Stand Dynamics of Pinyon-Juniper Woodlands After Hazardous Fuels Reduction Treatments in Arizona

Pinyon-juniper ecosystems occur extensively across western North America, and at the landscape scale, variation in structure and composition is influenced by topographic position, soils, disturbance history, and local climate. The persistent pinyon-juniper woodland is a common structural form, and though they are known to be infrequent-fire systems, there is increasing interest in implementation of hazardous fuels reduction treatments in woodlands, especially in the wildland-urban interface. Few studies have quantified stand dynamics following fuels reduction treatments in persistent woodlands or compared treatment outcomes to conditions that develop under natural disturbance and successional processes. In 2004, we established a randomized, replicated study in woodlands of northern Arizona, and monitored stand dynamics and understory responses to determine how stand-level changes differed between common fuels reduction approaches. We compared the resulting structure with a conceptual state-and-transition model. Results showed that, over the 11 yr after treatment, juniper tree densities decreased by 8.4% and 0.9% but increased by 14.0% and 27.3% in Control, Burn, Thin, and Thin + Burn treatments, respectively. Pinyon tree densities decreased by 1.1% and 3.3%, increased by 12.2%, and decreased 7.9% in Control, Burn, Thin, and Thin + Burn treatments, respectively. All treatments showed fuel load reductions throughout the 11-yr study period and minimal rebound of tree recruitment toward pretreatment conditions. Prescribed fire alone (Burn) maintained persistent woodland conditions. Thinning treatments substantially reduced small tree densities and, with the addition of prescribed fire, produced losses of large trees. Thinning with prescribed fire (Thin + Burn) tended to produce conditions qualitatively unlike those described by our state-and-transition model. Evaluation of these commonly used fuels treatments against our state-and-transition model suggested that concerns regarding loss of ecological integrity may be warranted.     

A 40-Year Record of Tree Establishment Following Chaining and Prescribed Fire Treatments in Singleleaf Pinyon (Pinus monophylla) and Utah Juniper (Juniperus osteosperma) Woodlands

Chaining and prescribed fire treatments have been widely applied throughout pinyon–juniper woodlands of the western United States in an effort to reduce tree cover and stimulate understory growth. Our objective was to quantify effects of treatment on woodland recovery rate and structure and the relative dominance of the two major tree species in our Great Basin study area, singleleaf pinyon (Pinus monophylla Torr. & Frém.) and Utah juniper (Juniperus osteosperma [Torr.] Little). We resampled plots after a 40-yr interval to evaluate species-specific differences in tree survivorship and establishment from posttreatment age structures. Tree age data were collected in 2008 within four chained sites in eastern Nevada, treated in 1958, 1962, 1968, and 1969 and originally sampled in 1971. The same data were collected at five prescribed burn sites treated in 1975 and originally sampled in 1976. All chained sites had greater juniper survival than pinyon survival immediately following treatment. Chained sites with higher tree survival also had the greatest amount of new tree establishment. During the interval between treatment and the 2008 sampling, approximately four more trees per hectare per year established following chaining than following fire. Postfire tree establishment only occurred for the initial 15 yr and was dominated by juniper. Establishment after chaining was dominated by juniper for the first 15 yr but by pinyon for 15–40 yr following treatment. Results support an earlier successional role for juniper than for pinyon, which is more dependent upon favorable microsites and facilitation from nurse shrubs. Repeated chaining at short intervals, or prescribed burning at infrequent intervals, will likely favor juniper dominance. Chaining at infrequent intervals (> 20–40 yr) will likely result in regained dominance of pinyon. Chaining treatments can be rapidly recolonized by trees and have the potential to create or amplify landscape-level shifts in tree species composition.     

Can Regeneration of Green Ash (Fraxinus pennsylvanica) be Restored in Declining Woodlands in Eastern Montana?

