Bruce A. Roundy

WHAT MAKES GREAT BASIN SAGEBRUSH ECOSYSTEMS INVASIBLE BY BROMUS TECTORUM

Ecosystem susceptibility to invasion by nonnative species is poorly understood, but evidence is increasing that spatial and temporal variability in resources has large-scale effects. We conducted a study in Artemisia tridentata ecosystems at two Great Basin locations examining differences in resource availability and invasibility of Bromus tectorum over elevation gradients and in response to direct and interacting effects of removal of perennial herbaceous vegetation and fire. We monitored environmental conditions, soil variables, and B. tectorum establishment and reproduction over two years. Soil water (measured as the number of days soil matric potential was .� 1.5 MPa) and nitrate availability (measured as micromoles of NO3 � sorbed to resin capsules per day in the ground) decreased with decreasing elevation. Lower-elevation sites had greater annual variability in soil water availability than upper-elevation sites did. Soil nitrate levels were highest at all elevations when soils were wettest; nitrate availability was not more variable at lower elevations. Removal of herbaceous perennials increased soil water and nitrate availability, but burning without removal had only minor effects. Bromus tectorum had low establishment, biomass, and seed production on high-elevation sites and on a mid-elevation site during a cold, short, growing season probably due to ecophysiological limitations resulting from cold temperatures. Establishment, biomass, and seed production were variable at low elevations and best explained by soil characteristics and spatial and temporal variation in soil water. Removal and fire had minor effects on emergence and survival, but biomass and seed production increased two to three times following removal, two to six times after burning, and 10-30 times following removal and burning. Our data indicate that invasibility varies across elevation gradients and appears to be closely related to temperature at higher elevations and soil water availability at lower elevations. High variability in soil water and lower average perennial herbaceous cover may increase invasion potential at lower elevations. Soil water and nitrate availability increase following either fire or removal, but on intact sites native perennials typically increase following fire, limiting B. tectorum growth and reproduction. Following resource fluctuations, invasibility is lowest on sites with relatively high cover of perennial herbaceous species (i.e., sites in high ecological condition).

Perspectives and Processes in Revegetation of Arid and Semiarid Rangelands

Range revegetation research has been dominated by empirical studies that provide some information about what works or does not work under a given set of conditions, but tell us little or nothing about the underlying ecological processes. Research has emphasized the establishment of vigorous exotic grasses on specific sites rather than the establishment of persistent, biologically diverse plant communities. A more mechanistic research approach is needed to better understand factors governing germination, seedling establishment, and plant community development in natural and synthetic systems to guide revegetation toward biological diversity. This paper evaluates selected aspects of the present knowledge of revegetation science on arid and semiarid lands, and attempts to identify areas for future research direction. Specific concepts and aspects of succession and plant community development, such as seedbed ecology, temporal and spatial patterns of resource availability and use, species life history traits, and species interactions are important areas of research. Continuous measurement of detailed environmental and biological data at the appropriate scale (down to the size of small seeds) will allow development of mechanistic models which can be used to predict plant establishment and community development for different environmental conditions.

Resilience and Resistance of Sagebrush Ecosystems: Implications for State and Transition Models and Management Treatments

