Comparison of Unmanned Aerial Vehicle Platforms for Assessing Vegetation Cover in Sagebrush Steppe Ecosystems

In this study, the use of unmanned aerial vehicles (UAVs) as a quick and safe method for monitoring biotic resources was evaluated. Vegetation cover and the amount of bare ground are important factors in understanding the sustainability of many ecosystems. Methods that improve speed and cost efficiency could greatly improve how biotic resources are monitored on western lands. Sagebrush steppe ecosystems provide important habitat for a variety of species including sage grouse and pygmy rabbit. Improved methods of monitoring these habitats are needed because not enough resource specialists or funds are available for comprehensive on-the-ground evaluations. In this project, two UAV platforms, fixed-wing and helicopter, were used to collect still-frame imagery to assess vegetation cover in sagebrush steppe ecosystems. This paper discusses the process for collecting and analyzing imagery from the UAVs to 1) estimate percentage of cover for six different vegetation types (shrub, dead shrub, grass, forb, litter, and bare ground) and 2) locate sage grouse using representative decoys. The field plots were located on the Idaho National Laboratory site west of Idaho Falls, Idaho, in areas with varying amounts and types of vegetation cover. A software program called SamplePoint was used along with visual inspection to evaluate percentage of cover for the six cover types. Results were compared against standard field measurements to assess accuracy. The comparison of fixed-wing and helicopter UAV technology against field estimates shows good agreement for the measurement of bare ground. This study shows that if a high degree of detail and data accuracy is desired, then a helicopter UAV may be a good platform to use. If the data collection objective is to assess broad-scale landscape level changes, then the collection of imagery with a fixed-wing system is probably more appropriate.     

A Process-Based Application of State-and-Transition Models: A Case Study of Western Juniper (Juniperus occidentalis) Encroachment

A threshold represents a point in space and time at which primary ecological processes degrade beyond the ability to self-repair. In ecosystems with juniper (Juniperus L. spp.) encroachment, ecological processes (i.e., infiltration) are impaired as intercanopy plant structure degrades during woodland expansion. The purpose of this research is to characterize influences of increasing juniper on vegetation structure and hydrologic processes in mountain big sagebrush–western juniper (Artemisia tridentata Nutt. subsp. vaseyana [Rydb.] Beetle–Juniperus occidentalis Hook.) communities and to identify and predict states and thresholds. Intercanopy plant cover and infiltration rates were sampled in relation to juniper canopy cover. Study plots, arranged in a randomized complete-block design, represented low shrub–high juniper, moderate shrub–moderate juniper, and high shrub–low juniper percentage of canopy cover levels at four primary aspects. In field plots, percentage of plant cover, bare ground, and steady-state infiltration rates were measured. In the laboratory, juniper canopy cover and topographic position were calculated for the same area using high-resolution aerial imagery and digital elevation data. Parametric and multivariate analyses differentiated vegetation states and associated abiotic processes. Hierarchical agglomerative cluster analysis identified significant changes in infiltration rate and plant structure from which threshold occurrence was predicted. Infiltration rates and percentage of bare ground were strongly correlated (r² = 0.94). Bare ground was highest in low shrub–high juniper cover plots compared to both moderate and high shrub–low juniper cover levels on south-, east-, and west-facing sites. Multivariate tests indicated a distinct shift in plant structure and infiltration rates from moderate to low shrub–high juniper cover, suggesting a transition across an abiotic threshold. On north-facing slopes, bare ground remained low, irrespective of juniper cover. Land managers can use this approach to anticipate and identify thresholds at various landscape positions.     

Abundance and guild structure of grasshoppers (Orthoptera: Acridoidea) in communally grazed and protected savanna

This study was conducted to determine how savanna grass sward modifications caused by heavy grazing pressure influenced the abundance and guild structure of grasshoppers. Heavily grazed communal land was compared with a lightly grazed area and a mowed airstrip, in adjacent protected land, in the Mpumalanga lowveld, South Africa. Plant species composition, height, aerial cover and greenness of grass in the herbaceous stratum were measured in representative sites. Total grasshopper abundance and relative abundance of grasshopper species were also assessed in each site. Grasshoppers were assigned to feeding and habitat functional groups for comparison among the three areas. The heavily grazed area, characterised by short vegetation and low aerial cover, high greenness of grass, and high frequency of forbs, was inhabited by grasshopper species associated with bare ground or short and/or sparse grass, that were non-graminivorous or soft grass feeders. The lightly grazed area, characterised by tall vegetation and high aerial cover, low greenness of grass, and low frequency of forbs, was inhabited by grasshopper species associated with long and/or thick grass, that were mixed feeders or tough grass feeders. The mowed area, characterised by short vegetation and low aerial cover, low greenness of grass, and low frequency of forbs, exhibited lower grasshopper abundance, species richness, and diversity than either of the grazed areas.     