Green ash (Fraxinus pennsylvanica Marsh.) dominates many deciduous woodlands at the western margin of its range in eastern Montana. Evidence suggests that the majority of green ash woodlands are in degraded condition with declining tree canopies and ground layers dominated by exotic grasses. The dense sod formed by these perennial grasses was hypothesized to interfere with green ash regeneration from seed. The purpose of this study was to test potential methods of restoring green ash regeneration in these declining woodlands. The effects of preseeding grazing and herbicide treatment and postgermination fertilizer on the recruitment, survival, and growth of green ash seedlings at each of four study sites typical of declining green ash woodlands in southeastern Montana were assessed. Six green ash trees at each of three sites were cut to examine the relationship of age, size, and health to sprouting ability and growth. Herbicide application had a positive effect on green ash recruitment and survival of green ash seedlings in woodlands with a dense sod of exotic grasses; seedling survival after 3–4 yr was ca. 10 times greater in herbicide-treated plots compared to controls or grazed plots. Seedlings grew slowly although fertilizer had a small positive effect on growth at one site. All coppiced trees produced basal sprouts, but sprout growth was severely curtailed at two of the three sites by deer browsing, suggesting that coppicing could increase tree canopy cover by replacing weakened trees with new and more vigorous boles and branches, but only where browsing by cattle and deer is reduced. Maintaining eastern Montana green ash woodlands in good condition should be given priority because restoration will be difficult.     

Quantifying Pinyon-Juniper Reduction within North America's Sagebrush Ecosystem

One of the primary conservation threats surrounding sagebrush (Artemisia spp.) ecosystems in the Intermountain West of the United States is the expansion and infilling of pinyon pine (Pinus edulis, P. monophylla) and juniper (Juniperus spp.) woodlands. Woodland expansion into sagebrush ecosystems has demonstrated impacts on sagebrush-associated flora and fauna, particularly the greater sage-grouse (Centrocercus urophasianus). These impacts have prompted government agencies, land managers, and landowners to ramp up pinyon-juniper removal efforts to maintain and restore sagebrush ecosystems. Accurately quantifying and analyzing management activities over time across broad spatial extents still poses a major challenge. Such information is vital to broad-scale planning and coordination of management efforts. To address this problem and aid future management planning, we applied a remote sensing change detection approach to map reductions in pinyon-juniper cover across the sage-grouse range and developed a method for rapidly updating maps of canopy cover. We found total conifer reduction over the past several yr (2011−2013 to 2015−2017) amounted to 1.6% of the area supporting tree cover within our study area, which is likely just keeping pace with estimates of expansion. Two-thirds of conifer reduction was attributed to active management (1.04% of the treed area) while wildfire accounted for one-third of all estimated conifer reduction in the region (0.56% of the treed area). Our results also illustrate the breadth of this management effort—crossing ownership, agency, and state boundaries. We conclude by identifying some key priorities that should be considered in future conifer management efforts based on our comprehensive assessment.     

Encounters with Pinyon-Juniper Influence Riskier Movements in Greater Sage-Grouse Across the Great Basin

Fine-scale spatiotemporal studies can better identify relationships between individual survival and habitat fragmentation so that mechanistic interpretations can be made at the population level. Recent advances in Global Positioning System (GPS) technology and statistical models capable of deconstructing high-frequency location data have facilitated interpretation of animal movement within a behaviorally mechanistic framework. Habitat fragmentation due to singleleaf pinyon (Pinus monophylla; hereafter pinyon) and Utah juniper (Juniperus osteosperma; hereafter juniper) encroachment into sagebrush (Artemisia spp.) communities is a commonly implicated perturbation that can adversely influence greater sage-grouse (Centrocercus urophasianus; hereafter sage-grouse) demographic rates. Using an extensive GPS data set (233 birds and 282,954 locations) across 12 study sites within the Great Basin, we conducted a behavioral change point analysis and constructed Brownian bridge movement models from each behaviorally homogenous section. We found the probability of encountering pinyon-juniper among adults was two and three times greater than that of yearlings and juveniles, respectively. However, the movement rate in response to the probability of encountering pinyon-juniper trees was 1.5 times greater for juveniles. Parameter estimates indicated a 6.1% increase in the probability of encountering pinyon-juniper coupled with a 6.2 km/hour increase in movement speed resulted in a 56%, 42% and 16% increase in risk of daily mortality, for juveniles, yearlings, and adults, respectively. The effect of pinyon-juniper encounters on survival was dependent on movement rate and differed among age class. Under fast speed movements (i.e., flight), mortality risk increased as encountering pinyon-juniper increased across all age classes. In contrast, slower speeds (i.e., average) yielded similar adverse effects for juveniles and yearlings but not for adults. This analytical framework supports a behavioral mechanism that explains reduced survival related to pinyon-juniper within sagebrush environments, whereby encountering pinyon-juniper stimulates riskier movements that likely increase vulnerability to visually acute predators.     

