding of current conditions, knowledge of the system dynamics and an ability to predict future conditions.

Sagebrush steppe has been actively manipulated for a variety of reasons over the past century, often with unpredicted outcomes. The uncertainty associated with sagebrush steppe management likely stems from a lack of knowledge concerning the population biology of big sagebrush, arguably the most important species in the sagebrush steppe ecosystem. Although many researchers have addressed the effects of disturbances in the greater sagebrush steppe ecosystem and have studied the recruitment and germination phases of the big sagebrush lifecycle extensively, very few have attempted to characterize the population dynamics of relatively undisturbed, mature stands.

The Sagebrush Demography Project is a research effort undertaken with the goal of providing enough INL Site-specific information about big sagebrush population biology to support development of an effective adaptive management strategy for sagebrush steppe plant communities. Characterizing the population dynamics of big sagebrush on the INL Site will allow us to determine if: undisturbed stands tend to have an even or uneven age structure, current stand condition can be used to predict future stand condition, stand die-offs are a result of advanced age, disturbance is necessary for stand regeneration and seed availability or germination restrictions limit establishment in poor-condition stands. Information gained from the results of this research will be integrated into the Conservation Management Plan as a part of a habitat management approach for obligate species. Specific benefits of this research on habitat management planning include: tools that can be used to determine the probability of future declines or improvements in stand condition, improved ability to target stands for monitoring or manipulation, increased confidence in identifying appropriate restoration techniques, reduced uncertainty about the results of stand manipulations and maximizing efficiency in monitoring, conservation and restoration efforts. In 2010, ring counts, data analysis and report writing will be completed to conclude the field investigation.

 

Objectives

The primary goal of the Sagebrush Demography Project is to support the Conservation Management Plan effort by facilitating the development of specific habitat management recommendations for sagebrush steppe at the INL Site and by providing guidance for assessing and monitoring the condition of big sagebrush stands. Two products will be developed for use in this conservation planning effort. The first is a comprehensive literature database and reprint collection containing sagebrush biology and ecology references pertinent to sagebrush steppe habitat management. The second product is a technical report resulting from a field investigation of sagebrush population biology (demography) at the INL Site. The specific objectives of the field study are to:

  • Characterize the typical stand age structure or range of stand age structures for mature sagebrush stands

  • Investigate how stand age structure relates to stand condition and shrub die-off for sagebrush

  • Examine the dynamics of sagebrush stand replacement in the absence of wildland fire.

 

This research effort was undertaken with the goal of providing enough INL Site-specific information about big sagebrush population biology to support the development of an effective adaptive management strategy for sagebrush steppe plant communities. Characterizing the population dynamics of big sagebrush on the INL Site will determine if: undisturbed stands tend to have an even or uneven age structure, current stand condition can be used to predict future stand condition, stand die-offs are a result of advanced age, disturbance is necessary for stand regeneration and seed availability or germination restrictions limit establishment in poor-condition stands. Information gained from results of this research will be integrated into the Conservation Management Plan as a part of a habitat management approach for obligate species.

During the summer of 2006, 14 stands of Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) were sampled. The vegetation data collected as a component of this study included shrub cover, sagebrush density, and individual shrub rank data for use in developing criteria for measuring stand condition. At each stand, cross section samples of sagebrush were also collected. The cross sections were labeled and archived for subsequent sanding and ring counts. The figure below shows a general schematic for vegetation and sagebrush cross section sampling at each plot.


Preparation and counts of annual rings in the sagebrush cross sections began in 2009. Cross sections were prepared by trimming each slab as close to the root/shoot interface as possible and sanding it through a series of three sandpaper grit sizes. Rings on each cross section were counted under a dissecting microscope twice; each count was made by a separate observer. When the ring count for a cross section differed by more than three rings for each observer, the cross section was counted by a third observer. When the ring count differed by three or fewer rings, an average of the first two counts was used. Counts were conducted along the radial arc of the cross section with the greatest number of rings. The center portion of the cross section was missing on several slabs due to growth pattern of the shrub and/or decomposition/delamination. When this occurred, the number of missing rings was estimated based on the average size of the rings in a particular cross section and the size of the area from which rings were missing. Approximately 90 percentof the slab processing and preparation and about 30 percent of the ring counts were completed in 2009.