Ground Cover—What Are the Critical Criteria and Why Does It Matter?

This publication is the result of concerns expressed regarding the definition and subsequent use of ground cover in rangeland monitoring. We reviewed 20 monitoring publications. All publications reviewed contained a definition of ground cover and/or direction on how to monitor ground cover. The majority of these publications also defined bare ground. In all cases, bare ground was defined as the opposite of ground cover.We identified critical criteria of ground cover based on the role it plays in soil conservation as it relates to water and wind erosion. Critical criteria identified included standing and nonstanding live vegetation, standing and nonstanding dead vegetation including litter, and rock. We compared these critical criteria to the 20 monitoring publications reviewed. We found 19 of these publications included the criteria standing live vegetation or similar words and standing dead vegetation or similar words in their definition and/or use of ground cover. The one source where standing live or dead vegetation or similar words were not included was “Indicators of Rangeland Health and Functionality in the Intermountain West.” This publication was produced by the US Department of Agriculture, Forest Service, Rocky Mountain Research Station. Ground cover was limited to basal vegetation, litter, moss/lichen, or rock. We also found inconsistencies in the definition and subsequent use of ground cover in Forest Service Handbook 2209.21–Rangeland Ecosystem Analysis and Monitoring Handbook, Intermountain Region.We contend a large volume of literature supports the inclusion of critical criteria as identified in this report as ground cover. These criteria are essential components contributing to resistance of water and wind erosion important to soil conservation. This review demonstrates the importance of accurately defining and subsequently including critical criteria in rangeland attributes including ground cover. This paper addresses standardizing terms and calculations used in determining ground cover.     

Departures of Rangeland Fractional Component Cover and Land Cover from Landsat-Based Ecological Potential in Wyoming,USA

Monitoring rangelands by identifying the departure of contemporary conditions from long-term ecological potential allows for the disentanglement of natural biophysical gradients driving change from changes associated with land uses and other disturbance types. We developed maps of ecological potential (EP) for shrub, sagebrush (Artemisia spp.), perennial herbaceous, litter, and bare ground fractional cover in Wyoming, USA. EP maps correspond to the potential natural vegetation cover expected by environmental conditions in the absence of anthropogenic and natural disturbance as represented by the greenest and least disturbed period of the Landsat archive. EP was predicted using regression tree models with inputs of soil maps and spectral data associated with the 75th percentile of the Normalized Difference Vegetation Index in the Landsat archive. We trained our EP models with 2015 component cover maps on ecologically intact sites with relatively lower bare ground than expected. We generated departure of vegetation cover by comparing the EP and 2015 fractional cover. The departures represent land cover change from potential land cover and/or within-state changes in 2015. Next, we converted EP and 2015 fractional cover maps into thematic land cover and evaluated departure to determine if it was great enough to result in land cover change. The 2015 conditions showed reduced shrub, sagebrush, litter, and perennial herbaceous cover and increased bare ground relative to EP. Known disturbances, such as energy development, fires, and vegetation treatments, are clearly visible on the departure maps, but not on EP component maps. The most frequent departure from EP land cover was shrubland conversion to grassland. Land cover departures can be explained only in small part by known disturbance, and instead are ostensibly related to climate and land management practices. These drivers result in land cover departures that broadened the ecotone between shrubland and grassland relative to EP.     