Abstract In sagebrush ecosystems invasion of annual exotics and expansion of piñon (Pinus monophylla Torr. and Frem.) and juniper (Juniperus occidentalis Hook., J. osteosperma [Torr.] Little) are altering fire regimes and resulting in large-scale ecosystem transformations. Management treatments aim to increase resilience to disturbance and enhance resistance to invasive species by reducing woody fuels and increasing native perennial herbaceous species. We used Sagebrush Steppe Treatment Evaluation Project data to test predictions on effects of fire vs. mechanical treatments on resilience and resistance for three site types exhibiting cheatgrass (Bromus tectorum L.) invasion and/or piñon and juniper expansion: 1) warm and dry Wyoming big sagebrush (WY shrub); 2) warm and moist Wyoming big sagebrush (WY PJ); and 3) cool and moist mountain big sagebrush (Mtn PJ). Warm and dry (mesic/aridic) WY shrub sites had lower resilience to fire (less shrub recruitment and native perennial herbaceous response) than cooler and moister (frigid/xeric) WY PJ and Mtn PJ sites. Warm (mesic) WY Shrub and WY PJ sites had lower resistance to annual exotics than cool (frigid to cool frigid) Mtn PJ sites. In WY shrub, fire and sagebrush mowing had similar effects on shrub cover and, thus, on perennial native herbaceous and exotic cover. In WY PJ and Mtn PJ, effects were greater for fire than cut-and-leave treatments and with high tree cover in general because most woody vegetation was removed increasing resources for other functional groups. In WY shrub, about 20% pretreatment perennial native herb cover was necessary to prevent increases in exotics after treatment. Cooler and moister WY PJ and especially Mtn PJ were more resistant to annual exotics, but perennial native herb cover was still required for site recovery. We use our results to develop state and transition models that illustrate how resilience and resistance influence vegetation dynamics and management options.

Understory Cover Responses to Piñon–Juniper Treatments Across Tree Dominance Gradients in the Great Basin

Abstract Piñon (Pinus spp.) and juniper (Juniperus spp.) trees are reduced to restore native vegetation and avoid severe fires where they have expanded into sagebrush (Artemisia tridentata Nutt.) communities. However, what phase of tree infilling should treatments target to retain desirable understory cover and avoid weed dominance? Prescribed fire and tree felling were applied to 8–20-ha treatment plots at 11 sites across the Great Basin with a tree-shredding treatment also applied to four Utah sites. Treatments were applied across a tree infilling gradient as quantified by a covariate tree dominance index (TDI = tree cover/[tree + shrub + tall perennial grass cover]). Mixed model analysis of covariance indicated that treatment × covariate interactions were significant (P < 0.05) for most vegetation functional groups 3 yr after treatment. Shrub cover was most reduced with fire at any TDI or by mechanical treatment after infilling resulted in over 50% shrub cover loss (TDI > 0.4). Fire increased cheatgrass (Bromus tectorum L.) cover by an average of 4.2% for all values of TDI. Cutting or shredding trees generally produced similar responses and increased total perennial herbaceous and cheatgrass cover by an average of 10.2% and 3.8%, at TDIs ≥ 0.35 and ≥ 0.45. Cheatgrass cover estimated across the region was < 6% after treatment, but two warmer sites had high cheatgrass cover before (19.2% and 27.2%) and after tree reduction (26.6% and 50.4%). Fuel control treatments are viable management options for increasing understory cover across a range of sites and tree cover gradients, but should be accompanied by revegetation on warmer sites with depleted understories where cheatgrass is highly adapted. Shrub and perennial herbaceous cover can be maintained by mechanically treating at lower TDI. Perennial herbaceous cover is key for avoiding biotic and abiotic thresholds in this system through resisting weed dominance and erosion.

Pinon-juniper reduction increases soil water availability of the resource growth pool

Abstract Managers reduce piñon (Pinus spp.) and juniper (Juniperus spp.) trees that are encroaching on sagebrush (Artemisia spp.) communities to lower fuel loads and increase cover of desirable understory species. All plant species in these communities depend on soil water held at > −1.5 MPa matric potential in the upper 0.3 m of soil for nutrient diffusion to roots and major growth in spring (resource growth pool). We measured soil water matric potentials and temperatures using gypsum blocks and thermocouples buried at 0.01–0.3 m on tree, shrub, and interspace microsites to characterize the seasonal soil climate of 13 tree-encroached sites across the Great Basin. We also tested the effects of initial tree infilling phase and tree control treatments of prescribed fire, tree cutting, and tree shredding on time of available water and soil temperature of the resource growth pool on nine sites. Both prescribed fire and mechanical tree reduction similarly increased the time that soil water was available (matric potential > −1.5 MPa) in spring, but this increase was greatest (up to 26 d) when treatments were applied at high tree dominance. As plant cover increased with time since treatment, the additional time of available water decreased. However, even in the fourth year after treatment, available water was 8.6 d and 18 d longer on treatments applied at mid and high tree dominance compared to untreated plots, indicating ongoing water availability to support continued increases in residual plants or annual invaders in the future. To increase resistance to invasive annual grasses managers should either treat at lower or mid tree dominance when there is still high cover of desirable residual vegetation or seed desirable species to use increased resources from tree reduction. This strategy is especially critical on warmer sites, which have high climate suitability to invasive species such as cheatgrass (Bromus tectorum L.)