Pinyon and Juniper Encroachment into Sagebrush Ecosystems Impacts Distribution and Survival of Greater Sage-Grouse

In sagebrush (Artemisia spp.) ecosystems, encroachment of pinyon (Pinus spp.) and juniper (Juniperus spp.; hereafter, “pinyon-juniper”) trees has increased dramatically since European settlement. Understanding the impacts of this encroachment on behavioral decisions, distributions, and population dynamics of greater sage-grouse (Centrocercus urophasianus) and other sagebrush obligate species could help benefit sagebrush ecosystem management actions. We employed a novel two-stage Bayesian model that linked avoidance across different levels of pinyon-juniper cover to sage-grouse survival. Our analysis relied on extensive telemetry data collected across 6 yr and seven subpopulations within the Bi-State Distinct Population Segment (DPS), on the border of Nevada and California. The first model stage indicated avoidance behavior for all canopy cover classes on average, but individual grouse exhibited a high degree of heterogeneity in avoidance behavior of the lowest cover class (e.g., scattered isolated trees). The second stage modeled survival as a function of estimated avoidance parameters and indicated increased survival rates for individuals that exhibited avoidance of the lowest cover class. A post hoc frailty analysis revealed the greatest increase in hazard (i.e., mortality risk) occurred in areas with scattered isolated trees consisting of relatively high primary plant productivity. Collectively, these results provide clear evidence that local sage-grouse distributions and demographic rates are influenced by pinyon-juniper, especially in habitats with higher primary productivity but relatively low and seemingly benign tree cover. Such areas may function as ecological traps that convey attractive resources but adversely affect population vital rates. To increase sage-grouse survival, our model predictions support reducing actual pinyon-juniper cover as low as 1.5%, which is lower than the published target of 4.0%. These results may represent effects of pinyon-juniper cover in areas with similar ecological conditions to those of the Bi-State DPS, where populations occur at relatively high elevations and pinyon-juniper is abundant and widespread.     

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.     

Change in Piñon-Juniper Woodland Cover Since Euro-American Settlement: Expansion Versus Contraction Associated with Soil Properties

Woodland and forest ecosystems across western North America have experienced increased density and expansion since the early 1900s, including in the widely distributed piñon-juniper vegetation type of the western United States. Fire suppression and grazing are often cited as the main drivers of these historic changes and have led to extensive tree-reduction treatments across the region. However, much of the scientific literature on piñon-juniper expansion dates back only to the early 1900s, which is generally half a century after Euro-American settlement. Yet US General Land Office (GLO) surveys provide valuable insight into the historical extent and density of woodland and forest ecosystems as surveyors would note where on the landscape they entered and exited woodlands or forests and provided qualitative estimates of relative tree density. This study uses these GLO surveys to establish piñon-juniper woodland extent in the late 19th century at the incipient stages of Euro-American settlement in southeastern Colorado and compares these data with 2017 aerial imagery of woodland cover. We found substantial amounts of woodland contraction, as well as expansion: ≈61% of historically dense woodland is now savanna or open (treeless), whereas ≈57% of historically open areas are now savannas or woodlands. The highest rates of expansion occurred on shallow, rocky soil types with low soil available water capacity, which support little herbaceous vegetation and were consequently less likely to be affected by fire suppression or grazing. Meanwhile, the significant contractions in woodland extent occurred on deeper, upland soils with higher soil available water capacity, which were likely where early settlement and tree cutting was most prevalent. Our results provide mixed support for the widespread assumption of woodland expansion since Euro-American settlement in southeast Colorado and suggest that the expansion that has occurred in our study area is unlikely a result of past grazing or fire suppression.     

