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The effects of fire on biological soil crusts and their subsequent recovery in pinyon-juniper

Researchers:

Steven D. Warren, Warner College of Natural Resources, Colorado State University, 970-491-7478, sdwarren@cemml.colostate.edu 

Larry L. St. Clair, Director, Monte L. Bean Museum, Brigham Young University, 801-422-6211, Larry_StClair@byu.edu

Jeffrey R. Johansen, Department of Biology, John Carroll University

Paul Kugrens, Department of Biology, Colorado State University (deceased)

Study Dates:

September 2003 to September 2008

SageSTEP Study Plots:

Onaqui sage/woodland

Study Summary:

Biological soil crusts, comprised primarily of cyanobacteria, algae, lichens, and mosses living at or near the soil surface, are a key component of pinyon-juniper woodlands. They influence soil stability, hydrology, and cycling of macro- and micronutrients. They also affect the dispersal and germination of vascular plant seeds, as well as the survival and growth of seedlings. Research on the impacts of fire on biological crusts in pinyon-juniper woodlands is lacking, leaving managers to guess what effects they might be having on these important organisms when they conduct a controlled burn. To help provide answers, a study was conducted from 2006 to 2008 on the SageSTEP Onaqui woodland prescribed fire plot looking at the effects of fire on biological soil crust communities in pinyon-juniper woodlands. Results from this study provide information about the effects of fire on biological soil crusts and suggestions for managing encroaching woodlands in a manner that will not have long-term negative effects on the soil crust community.

The objectives of this study were to 1) determine the nature and extent of fire-related damage to biological soil crust communities by documenting changes in their cover, biomass, species composition and ecosystem function after a prescribed burn, and 2) document recovery dynamics over two subsequent years. A prescribed fire was implemented at the Onaqui woodland study site in October 2006 as part of the SageSTEP research project. Due to sub-optimal conditions, the fire did not successfully burn in mid- or late-successional tree stands; therefore, data collection for the biological crust study was limited to the early successional portion of the study site.

Typical views of juniper (left) and sagebrush (right) vegetation patch types present at the study site.
Interspaces are visible in both photographs.

Prior to the burn, 0.5 x 0.5 m plots were established in the three vegetation patch types present in early succesional juniper woodlands: juniper understory, sagebrush understory, and the interspaces between the scattered shrubs and trees. Aluminum tags painted with heat-sensitive paint were used to measure fire intensity in the plots. After the burn, a duplicate set of plots was established in an unburned area for comparison purposes. Data were collected in burned and unburned plots soon after the prescribed fire took place in October 2006, and again in September 2007 and June 2008. Data collected included cover  of vascular plants, mosses and lichens; algal biomass, density and species identities; and nitrogen fixation rates.

In general, analyses of the data indicated that while the burn negatively affected some components of the crust community in some parts of the early successional stage of the juniper woodland, the overall impact on the crusts was minimal. Results showed that mosses were rare under juniper trees, so the effects of the fire were negligible there. The burn significantly reduced the cover of mosses under sagebrush and in shrub interspaces. Lichens were uncommon under juniper and sagebrush, and were more common in shrub interspaces, but because the fire was spotty and of low intensity in the interspaces, they were minimally affected. The burn significantly reduced the biomass of green algae and cyanobacteria under juniper and sagebrush, but it was unaffected in the shrub interspaces. Similar trends were seen in algal density. Nitrogen fixation was significantly reduced under juniper trees but not under sagebrush or in the interspaces. Nitrogen fixation was approximately an order of magnitude greater in the shrub interspaces than beneath juniper and sagebrush. Because the interspaces were generally not affected by the burn, there was no significant impact on nitrogen fixation.

For management plans that include prescribed fires to reduce juniper, it appears that burning of early successional juniper woodland is appropriate because most affected trees were killed. Control of sagebrush can likewise be accomplished by low intensity, cool season fires without eliminating the crust component. Due to the spotty nature of the fire in the shrub interspaces where the best-developed biological soil crusts occur, they were only minimally affected by the fire and may provide a good source of algal inoculants to re-colonize the soil in the juniper and sagebrush vegetation patch types that were more affected by the fire.

The data from this study suggest that intense fires should be avoided due to the potential for greater encroachment into the shrub interspaces that contain the most developed crust community. This information, in addition to the fact that late successional juniper woodlands are difficult to burn, suggests that burning of early successional juniper woodlands may be a preferred method for controlling juniper encroachment on western rangelands. A final report for this project can be viewed at http://www.firescience.gov/JFSP_Search_ProjectID_Results.cfm.

 

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