Postfire Succession in Big Sagebrush Steppe With Livestock Grazing

Prescribed fire in rangeland ecosystems is applied for a variety of management objectives, including enhancing productivity of forage species for domestic livestock. In the big sagebrush (Artemisia tridentata Nutt.) steppe of the western United States, fire has been a natural and prescribed disturbance, temporarily shifting vegetation from shrub–grass codominance to grass dominance. There is limited information on the impacts of grazing to community dynamics following fire in big sagebrush steppe. This study evaluated cattle grazing impacts over four growing seasons after prescribed fire on Wyoming big sagebrush (Artemisia tridentata subsp. Wyomingensis [Beetle & Young] Welsh) steppe in eastern Oregon. Treatments included no grazing on burned and unburned sagebrush steppe, two summer-grazing applications after fire, and two spring-grazing applications after fire. Treatment plots were burned in fall 2002. Grazing trials were applied from 2003 to 2005. Vegetation dynamics in the treatments were evaluated by quantifying herbaceous canopy cover, density, annual yield, and perennial grass seed yield. Seed production was greater in the ungrazed burn treatments than in all burn–grazed treatments; however, these differences did not affect community recovery after fire. Other herbaceous response variables (cover, density, composition, and annual yield), bare ground, and soil surface litter did not differ among grazed and ungrazed burn treatments. All burn treatments (grazed and ungrazed) had greater herbaceous cover, herbaceous standing crop, herbaceous annual yield, and grass seed production than the unburned treatment by the second or third year after fire. The results demonstrated that properly applied livestock grazing after low-severity, prescribed fire will not hinder the recovery of herbaceous plant communities in Wyoming big sagebrush steppe.     

Responses of Chaparral and Oak Woodland Plant Communities to Fuel-Reduction Thinning in Southwestern Oregon

Fire suppression has led to large fuel accumulations in many regions of the United States. In response to concerns about associated wildfire hazards, land managers in the western United States are carrying out extensive fuel-reduction thinning programs. Although reductions in cover by woody vegetation seem likely to cause changes in herbaceous communities, few published studies have reported on consequences of such treatments for native or exotic plant species. We compared vegetation and abiotic characteristics between paired thinned and unthinned chaparral and oak woodland communities of southwestern Oregon 4–7 yr posttreatment and contrasted impacts of manual vs. mechanical treatments. Herbaceous cover increased on thinned sites, but species richness did not change. Herbaceous communities at thinned sites had an early postdisturbance type of composition dominated by native annual forbs and exotic annual grasses; cover by annuals was nearly twice as high on treated as on untreated sites. Absolute and proportional cover of native annual forbs increased more than any other trait group, whereas exotic annual forbs and native perennial forbs declined. Exotic annual grass cover (absolute and proportional) increased, whereas cover by native perennial grasses did not. Shrub reestablishment was sparse after thinning, probably because of a lack of fire-stimulated germination. Manual and mechanical treatment impacts on abiotic site conditions differed, but differences in vegetation impacts were not statistically significant. Fuel-reduction thinning may have some unintended negative impacts, including expansion of exotic grasses, reductions in native perennial species cover, persistent domination by annuals, and increased surface fuels. Coupled with sparse tree or shrub regeneration, these alterations suggest that ecological-state changes may occur in treated communities. Such changes might be mitigated by retaining more woody cover than is currently retained, seeding with native perennials after treatment, or other practices; further research is needed to inform management in these ecosystems.     

Spatial Predictions of Cover Attributes of Rangeland Ecosystems Using Regression Kriging and Remote Sensing

Sound rangeland management requires accurate information on rangeland condition over large landscapes. A commonly applied approach to making spatial predictions of attributes related to rangeland condition (e.g., shrub or bare ground cover) from remote sensing is via regression between field and remotely sensed data. This has worked well in some situations but has limited utility when correlations between field and image data are low and it does not take advantage of all information contained in the field data. I compared spatial predictions from generalized least-squares (GLS) regression to a geostatistical interpolator, regression kriging (RK), for three rangeland attributes (percent cover of shrubs, bare ground, and cheatgrass [Bromus tectorum L.]) in a southern Idaho study area. The RK technique combines GLS regression with spatial interpolation of the residuals to improve predictions of rangeland condition attributes over large landscapes. I employed a remote-sensing technique, object-based image analysis (OBIA), to segment Landsat 5 Thematic Mapper images into polygons (i.e., objects) because previous research has shown that OBIA yields higher image-to-field data correlations and can be used to select appropriate scales for analysis. Spatial dependence, the decrease in autocorrelation with increasing distance, was strongest for percent shrub cover (samples autocorrelated up to a distance [i.e., range] of 19 098 m) but present in all three variables (range of 12 646 m and 768 m for bare ground and cheatgrass cover, respectively). As a result, RK produced more accurate results than GLS regression alone for all three attributes when predicted versus observed values of each attribute were measured by leave-one-out cross validation. The results of RK could be used in assessments of rangeland conditions over large landscapes. The ability to create maps quantifying how prediction confidence changes with distance from field samples is a significant benefit of regression kriging and makes this approach suitable for landscape-level management planning.     