Long-Term Vegetation Trends on Grazed and Ungrazed Chihuahuan Desert Rangelands

Long-term information on the effects of managed grazing versus excluded grazing effects on vegetation composition of desert rangelands is limited. Our study objectives were to evaluate changes in frequency of vegetation components and ecological condition scores under managed livestock grazing and excluded livestock grazing over a 38-yr period at various locations in the Chihuahuan Desert of southwestern New Mexico. Sampling occurred in 1962, 1981, 1992, 1998, 1999, and 2000. Range sites of loamy (1), gravelly (2), sandy (2), and shallow sandy (2) soils were used as replications. Black grama (Bouteloua eriopoda Torr.) was the primary vegetation component at the seven locations. Dyksterhuis quantitative climax procedures were used to determine trends in plant frequency based on a 1.91-cm loop and rangeland ecological condition scores. Frequency measures of total perennial grass, black grama, tobosa (Hilaria mutica Buckley), total shrubs, honey mesquite (Prosopis glandulosa Torr.), and other vegetation components were similar on both grazed and ungrazed treatments (P > 0.1) at the beginning and end of the study. The amount of change in rangeland ecological condition scores was the same positive increase (14%) for both grazed and ungrazed treatments. Major changes (P < 0.1) occurred within this 38-yr study period in ecological condition scores and frequency of total perennial grasses and black grama in response to annual fluctuations in precipitation. Based on this research, managed livestock grazing and excluded livestock grazing had the same long-term effects on change in plant frequency and rangeland ecological condition; thus, it appears that managed livestock grazing is sustainable on Chihuahuan desert rangelands receiving over 25 cm annual precipitation.     

Effects of Long-Term Livestock Grazing on Fuel Characteristics in Rangelands: An Example From the Sagebrush Steppe

Livestock grazing potentially has substantial influence on fuel characteristics in rangelands around the globe. However, information quantifying the impacts of grazing on rangeland fuel characteristics is limited, and the effects of grazing on fuels are important because fuel characteristics are one of the primary factors determining risk, severity, continuity, and size of wildfires. We investigated the effects of long-term (70+ yr) livestock grazing exclusion (nongrazed) and moderate levels of livestock grazing (grazed) on fuel accumulations, continuity, gaps, and heights in shrub-grassland rangelands. Livestock used the grazed treatment through 2008 and sampling occurred in mid- to late summer in 2009. Nongrazed rangelands had over twofold more herbaceous standing crop than grazed rangelands (P < 0.01). Fuel accumulations on perennial bunchgrasses were approximately threefold greater in nongrazed than grazed treatments. Continuity of fuels in nongrazed compared to grazed treatments was also greater (P < 0.05). The heights of perennial grass current year’s and previous years’ growth were 1.3-fold and 2.2-fold taller in nongrazed compared to grazed treatments (P < 0.01). The results of this study suggest that moderate livestock grazing decreases the risk of wildfires in sagebrush steppe plant communities and potentially other semi-arid and arid rangelands. These results also suggest wildfires in moderately grazed sagebrush rangelands have decreased severity, continuity, and size of the burn compared to long-term nongrazed sagebrush rangelands. Because of the impacts fuels have on fire characteristics, moderate levels of grazing probably increase the efficiency of fire suppression activities. Because of the large difference between fuel characteristics in grazed and nongrazed sagebrush rangelands, we suggest that additional management impacts on fuels and subsequently fires need to be investigated in nonforested rangelands to protect native plant communities and prioritize management needs.     