Response of conifer-encroached shrublands in the Great Basin to prescribed fire and mechanical treatments

Abstract In response to the recent expansion of piñon and juniper woodlands into sagebrush-steppe communities in the northern Great Basin region, numerous conifer-removal projects have been implemented, primarily to release understory vegetation at sites having a wide range of environmental conditions. Responses to these treatments have varied from successful restoration of native plant communities to complete conversion to nonnative invasive species. To evaluate the general response of understory vegetation to tree canopy removal in conifer-encroached shrublands, we set up a region-wide study that measured treatment-induced changes in understory cover and density. Eleven study sites located across four states in the Great Basin were established as statistical replicate blocks, each containing fire, mechanical, and control treatments. Different cover groups were measured prior to and during the first 3 yr following treatment. There was a general pattern of response across the wide range of site conditions. There was an immediate increase in bare ground and decrease in tall perennial grasses following the fire treatment, but both recovered by the second or third growing season after treatment. Tall perennial grass cover increased in the mechanical treatment in the second and third year, and in the fire treatment cover was higher than the control by year 3. Nonnative grass and forb cover did not increase in the fire and mechanical treatments in the first year but increased in the second and third years. Perennial forb cover increased in both the fire and mechanical treatments. The recovery of herbaceous cover groups was from increased growth of residual vegetation, not density. Sagebrush declined in the fire treatment, but seedling density increased in both treatments. Biological soil crust declined in the fire treatment, with no indications of recovery. Differences in plant response that occurred between mechanical and fire treatments should be considered when selecting management options.

Assessment of Range Planting as a Conservation Practice

ABSTRACT Natural Resource Conservation Service Range Planting — Conservation Practice Standards provide guidelines for making decisions about seedbed preparation, planting methods, plant materials selection, seeding rate, seeding depth, timing of seeding, postplanting management, and weed control. Adoption of these standards is expected to contribute to successful improvement of vegetation composition and productivity of grazed plant communities. Also expected are some specific conservation effects, such as improved forage for livestock; improved forage, browse, or cover for wildlife; improved water quality and quantity; reduced wind or water erosion; and increased carbon sequestration. The success of specific conservation practices and the magnitude of conservation effects are highly dependent on ecological-site characteristics, the initial degree of deviation from desired site characteristics, and weather, all of which are highly variable in both time and space. Previous research has produced few studies directly linking range planting conservation practices to conservation effects. Assessment of conservation effects attributed to rangeland planting practices must, therefore, be separated into two components: 1) evidence of the degree to which specific management practices have been shown to result in desirable vegetation change and 2) evidence supporting positive conservation effects of alternative vegetation states. The aggregate literature generally supports both 1) the existing conservation practice recommendations for rangeland seeding and 2) the inherent assumption that if these practices are successful, they will result in beneficial conservation effects. High spatial and temporal variability in these systems, however, may limit the success of generic or prescriptive management practices. Current conservation practice recommendations could be improved by incorporating more direct linkages to the ecologically based technical literature, more up-to-date information on adaptive management strategies in highly variable rangeland systems, and integration of monitoring strategies designed to directly test the efficacy of specific conservation practices.