Long-Term Effects of Aeration and Fire on Invasion of Exotic Grasses in Mixed-Brush Plant Communities

Invasion of exotic grasses into grasslands dominated by native plants changes fire cycles and reduces biodiversity. Brush management practices that create soil disturbance, such as aeration, may potentially result in invasion of exotic grasses and replacement of native vegetation. We tested the hypothesis that a long-term effect of aeration and prescribed burning is an increase in exotic grasses. The study was conducted at the Chaparral Wildlife Management Area in the western south Texas plains where four treatments were evaluated: aeration, warm-season burn, aeration followed by a warm-season burn, and no treatment (control). The experimental design was a randomized complete block with four replicates. We estimated percentage canopy cover of exotic grasses, native grasses, forbs, litter, bare ground, and woody and succulent plants in 2007. There was a multivariate main effect among treatments for the dependent variables absolute canopy cover of exotic grasses, native grasses, forbs, litter, and bare ground (Wilks's Lambda F_15,179.84 = 2.78, P = 0.001). Variables that contributed to the significant overall effect included litter (F_3,69 = 4.32, P = 0.008) and native grasses (F_3,69 = 6.11, P = 0.001). The multivariate main effect of treatment was significant (Wilks's Lambda F_9,180.25 = 2.04, P = 0.038) for the relative canopy cover of herbaceous species. Relative cover of exotic grasses was 31% higher (P = 0.024) in control than in the prescribed burn treatment. Native grasses relative cover was 30% higher (P = 0.003) in prescribed burn than in the control treatment. We did not detect differences among treatments in the percentage of total woody and succulent plants canopy cover (P = 0.083). Under the environmental conditions at the time of the study, aeration and/or prescribed burning do not increase exotic grasses.     

Assessing Greater Sage-Grouse Breeding Habitat With Aerial and Ground Imagery

Anthropogenic disturbances, wildfires, and weedy-plant invasions have destroyed and fragmented many sagebrush (Artemisia L. spp.) habitats. Sagebrush-dependent species like greater sage-grouse (Centrocercus urophasianus) are vulnerable to these changes, making habitat monitoring essential to effective management. Conventional ground inventory methods are time consuming (expensive) and have lower data collection potentials than remote sensing. Our study evaluated the feasibility of ground (0.3-mm ground surface distance [GSD]) and aerial imagery (primarily, 1-mm GSD) to assess ground cover for big sagebrush (Artemisia tridentata Nutt.) and other vegetation functional groups important in sage-grouse breeding habitat (lekking, nesting, and brood rearing). We surveyed ∼ 526 km² of the upper Powder River watershed in Natrona County, Wyoming, USA, a region dominated by Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle & Young) communities interspersed with narrow riparian corridors. Our study area was used year-round by sage-grouse and included 16 leks. In June 2010, we acquired aerial images (1-mm resolution) for 3 228 systematic sampling locations; additional images were acquired as rapid-succession bursts where aerial transects crossed riparian areas and for 39 riparian and 39 upland ground locations (0.3-mm resolution) within 3.2-km of leks. We used SamplePoint software to quantify cover for plant taxa and functional groups using all ground images and a systematic sampling of aerial images. Canopy cover of sage-grouse food forbs—as averaged across aerial and ground imagery around all leks—was 1.8% and 7.8% in riparian and 0.5% and 4.0% in upland areas, respectively. Big sagebrush cover was 8.7% from upland aerial images and 9.4% from upland ground images. Aerial and ground imagery provided similar values for bare ground and shrubs in riparian and upland areas, whereas ground imagery provided finer-scale herbaceous-cover data that complemented the aerial imagery. These and other image-derived archival data provide a practical basis for landscape-scale management and are a cost-effective means for monitoring extensive sagebrush habitats.     

Cover Estimations Using Object-Based Image Analysis Rule Sets Developed Across Multiple Scales in Pinyon-Juniper Woodlands