Effects of Grazing on Population Growth Characteristics of Caragana stenophylla Along a Climatic Aridity Gradient

Shrubs are important plant species in grassland ecosystems worldwide, and their density and cover have been gradually increasing globally. However, the interaction effect of grazing and aridity on population recruitment and population growth of shrub species in grasslands has not been examined explicitly. We examined sapling establishment, sexual recruitment, population mortality, and population growth of Caragana stenophylla along a climatic aridity gradient and a grazing intensity gradient in the Inner Mongolia Steppe, using manipulative field experiments. Sapling establishment, sapling height, and sexual recruitment of C. stenophylla decreased as climatic aridity and grazing intensity increased. The negative effects of grazing on sapling establishment and sexual recruitment gradually increased as climatic aridity increased. The effect of climatic aridity and grazing on population mortality was influenced by sexual recruitment. In the combined treatments of climatic aridity and grazing, population mortality was relatively high when sexual recruitment was relatively high, while population mortality increased as climatic aridity and grazing increased when sexual recruitment was relatively low. C. stenophylla population increased under relatively low drought stress and mild grazing but declined under strong drought stress and/or severe grazing. Our results suggested that to maintain viable Caragana populations, appropriate grazing policies must be made according to climate aridity gradient.     

Effects of a Rest–Rotation Grazing System on Wintering Elk Distributions at Wall Creek,Montana

The large-scale influence of livestock grazing in the western United States generates a need to integrate landscape management to incorporate both wildlife and livestock. The purpose of this project was to evaluate the effects of four different grazing cells (spring grazing, summer growing-season grazing, fall grazing, and resting) on wintering elk resource selection within the Wall Creek range in southwest Montana. We collected biweekly observations of elk (Cervus elaphus) numbers and distributions across the winter range from 1988 to 2007. Using a matched-case control logistic regression model to estimate selection coefficients, we evaluated the effects of annual green-up conditions, winter conditions, landscape features, and grazing treatment on elk group resource selection within the grazing system. We found that within the grazing system, elk groups preferentially selected for rested pastures over pastures that were grazed during the previous spring (1 May–1 June), summer (1 June–15 July), and fall (15 September–30 September). The strength of selection against the pasture grazed during the summer growing season was strongest, and pastures grazed during the spring and fall were selected for over the pasture grazed during the summer. The number of elk utilizing the grazing system increased in the 19 yr following implementation of the grazing system; however, total elk herd size also increased during this time. We found no evidence that the proportion of the elk herd utilizing the grazing system changed following implementation of the rest–rotation grazing system. Wintering elk group preference for rested pastures suggests rested pastures play an important role in rotation grazing systems by conserving forage for wintering elk. Additionally, rested pastures provide important cover for a host of other wildlife species. We recommend wildlife managers maintain rested pastures within rotation grazing systems existing on ungulate winter range.     

Effect of Pinyon–Juniper Tree Cover on the Soil Seed Bank

As pinyon–juniper (specifically, Pinus monophylla and Juniperus osteosperma) woodlands in the western United States increase in distribution and density, understory growth declines and the occurrence of crown fires increases, leaving mountainsides open to both soil erosion and invasion by exotic species. We examined if the loss in understory cover that occurred with increasing tree cover was reflected in the density and diversity of the seed bank. Seed banks in stands with low, medium, and high tree cover were measured in late October for 2 yr. Multivariate analyses indicated that cover and diversity of standing vegetation changed as tree cover increased. However, the seed bank did not differ in overall seed density or species diversity because seeds of the 13 species that comprised 86% of the seed bank occurred in similar density across the tree-cover groups. Sixty-three percent of the species that were in the seed bank were absent from the vegetation (mostly annual forbs). In addition, 49% of the species that occurred in the standing vegetation were not in the seed bank (mostly perennial forbs and shrubs). Only Artemisia tridentata, Bromus tectorum, and Collinsia parviflora displayed positive Spearman rank correlations between percent cover in the vegetation and density in the seed bank. Thus, much of the standing vegetation was not represented in the seed bank, and the few species that dominated the seed bank occurred across varying covers of pinyon–juniper.     