Fire Rehabilitation Using Native and Introduced Species: A Landscape Trial

Abstract Following the 1999 Railroad Fire in Tintic Valley, Utah, we initiated a large-scale fire rehabilitation study comparing a predominately introduced species seed mix used by the US Department of Interior–Bureau of Land Management (BLM), a mix of native and introduced species provided by the US Department of Agriculture–Agricultural Research Service (ARS), and 2 native seed mixes (high and low diversity). Mixes were seeded with a rangeland drill on the big sagebrush (Artemisia tridentata var. wyomingensis [Beetle & A. Young] Welsh) study area whereas the pinyon–juniper (Pinus edulis Engelm.–Juniperus osteosperma [Torr.] Little) woodland study area was aerially seeded followed by 1-way chaining. On drill-seeded plots and by the third year after seeding the native high-diversity mix (16.4 kg pure live seed [PLS]·ha−1) had the highest seeded species cover (11.5%) and density (14 plants·m−2). Both the BLM (9.3 kg PLS·ha−1) and ARS (9.1 kg PLS·ha−1) seed mixes had higher seeded species cover (BLM = 8.5%, ARS = 8.2%) and density (BLM = 8.4 and ARS = 7.2 plants·m−2) than plots seeded to the low-diversity native mix (8 kg PLS·ha−1, cover = 3.8%, density = 3.6 plants·m−2). Indian ricegrass (Achnatherum hymenoides [Roemer and J. A. Schultes] Barkworth ‘Nezpar’) in the native high-diversity mix was especially successful on the sandy soils of the drill site, whereas seeds of other species may have been buried too deep for optimum emergence. Aerially-seeded and chained plots had similar and successful seeded species frequency, cover, and density (third-year average = 10.6% cover, 17.2 plants·m−2) among all species mixes. All seeded plots had lower cover of annual species than unseeded plots, indicating that revegetation is necessary to reduce weed invasion following catastrophic wildfire in big sagebrush communities lacking residual perennial understory vegetation.

Prediction of cheatgrass field germination potential using wet thermal accumulation.

Abstract Invasion and dominance of weedy species is facilitated or constrained by environmental and ecological factors that affect resource availability during critical life stages. We compared the relative effects of season, annual weather, site, and disturbance on potential cheatgrass (Bromus tectorum L.) germination in big sagebrush (Artemisia tridentata Nutt.) communities. Soil water status and temperature in the seedbed were measured continuously for 4 years on 9 big sagebrush sites in Nevada and Utah. Field plots at lower-, middle-, and upper-elevation sites were either undisturbed, or were burned, sprayed with herbicide, or both sprayed and burned. Spraying removed perennial herbaceous vegetation, whereas burning removed sagebrush. We used thermal-germination data from laboratory incubation studies of 18 cheatgrass seedlots and field soil moisture and temperature measurements to model and predict potential germination in the field plots for periods when seedbeds were continuously wet (above −0.5, −1, or −1.5 MPa) and across intermittent wet and dry periods. Season had the greatest effect on potential cheatgrass germination, followed by annual weather, and site variables (elevation and location); the effects of disturbance were minimal. Potential germination was predicted for most sites and years in spring, a majority of sites and years in fall, and few sites or years in winter. Even though disturbance has limited effects on potential germination, it can increase cheatgrass invasion and dominance by reducing perennial herbaceous species resource use and allowing increased cheatgrass growth and reproduction.

The Sagebrush Steppe Treatment Evaluation Project (SageSTEP): a test of state-and-transition theory

The Sagebrush Steppe Treatment Evaluation Project (SageSTEP) is a comprehensive, integrated, long-term study that evaluates the ecological effects of fire and fire surrogate treatments designed to reduce fuel and to restore sagebrush (Artemisia spp.) communities of the Great Basin and surrounding areas. SageSTEP has several features that make it ideal for testing hypotheses from state-and-transition theory: it is long-term, experimental, multisite, and multivariate, and treatments are applied across condition gradients, allowing for potential identification of biotic thresholds. The project will determine the conditions under which sagebrush steppe ecological communities recover on their own following fuel treatment versus the communities crossing ecological thresholds, which requires expensive active restoration.