Numerous studies have been conducted that evaluate the utility of remote sensing for monitoring and assessing vegetation and ground cover to support land management decisions and complement ground measurements. However, few comparisons have been made that evaluate the utility of object-based image analysis (OBIA) to accurately classify a landscape where rule sets (models) have been developed at various scales. In this study, OBIA rule sets used to estimate land cover from high–spatial resolution imagery (0.06-m pixel) on Pinus L. (pinyon) and Juniperus L. (juniper) woodlands were developed using eCognition Developer at four scales with varying grains—1) individual plot, 2) individual sites, 3) regions (western juniper vs. Utah juniper sites), and 4) pinyon-juniper woodland network (all plots)—that were within the same study extent. Color-infrared imagery was acquired over five sites in Oregon, California, Nevada, and Utah with a Vexcel UltraCamX digital camera in June 2009. Ground cover measurements were also collected at study sites in 2009 on 80 0.1-ha plots. Correlations between OBIA and ground measurements were relatively high for individual plot and site rule sets (ranging from r = 0.52 to r = 0.98). Correlations for regional and network rule sets were lower (ranging from r = 0.24 to r = 0.63), which was expected due to radiance differences between the images as well as vegetation differences found at each site. All site and plot OBIA average cover percentage estimates for live trees, shrubs, perennial herbaceous vegetation, litter, and bare ground were within 5% of the ground measurements, and all region and network OBIA average cover percentage estimates were within 10%. The trade-off for decreased accuracy over a larger area (region and network rule sets) may be useful to prioritize management strategies but will unlikely capture subtle shifts in understory plant communities that site and plot rule sets often capture.     

Seasonal Timing of Fire Alters Biomass and Species Composition of Northern Mixed Prairie

Fire plays a central role in influencing ecosystem patterns and processes. However, documentation of fire seasonality and plant community response is limited in semiarid grasslands. We evaluated aboveground biomass, cover, and frequency response to summer, fall, and spring fires and no fire on silty and clayey sites in semiarid, C₃-dominated grassland. The magnitude of change in biomass between years was greater than any differences among fire treatments. Still, differences existed among seasons of fire. Summer fire reduced non-native annual forb frequency (3% vs. 10% ± 2%) and Hesperostipa comata, reduced native annual forbs the first year, increased Poa secunda and bare ground, and increased Vulpia octoflora the second year. Fall fire increased grass biomass (1224 vs. 1058 ± 56 kg ? ha^? 1), but fall fire effects were generally similar to those of summer fire. Spring fire effects tended to be intermediate between no fire and summer and fall fire with the exception that spring fire was most detrimental to H. comata the first growing season and did not increase bare ground. All seasons of fire reduced litter, forb biomass, and frequency of Bromus japonicus and Artemisia spp., and they reduced H. comata, V. octoflora, and native annual forbs the first year, but increased basal cover of C₃ perennial grasses (2.2% vs. 0.6% ± 0.4%). Fire during any season increased dominance of native species compared with no fire (6.6% vs. 2.0% ± 1.0% basal cover) and maintained productivity. Seasonal timing of fire manipulated species composition, but increased C₃ perennial grass cover and native species dominance with fire during any season indicated that using fire was more important than the season in which it occurred. In addition, fire effects on the vegetation components tended to be counter to previously observed effects of grazing, suggesting fire and grazing may be complementary.     

Effects of Defoliation,Litter, and Moss on Bromus arvensis in a Northern Mixed-Grass Prairie

Exotic winter annual grasses (Bromus spp.) are a problem in North American rangelands. Defoliation, litter, and mosses are thought to regulate invasive annual Bromus species. We conducted a field experiment that tested effects of mechanical mowing and fungicide applications on Bromus arvensis, other and total graminoids, forbs, litter, and moss. Treatments caused litter biomass and moss cover to vary, which enabled testing whether litter and mosses explain variation in B. arvensis biomass. Two yr after cessation of experimental treatments, mowing treatments caused persistent reductions in B. arvensis, total graminoid, and litter biomasses but had no effect on other graminoid and forb biomasses. We detected a positive relationship between litter and B. arvensis. Fungicide applications increased moss cover and other and total graminoid biomasses, thereby suggesting mosses and several graminoids were released from the suppressive effects of biota (e.g., lichen, pathogenic fungi) susceptible to the fungicide. We found no relationship, however, between moss cover and B. arvensis. In temperate and semiarid ecosystems, mowing during flowering and before seed drop coupled with removal of clippings is likely to help control invasive bromes and fungicide additions may increase grass production.     