Sage Grouse Groceries: Forb Response to Piñon-Juniper Treatments

Juniper and piñon coniferous woodlands have increased 2- to 10-fold in nine ecoregions spanning the Intermountain Region of the western United States. Control of piñon-juniper woodlands by mechanical treatments and prescribed fire are commonly applied to recover sagebrush steppe rangelands. Recently, the Sage Grouse Initiative has made conifer removal a major part of its program to reestablish sagebrush habitat for sage grouse (Centrocercus urophasianus) and other species. We analyzed data sets from previous and ongoing studies across the Great Basin characterizing cover response of perennial and annual forbs that are consumed by sage grouse to mechanical, prescribed fire, and low-disturbance fuel reduction treatments. There were 11 sites in western juniper (Juniperus occidentalis Hook.) woodlands, 3 sites in singleleaf piñon (Pinus monophylla Torr. & Frém.) and Utah juniper (Juniperus osteosperma [Torr.] Little), 2 sites in Utah juniper, and 2 sites in Utah juniper and Colorado piñon (Pinus edulis Engelm). Western juniper sites were located in mountain big sagebrush (A. tridentata ssp. vaseyana) steppe associations, and the other woodlands were located in Wyoming big sagebrush (A. tridentata ssp. wyomingensis) associations. Site potential appears to be a major determinant for increasing perennial forbs consumed by sage grouse following conifer control. The cover response of perennial forbs, whether increasing (1.5- to 6-fold) or exhibiting no change, was similar regardless of conifer treatment. Annual forbs favored by sage grouse benefitted most from prescribed fire treatments with smaller increases following mechanical and fuel reduction treatments. Though forb abundance may not consistently be enhanced, mechanical and fuel reduction conifer treatments remain good preventative measures, especially in phase 1 and 2 woodlands, which, at minimum, maintain forbs on the landscape. In addition, these two conifer control measures, in the short term, are superior to prescribed fire for maintaining the essential habitat characteristics of sagebrush steppe for sage grouse.     

Grazing and Burning Japanese Brome (Bromus Japonicus) on Mixed Grass Rangelands

Japanese brome (Bromus japonicus Thunb. ex Murr.) is an introduced, annual cool-season grass adapted to the central and northern Great Plains. Japanese brome has negatively impacted perennial grasses and decreased seasonal animal gains. Prescribed spring burning and defoliation have been effective in reducing brome density or cover, but little information directly compares the two common strategies. The objectives of this study were to 1) compare annual spring burning and grazing to reduce Japanese brome populations; and 2) evaluate trends of vegetative composition and biomass in burned, grazed, and unburned rangelands infested with Japanese brome. Paddocks with Japanese brome were assigned to one of four treatments: 1) annual prescribed spring burning, 2) spring grazing, 3) a combination of annual spring burning and grazing, and 4) an idle control. Treatments were applied annually from 2000 to 2004. Japanese brome density was greatest in the idle control in all years, even when low winter and spring precipitation limited Japanese brome recruitment. Late spring Japanese brome density was similar in all treatments with grazing or burning in four of the five seasons. Spring burning resulted in less than 65% litter cover the last 3 years, whereas the idle control and spring grazing had over 80% litter cover the last 4 years. Western wheatgrass (Pascopyrum smithii [Rydb.] A. Löve) decreased with spring grazing in burned and unburned paddocks. Buffalograss (Bouteloua dactyloides [Nutt] J. T. Columbus) composition decreased in the idle control treatment. Blue grama (Bouteloua gracilis [Willd. ex Kunth] Lag. ex Griffiths) and sideoats grama (Bouteloua curtipendula [Michx.] Torr.) composition varied by year. Even though annual burning and spring grazing were equally effective in limiting Japanese brome density and biomass compared to the idle control, Japanese brome was still present after 5 years, which indicates the difficulty of eradicating Japanese brome from ecosystems where it has become naturalized.     

