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Natural and Assisted Recovery of Sagebrush (Artemisia tridentata) in Idaho’s Big Desert: Effects of Seeding Treatments and Livestock Grazing on Successional Trajectories of Sagebrush Communities

Background
Averaged over the last 10 years, approximately 95,000 ha (235,000 acres) of lands managed by the BLM in Idaho have burned annually. The BLM and other managers of Idaho rangelands, including the INEEL, must decide whether the burned areas need stabilization and rehabilitation treatments to prevent soil erosion and inhibit the invasion of exotic species such as cheatgrass (Bromus tectorum). Most of these rangelands have historically been dominated by big sagebrush (Artemisia tridentata), which does not resprout after fire. Sagebrush provides critical food and habitat for sage grouse, a species proposed for listing under the Endangered Species Act. With the accelerating loss of native sagebrush communities and habitat for sage grouse and other sagebrush-obligate species, sagebrush reseeding following fire has become an important consideration, as has the issue of livestock grazing impacts on recovering native vegetation and seeded areas. In the last three years, approximately 70 percent of the sage grouse habitat in eastern Idaho's Big Desert has been burned by wildfire. Fire suppression and rehabilitation costs are rising, and the threats to human life and property are increasing in eastern Idaho.

This study has been divided into three components to address management concerns relative to: (1) native plant recovery in good ecological condition rangeland, (2) success of aerial seeding sagebrush, and (3) whether livestock grazing affects recovery on sagebrush steppe rangelands. These three components will provide new scientific information that addresses current management concerns relative to wildfire impacts and rehabilitation treatments on the eastern Snake River Plain. These studies are designed to establish long-term, replicated monitoring sites that can be reread in the future to provide additional information to managers about post-fire recovery and rehabilitation success. These studies will also provide insight into restoring sagebrush and understory herbaceous species for sage grouse and other sagebrush obligate wildlife species and domestic livestock in the Great Basin.

Objectives
The overall objectives of this research project are to examine some of the key factors that influence trajectories of community diversity and structure following wildfire in sagebrush-steppe ecosystems. Specifically, the factors that influence the recovery of these systems following fire and the replacement of native plant communities with vegetation dominated by cheatgrass
(B. tectorum) will be examined. The three basic research objectives were to:

  • Describe post-wildfire trajectories in community composition and structure in areas in good ecological condition;
  • Compare sagebrush recruitment on areas that have been aerially seeded to areas relying on natural recruitment processes; and
  • Determine whether trajectories of community composition and structure differ between areas returned to grazing after fire and areas where grazing is excluded.

Accomplishments through 2004

The trend in seedling density measured in 2003 on the 1994 burn pointed to the role of wind in seed dispersal and therefore it was determined to collect better data on that distribution pattern. To collect that data, six, 1000 m transects were used extending downwind from the upwind edge of the fire. Fifteen meter radius plots were surveyed at 50 m intervals along those transects. The center of the first plot on each transect was set 20 m from the fire edge. These surveys were conducted in burns from 1994, 1995, 1996, 1999 and 2000. This data will be used to look for changes in that distribution pattern with age since fire, possibly allowing us to determine a rate of spread into the burn.

Because those fires cover a period spanning from relative wet conditions through the current drought, this data can be used to further test the idea that sagebrush establishment is controlled by climatic conditions and not seed dispersal alone.

To address the first and third objectives, paired research plots were established in a portion of the area burned by the 2000 Tin Cup Fire. Grazing exclosure fences were constructed around one plot from each pair. The exclosed plot will be used to address questions related to recovery of vegetation in ungrazed sagebrush steppe rangeland. The unfenced plot will be used to examine the role of livestock grazing on that recovery. In all of these plots, plant cover, species richness and diversity were measured. Permanent photoplots and photopoints were established and photographed.

To address the first objective further, plots for addressing plant density and species richness in some of the older burned areas on the INEEL were established.

Grazing treatments were initiated in 2003 so utilization measurements were initiated at that time. Utilization was measured with the Ocular Estimate Method. Key species (one grass and one forb) were selected for each plot. Selection criteria included consideration of the most abundant species that had actually been grazed and for which there were sufficient numbers of individuals in the plot to obtain a reasonable sample.