Postfire Recovery of Sagebrush Communities: Assessment Using Spot-5 and Very Large-Scale Aerial Imagery

Much interest lies in long-term recovery rates of sagebrush communities after fire in the western United States, as sagebrush communities comprise millions of hectares of rangelands and are an important wildlife habitat. Little is known about postfire changes in sagebrush canopy cover over time, especially at a landscape scale. We studied postfire recovery of shrub canopy cover in sagebrush-steppe communities with the use of spectral mixture analysis. Our study included 16 different fires that burned between 1937 and 2005 and one unburned site at the US Sheep Experiment Station in eastern Idaho. Spectral mixture analysis was used with September 2006 Systeme Pour l’Observation de la Terre-5 (SPOT-5) satellite imagery to estimate percent shrub canopy cover within pixels. Very large-scale aerial (VLSA) imagery with 24-mm resolution was used for training and validation. SPOT-5 image classification was successful and the spectral mixture analysis estimates of percent shrub canopy cover were highly correlated with the shrub canopy cover estimates in the VLSA imagery (R² = 0.82; P < 0.0001). Additional accuracy assessment of shrub classification produced 85% overall accuracy, 98% user’s accuracy, and 78% producer’s accuracy. This successful application of spectral mixture analysis has important implications for the monitoring and assessment of sagebrush-steppe communities. With the use of the percent shrub canopy cover estimates from the classified SPOT-5 imagery, we examined shrub canopy recovery rates since different burn years. With the use of linear-plateau regression, it was determined that shrub cover in mountain big sagebrush (Artemisia tridentata Nutt. subsp. vaseyana [Rydb.] Beetle) communities recovered approximately 27 yr after fire, with an average shrub cover of 38%. These results are consistent with other field-based studies in mountain big sagebrush communities.     

Grazing History Effects on Rangeland Biomass,Cover and Diversity Responses to Fire and Grazing Utilization

Exclusion of large grazers from rangelands that evolved with significant grazing pressure can alter natural processes and may have legacy effects by changing magnitude or direction of community responses to subsequent disturbance. Three moderately grazed pastures were paired with 12-ha areas with 15 yr of livestock exclusion. Six treatments were assigned to each in a 2 x 3 factorial arrangement of fire (fall fire or no fire) and grazing utilization (0%, 50%, or 75% biomass removal) to determine grazing history effects on rangeland response to subsequent disturbance. Livestock exclusion increased C₃ perennial grass (1 232 vs. 980 ± 50 kg ? ha^-1) and forbs (173 vs. 62 ± 19 kg ? ha^-1) and reduced C₄ perennial grass (36 vs. 180 ± 25 kg ? ha^-1) with no effect on total current-year biomass. Diversity was greater in pastures than exclosures (H’ = 1.5400 vs. 1.3823 ± 0.0431). Every biomass, cover, and diversity measure, except subshrub biomass, was affected by fire, grazing utilization, or both. Contrary to expectations, grazing history only interacted with fire effects for old standing dead material and interactions with grazing utilization were limited to old dead, bare ground, richness and dominance. Fire by grazing utilization interaction was limited to bare ground. Fire reduced annual grass (64 vs. 137 ± 29 kg ? ha^-1), forbs (84 vs. 133 ± 29 kg ? ha^-1), and diversity (H’ = 1.3260 vs. 1.5005 ± 0.0537) with no difference in total current-year biomass (1 557 vs. 1 594 ± 66 kg ? ha^-1). Grazing to 75% utilization reduced total current-year biomass (1 467 vs. 1 656 ± 66 kg ? ha^-1) and dominance (0.4824 vs. 0.5584 ± 0.0279). Grazing history affected starting points for most variables, but changes caused by grazing utilization or fire were similar between pastures and exclosures, indicating management decisions can be made based on independent knowledge of grazing or fire effects.     

Disturbance Type and Sagebrush Community Type Affect Plant Community Structure After Shrub Reduction

Treatments to reduce shrub cover are commonly implemented with the objective of shifting community structure away from shrub dominance and toward shrub and perennial grass codominance. In sagebrush (Artemisia L.) ecosystems, shrub reduction treatments have had variable effects on target shrubs, herbaceous perennials, and non-native annual plants. The factors mediating this variability are not well understood. We used long-term data from Utah’s Watershed Restoration Initiative project to assess short-term (1 − 4 yr post-treatment) and long-term (5 − 12 yr post-treatment) responses of sagebrush plant communities to five shrub reduction treatments at 94 sites that span a range of abiotic conditions and sagebrush community types. Treatments were pipe harrow with one or two passes, aerator, and fire with and without postfire seeding. We analyzed effect sizes (log of response ratio) to assess responses of sagebrush, perennial and annual grasses and forbs, and ground cover to treatments. Most treatments successfully reduced sagebrush cover over the short and long term. All treatments increased long-term perennial grass cover in Wyoming big sagebrush (A. tridentata Nutt. ssp. wyomingensis Beetle & Young) communities, but in mountain big sagebrush (ssp. vaseyana [Rydb.] Beetle) communities, perennial grasses increased only when seeded after fire. In both sagebrush communities, treatments generally resulted in short-term, but not long-term, increases in perennial forb cover. Annual grasses (largely invasive cheatgrass, Bromus tectorum L.) increased in all treatments on sites dominated by mountain big sagebrush but stayed constant or decreased on sites dominated by Wyoming big sagebrush. This result was unexpected because sites dominated by Wyoming big sagebrush are typically thought to be less resilient to disturbance and less resistant to invasion than sites dominated by mountain big sagebrush. Together, these results indicate some of the benefits, risks, and contingent outcomes of sagebrush reduction treatments that should be considered carefully in any future decisions about applying such treatments.     

