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Quantifying and predicting fuels and the effects of reduction treatments along successional and invasion gradients in sagebrush habitats - JFSP final report

Author(s): Douglas J. Shinneman, David S. Pilliod, Robert S. Arkle, Nancy F. Glenn
Year Published: 2015

Sagebrush shrubland ecosystems in the Great Basin are prime examples of how altered successional trajectories can create dynamic fuel conditions and, thus, increase uncertainty about fire risk and behavior. Although fire is a natural disturbance in sagebrush, post-fire environments are highly susceptible to conversion to an invasive grass-fire regime (often referred to as a 'grass-fire cycle'). After fire, native shrub-steppe plants are often slow to regenerate, whereas nonnative annuals, especially cheatgrass (Bromus tectorum) and medusahead (Taeniatherumcaput-medusae), can establish quickly and outcompete native species. Once fire-prone annuals become established, fire occurrences increase, further promoting dominance of nonnative species. The invasive grass-fire regime also alters nutrient and hydrologic cycles, pushing ecosystems beyond ecological thresholds toward steady-state, fire-prone, nonnative communities. These changes affect millions of hectares in the Great Basin and increase fire risk, decrease habitat quality and biodiversity, accelerate soil erosion, and degrade rangeland resources for livestock production. In many sagebrush landscapes, constantly changing plant communities and fuel conditions hinder attempts by land managers to predict and control fire behavior, restore native communities, and provide ecosystem services (e.g., forage production for livestock). We investigated successional and nonnative plant invasion states and associated fuel loads in degraded sagebrush habitat in a focal study area, the Morley Nelson Snake River Birds of Prey National Conservation Area (hereafter the NCA), in the Snake River Plain Ecoregion of southern Idaho. We expanded our inference by comparing our findings to similar data collected throughout seven major land resource areas (MLRAs) across the Great Basin (JFSP Project 'Fire Rehabilitation Effectiveness: A Chronosequence Approach for the Great Basin' [09-S-02-1]). We used acombination of field-sampling, experimental treatments, and remotely sensed data to address the following questions: (1) How do fuel loads change along gradients of succession and invasion in sagebrush ecological sites? (2) How do fuel reduction treatments influence fuels in invaded areas formerly dominated by sagebrush? (3) How do fuel loads vary across landscapes and which remote sensing techniques are effective for characterizing them? For Question 1, we sampled 148 1-ha sites over three years (2012-2014) to quantify and characterize fuel loadings across a comprehensive invasion-successional gradient in the NCA. Fifty-seven of these sites were sampled annually for three years to capture inter-annual variability in plant cover and biomass. To capture invasion and successional gradients, we sampled stratified random locations within unburned, burned-treated, and burned-untreated areas. We found that, although fuel loads and cover types varied across the landscape, herbaceous fuel loads were substantially higher in sites dominated by invasive species than in late-successional sagebrush stands. Moreover, the greatest variability in fine fuel loadings among sample years was found within highly invaded stands, highlighting both the interannual variability in fuel loadings, and the need to track fuel conditions in more degraded landscapes. These findings were consistent with findings from a project examining the effects of seeding treatments on plant cover, composition, and fuel structure across the Great Basin (JFSP Project 09-S-02-1), suggesting broad applicability to sagebrush-steppe ecological sites. For Question 2, we treated 48 experimental plots in nonnative plant-dominated communities located within three large replicate blocks with a full-factorial, completely randomized combination of the following management practices: mowing, mowing + herbicide, herbicide application, control (no treatment). Half of all plots were seeded with native species. We also out-planted live big sagebrush seedlings in some of the plots the following growing season. We found little or no difference in fuel loadings among treatment types, suggesting that treatment effects on fuels either disappeared within the first year, or were overshadowed by effects of inter-annual variability in precipitation. Seeding treatments did not result in detectable establishment for any species, while out-planted sagebrush seedlings survived for a limited duration during the growing season, likely due to drought conditions. Survival probabilities for sagebrush seedlings did increase with mowing, except when followed by seeding, probably because the soil disturbance from the minimum-till drill led to less bare ground cover (and hence, more competition with ruderal plants). Treatments had no significant effects on soil C decomposition or N mineralization rates. Thus, changes in soil nutrients were unlikely to explain observed treatment effects, or the lack thereof. For Question 3, we coupled our non-experimental field sites with terrestrial laser scanning, (TLS) sampling, airborne lidarimagery, and multispectral satellite imagery. These data were analyzed to determine which technologies were most effective for capturing key fuel-related vegetation characteristics, and to derive biomass estimates for different plant communities at various spatial scales and resolutions. We found that TLS, airborne lidar, and satellite-based imagery each have unique contributions for characterizing fuel-related vegetation characteristics across the landscape. Lidar (both TLS and airborne) can provide spatially explicit baseline data on the terrain surface and vegetation structure, including shrub biomass and cover. TLS also proved successful at capturing the structural characteristics of low-lying vegetation over plot-sized (1-ha) areas. While less precise than lidar, Landsat 8 satellite imagery was found to accurately map shrub biomass across the NCA. Herbaceous cover and biomass were challenging to map accurately with either satellite or airborne lidar imagery, but we did find that Landsat 8 was more accurate, less costly, and more repeatable than current airborne lidar technology. Fine-scale quantification of herbaceous cover and biomass were best mapped with TLS. The results of all three of our investigations may be applicable to a variety of sagebrush-steppe conditions, ranging from late-successional sagebrush stands to degraded, annual grass-dominated communities that are increasingly characteristic of much of the Snake River Plain and Great Basin. We provide detailed datasets, analyses, results and interpretations to the public and our agency partners in several publications, database tools, and remotely sensed data products (see Deliverables Crosswalk Table).

Citation: Shinneman, Douglas J.; Pilliod, David S.; Arkle, Robert S.; Glenn, Nancy F. 2015. Quantifying and predicting fuels and the effects of reduction treatments along successional and invasion gradients in sagebrush habitats - Final Report to the Joint Fire Science Program. JFSP Project No. 11-1-2-30. Boise, ID: US Geological Survey. 44 p.
Topic(s): Fire Effects, Management Approaches, Risk & Uncertainty
Ecosystem(s): None
Document Type: Technical Report or White Paper
NRFSN number: 15504
FRAMES RCS number: 21907
Record updated: Oct 9, 2017