Results

Heterogeneity chi-square tests were used to determine whether seedling counts were evenly, or uniformly, distributed among the twenty positions along each transect line. Overall, seedling counts were not uniformly distributed among the twenty plot positions along each transect for 27 of the 29 transects. Seedling counts were not comparable at the same position along each transect line within each burn, or stated differently, seedling count patterns were different along each transect within a burn. Thus, the highest seedling counts in any single transect did not necessarily occur at positions near the burn edge. In fact, the positions with the highest seedling counts were quite variable from one transect to the next.

A Poisson distribution was calculated for the frequency of occurrence of seedling counts among the 120 plots for each burn. The calculated Poisson distribution was then statistically compared to the actual frequency of occurrence of seedling counts among the plots using a Chi-Square Goodness of Fit Test to determine whether or not sagebrush seedlings occur randomly within the burned area. If the actual frequency of seedling occurrence among plots departed significantly from a random distribution, a variance to mean ratio was calculated to determine whether the seedling spatial pattern was clumped or uniform. However, there was already certainty the distribution would not be uniform because the possibility had been ruled out by the heterogeneity analysis. The variance to mean ratio for the sagebrush seedlings/plot on the all burns indicated either clumped or highly clumped distributions.

Species Richness, Density and Frequency - A total of 79 plant species were encountered in the ten pairs of plots (20 plots). Three species found in 2003 were not found in 2004. All three were perennials and two were native species. Twelve species not found in 2003 were found in 2004. Seven of those twelve were present in 2002, but not in 2003. Five were species not previously found in these plots. All of the species added in 2004 were native and ten of those were annuals.

Coefficient of Community is the percentage of total species that the two communities have in common. It was calculated here as to compare the two plots of each pair for similarity in terms of the species present. Coefficient of Community varied from 0.72 to 0.90. The Coefficient of Community went up in all but one plot pair (Plot 10).

Species/area curves were plotted based on the species counted in the nested plots. The y-intercept and slope increased in nearly all of the species/area curves. This is likely due to the increases in species richness and frequency.

Utilization - Utilization of grasses on each plot ranged from 0.0 to 1.4 percent with an average utilization of 0.26 percent. Forb utilization ranged from 0.0 to 6.7 percent with an average utilization of 1.9 percent. These numbers are obviously very low and indicate that the needed grazing treatment was not achieved on these plots in the spring of 2004.

Plant Cover - Mean total cover was 11.7, 13.6 and 20.1 percent in 2002, 2003 and 2004 respectively. Native perennial grass basal cover increased from 1.5 percent in 2002 to 2.3 percent in 2003 and remained unchanged through 2004. Percent aerial cover of native perennial forbs remained relatively stable with a high of 4.1 in 2003 and a low of 3.6 in 2004. Cover by introduced species (primarily annual forbs) was 0.6, 0.3 and 1.9 percent in 2002, 2003 and 2004 respectively. Aerial cover by native annual and biennial forbs was 0.6, 0.0 and 3.2 percent in 2002, 2003 and 2004 respectively. The increase in cover by annual and biennial species in 2004 may have been the result of higher precipitation in May and June of 2004 compared to 2002 and 2003.

Shannon-Weiner Diversity Indices for each plot ranged from 1.20 to 2.75. The diversity index went up on most of the plots between 2003 and 2004. The Morisita's Similarity Indices for comparing plots within a pair ranged from 62.62 to 98.52. The similarity index went up on four pairs of plots (2, 4, 7 and 9) and went down on six plot pairs (1, 3, 5, 6, 8 and 10).

Older Burned Area Plots - Of the 32 plots planned for this study, surveys were completed on 18 during 2003 and the remainder was surveyed in 2004. A total of 101 species were encountered on the 32 plots. There were more species of annual forbs in 2004 than 2003 and they were present in higher densities in 2004. There is a trend for big sagebrush density and frequency to be higher in the older burns than the more recent ones. Green rabbitbrush density and frequency varied greatly from plot to plot and from fire to fire with no apparent relationship to fire age. Cheatgrass frequency averaged about 0.5, but the density rarely exceeded 10 plants m-2. There was no relationship between cheatgrass density or frequency with fire age. Halgeton density and frequency were higher on the areas burned in 1994 and 1996.