高寒草甸不同植被功能群盖度对模拟气候变化的初期响应

为进一步研究高寒生态系统植被群落对气候变化响应方式和适应机理,我们在青藏高原东北祁连山南坡,沿海拔梯度对嵩草(Kobresia bellardii)草甸、灌丛草甸、杂草草甸和稀疏植被4种植被类型进行双向移栽实验,以期获得高寒草甸植被群落较为详尽而真实的响应特征。4个功能群(莎草类、禾本类、豆科类和杂草类)绝对盖度的方差分析结果表明:高寒草甸生态系统植被群落对气候变化较为敏感。莎草类植被盖度与土壤含水量线性负相关,在3600 m处最大。禾本类和豆科类分别与土壤含水量和气温线性正相关,两者在3200 m处最为适宜。海拔与杂草类盖度的二次方程可解释其63%的变异。莎草类与禾本类、豆科类植被盖度存在较弱的补偿作用,而禾本类与豆科类之间具有显著的补充作用。对比80年代的植被盖度,嵩草草甸优势种对模拟降温的响应暗示其稳定性较高。     

Vegetation,Hydrologic, and Erosion Responses of Sagebrush Steppe 9 Yr Following Mechanical Tree Removal

Land managers across the western United States are faced with selecting and applying tree-removal treatments on pinyon (Pinus spp.) and juniper (Juniperus spp.) woodland-encroached sagebrush (Artemisia spp.) rangelands, but current understanding of long-term vegetation and hydrological responses of sagebrush sites to tree removal is inadequate for guiding management. This study applied a suite of vegetation and soil measures (0.5 ? 990 m²), small-plot rainfall simulations (0.5 m²), and overland flow experiments (9 m²) to quantify the effects of mechanical tree removal (tree cutting and mastication) on vegetation, runoff, and erosion at two mid- to late-succession woodland-encroached sagebrush sites in the Great Basin, United States, 9 yr after treatment. Low amounts of hillslope-scale shrub (3 ? 15%) and grass (7 ? 12%) canopy cover and extensive intercanopy (area between tree canopies) bare ground (69 ? 88% bare, 75% of area) in untreated areas at both sites facilitated high levels of runoff and sediment from high-intensity (102 mm ? h^? 1, 45 min) rainfall simulations in interspaces (~ 45 mm runoff, 59 ? 381 g ? m^? 2 sediment) between trees and shrubs and from concentrated overland flow experiments (15, 30, and 45 L ? min^? 1, 8 min each) in the intercanopy (371 ? 501 L runoff, 2 342 ? 3 015 g sediment). Tree cutting increased hillslope-scale density of sagebrush by 5% and perennial grass cover by twofold at one site while tree cutting and mastication increased hillslope-scale sagebrush density by 36% and 16%, respectively, and perennial grass cover by threefold at a second more-degraded (initially more sparsely vegetated) site over nine growing seasons. Cover of cheatgrass (Bromus tectorum L.) was < 1% at the sites pretreatment and 1 ? 7% 9 yr after treatment. Bare ground remained high across both sites 9 yr after tree removal and was reduced by treatments solely at the more degraded site. Increases in hillslope-scale vegetation following tree removal had limited impact on runoff and erosion for rainfall simulations and concentrated flow experiments at both sites due to persistent high bare ground. The one exception was reduced runoff and erosion within the cut treatments for intercanopy plots with cut-downed-trees. The cut-downed-trees provided ample litter cover and tree debris at the ground surface to reduce the amount and erosive energy of concentrated overland flow. Trends in hillslope-scale vegetation responses to tree removal in this study demonstrate the effectiveness of mechanical treatments to reestablish sagebrush steppe vegetation without increasing cheatgrass for mid- to late-succession woodland-encroached sites along the warm-dry to cool-moist soil temperature ? moisture threshold in the Great Basin. Our results indicate improved hydrologic function through sagebrush steppe vegetation recruitment after mechanical tree removal on mid- to late-succession woodlands can require more than 9 yr. We anticipate intercanopy runoff and erosion rates will decrease over time at both sites as shrub and grass cover continue to increase, but follow-up tree removal will be needed to prevent pinyon and juniper recolonization. The low intercanopy runoff and erosion measured underneath isolated cut-downed-trees in this study clearly demonstrate that tree debris following mechanical treatments can effectively limit microsite-scale runoff and erosion over time where tree debris settles in good contact with the soil surface.     

Hydrologic Vulnerability of Sagebrush Steppe Following Pinyon and Juniper Encroachment

Woodland encroachment on United States rangelands has altered the structure and function of shrub steppe ecosystems. The potential community structure is one where trees dominate, shrub and herbaceous species decline, and rock cover and bare soil area increase and become more interconnected. Research from the Desert Southwest United States has demonstrated areas under tree canopies effectively store water and soil resources, whereas areas between canopies (intercanopy) generate significantly more runoff and erosion. We investigated these relationships and the impacts of tree encroachment on runoff and erosion processes at two woodland sites in the Intermountain West, USA. Rainfall simulation and concentrated flow methodologies were employed to measure infiltration, runoff, and erosion from intercanopy and canopy areas at small-plot (0.5 m²) and large-plot (13 m²) scales. Soil water repellency and vegetative and ground cover factors that influence runoff and erosion were quantified. Runoff and erosion from rainsplash, sheet flow, and concentrated flow processes were significantly greater from intercanopy than canopy areas across small- and large-plot scales, and site-specific erodibility differences were observed. Runoff and erosion were primarily dictated by the type and quantity of ground cover. Litter offered protection from rainsplash effects, provided rainfall storage, mitigated soil water repellency impacts on infiltration, and contributed to aggregate stability. Runoff and erosion increased exponentially (r² = 0.75 and 0.64) where bare soil and rock cover exceeded 50%. Sediment yield was strongly correlated (r² = 0.87) with runoff and increased linearly where runoff exceeded 20 mm·h^?1. Measured runoff and erosion rates suggest tree canopies represent areas of hydrologic stability, whereas intercanopy areas are vulnerable to runoff and erosion. Results indicate the overall hydrologic vulnerability of sagebrush steppe following woodland encroachment depends on the potential influence of tree dominance on bare intercanopy expanse and connectivity and the potential erodibility of intercanopy areas.     

Long-Term Vegetation Productivity and Trend Under Two Stocking Levels on Chihuahuan Desert Rangeland

Vegetation changes were evaluated over an 11-year period (1995–2005) on 2 light- and 2 conservative-stocked Chihuahuan Desert pastures in south central New Mexico. Grazing treatments were applied to the pastures over a 5-year period from 1997 through 2001. Pastures were not grazed in the 1995–1996 and 2002–2005 periods due to drought. During the 1997–2001 grazing period, grazing use of primary forage species averaged 29% and 40% on light- and conservative-stocked rangelands, respectively. Grazing intensity was consistently higher on conservative-stocked than light-stocked pastures. During our study heavy grazing occurred only in 1 year on pastures with conservative stocking. There were no differences in species or species categories (grasses, forbs, shrubs) of autumn standing crop and basal cover between light-and conservative-stocked pastures. Standing crop of total vegetation and perennial grasses showed large fluctuations among the years due to variable rainfall. Under both treatments, total herbaceous standing crop was unchanged, but perennial grass standing crop declined by over 50% when the last 3 years of study were compared with the first 3 years of study. Broom snakeweed (Gutierrezia sarothrae Pursh), a poisonous half shrub, increased in standing crop and cover during the study. Basal cover of total perennial grasses declined less under light than conservative stocking during the study period. However, climatic conditions exerted the overriding influence on vegetation standing crop and basal cover. Our study indicates that light stocking in the Chihuahuan Desert does not increase perennial grass production compared to conservative grazing but it could have a small benefit in maintaining perennial grass cover during drought. We believe our findings have broad application in the Chihauhuan Desert, but caution they might not apply well to other arid rangeland types.