Project Conclusions

Natural Sagebrush Seedling Recovery - Sagebrush seedling spatial distribution was clumped in all five of the burns sampled. The 1994 and 2000 burns most closely approximate the exponential decay model that has been proposed for seedling distribution from the edge of a burn scar; those burns also have the highest variance to mean ratios, indicating the more strongly clumped distributions (likely driven by plots with higher seedling counts close to the burn edge). However, all of the burns sampled exhibited clumped seedling distributions, indicating that seedlings establish in clusters in burns that do not show a strong edge effect. Additionally, groups of seedlings may become established several hundred meters from the burn edge, as seen in all burns sampled except the 2000 burn. The clustered pattern may be influenced by seed from remnant islands. The clustered pattern may also reflect soils, topography, and microclimate.

The mechanism is likely a combination of remnant islands and an appropriate microhabitat, though the relative importance of each factor is unclear at this point. Most importantly, sagebrush seed does appear to become dispersed and seedlings established the interior of the burn scars that were sampled in substantial numbers and within a relatively short time frame. However, the presence of sagebrush seedlings in the interior of burn scars does not necessarily indicate a short recovery period; it simply indicates that initial seed dispersal and establishment may not be as limited as previously thought. These results also demonstrate that there must be seed dispersal from the fire edge well into the burn area (something greater than 1000 m). That annual seed rain may be important for providing seed each year far into the burn to take advantage of a climate-driven establishment event.

Natural Recovery Trajectory - Consistent increases in cover by native species have been observed during each year of the study. This happened concurrent with a severe drought. Total annual precipitation since the fire in 2000 ranged from 110 to 155 mm while the long term mean annual precipitation at the INEEL is 220 mm. It was also interesting to see the return of native annuals in 2004 acting as ephemerals responding to summer conditions that were wetter than normal. During 2004, the plots received a substantial portion of the total annual precipitation during late spring. This is in contrast to 2003 when there was no precipitation between early May and late August. At this time, it appears that none of the plots are at risk of a recovery trajectory to anything other than communities dominated by native perennial species.

When that trajectory is extended by considering the data collected in the older burn plots a similar pattern is seen. It was interesting to find no sagebrush on the survey plots in an area that burned more than 55 years ago. This along with previous data collected at the INEEL indicate that natural recovery times for Wyoming big sagebrush may be much longer than previously estimated. Similarly the lack of an apparent relationship between time since burn and green rabbitbrush frequency and density suggests that the assumption that rabbitbrush acts as a successional species may need to be reconsidered.

Recovery Trajectory With Livestock Grazing - Unfortunately, meaningful grazing treatments were not achieved during 2004. Cattle were released onto the allotment around July 1, 2004, and removed in early August. When it was found that some of the plots would be grazed, albeit later than expected, it was attempted to capture as much information as possible about that event. It was hoped to be able to extend the study for at least one more year in an effort to determine if livestock grazing has an effect. To that end, utilization was re-measured on the ten pairs of plots on August 10, 11 and 12, 2004. In addition, six new grazed plots were established in an area that had received moderate to heavy utilization. Utilization was measured on those plots as well. A plan is in place to continue surveying the ten paired plots and the newly established plots should funding come available.

Investigators

  • Mike Pellant, Idaho State Office of the Bureau of Land Management, Boise, ID
  • Roger D. Blew, Amy D. Forman, and Jackie Hafla, Environmental Surveillance, Education and Research Program, S.M. Stoller Corporation, Idaho Falls, ID
  • Robert Jones, U.S. Department of Energy Idaho Operations Office, Idaho Falls, ID
  • Greg White, Idaho National Engineering and Environmental Laboratory, Bechtel BWXT Idaho, LLC, Idaho Falls, ID
  • Alan Sands, The Nature Conservancy, Boise, ID

Funding Sources

  • U.S. Bureau of Land Management
  • U.S. Department of Energy Idaho Operations Office
  • The Nature Conservancy

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