Spatial Patterns of Pinyon–Juniper Woodland Expansion in Central Nevada

The expansion of the pinyon–juniper (Pinus monophylla Torr. & Frém.–Juniperus osteosperma Torr.) woodland type in the Great Basin has been widely documented, but little is known concerning how topographic heterogeneity influences the temporal development of such vegetation changes. The goals of this study were to quantify the overall rates of pinyon–juniper expansion over the past 3 decades, and determine the landscape factors influencing patterns of expansion in central Nevada. Aerial panchromatic photos (1966–1995) were used to quantify changing distribution of pinyon–juniper woodland, over multiple spatial scales (0.002-, 0.02-, and 0.4-ha median patch sizes), and for discrete categories of elevation, slope aspect, slope steepness, hillslope position, and prior canopy cover class. An object-oriented multiscale segmentation and classification scheme, based on attributes of brightness, shape, homogeneity, and texture, was applied to classify vegetation. Over the 30-year period, the area of woodland has increased by 11% over coarse, ecotonal scales (0.4-ha scale) but by 33% over single-tree scales (20-m² scale). Woodland expansion has been dominated by infilling processes where small tree patches have established in openings between larger, denser patches. Infilling rates have been greatest at lower elevations, whereas migration of the woodland belt over coarser scales has proceeded in both upslope and downslope directions. Increases in woodland area were several times greater where terrain variables indicated more mesic conditions. Management treatments involving removal of trees should be viewed in a long-term context, because tree invasion is likely to proceed rapidly on productive sites.     

Grazing-Induced Changes to Biological Soil Crust Cover Mediate Hillslope Erosion in Long-Term Exclosure Experiment

Dryland ecosystems are particularly vulnerable to erosion generated by livestock grazing. Quantifying this risk across a variety of landscape settings is essential for successful adaptive management, particularly in light of a changing climate. In the Upper Colorado River Basin, there are nearly 25 000 km² of rangelands with underlying soils derived from Mancos Shale, an erodible and saline geologic parent material. Salinity is a major concern within the Colorado River watershed, much of which is attributed to runoff and leaching from Mancos Shale deposits. In a 60-yr paired-watershed experiment in western Colorado, we used silt fences to measure differences in saline hillslope erosion, including both total sediment yield and concentrations of primary saline constituents (Na and Se), in watersheds that were either exposed to grazing or where livestock was excluded. After accounting for the strong effects of soil type, slope, and antecedent precipitation, we found that grazing increased sediment loss by ≈50% across our 8-yr time series (0.1–1.5 tn ha⁻¹), consistent with levels reported at the watershed scale in early published work from studies at the same location. Eroded sediment Se levels were low and unaffected by grazing history, but Na concentrations were significantly reduced on grazed hillslopes, likely due to depletion of surface Na in soils exposed to chronic soil disturbance by livestock. Variable selection and path analysis identified that biological soil crust (BSC) cover, more than any other variable, explained the differences in sediment yields between grazed and ungrazed watersheds, partially through the enhancement of soil aggregate stability. Our results suggest that BSC cover should be granted heightened consideration in rangeland decision support tools (e.g., state-and-transition models) and that measures to reduce surface disturbance from livestock such as altering the timing or intensity of grazing may be effective for reducing downstream impacts.     

An Assessment of Nonequilibrium Dynamics in Rangelands of the Aru Basin,Northwest Tibet,China

An assessment of nonequilibrium rangeland dynamics was conducted in the Aru basin, a semiarid site located in the very dry northwest part of the Chang Tang Nature Reserve, Tibet, China. A grazing gradient approach was used to examine the effects of different livestock grazing intensities on vegetation, providing data to determine if plant–herbivore interaction has been a major structuring force of the plant community and thus to indicate what type of dynamic might apply in the study area. No significant differences were found between a highly grazed site and a lightly grazed site in vegetation cover, standing biomass, and Shannon–Wiener species diversity index of total, graminoid, forb, and tomtza (Oxytropis glacialis Benth. ex Bunge) functional groups, with the exception that tomtza coverage was significantly higher at the highly grazed (1.04%) than at the lightly grazed site (0.02%). Grazing intensity alone did not explain a significant amount of variation in the plant species data. These results indicate that a dominance of nonequilibrium dynamics appears to be the case in the basin, probably one of the least-arid sites in the northwest Chang Tang region of the Tibetan Plateau. Thus, opportunistic livestock management strategies adapted to variable vegetation production from year to year, rather than the setting of a rigid stocking rate that assumes a stable carrying capacity, is probably the most plausible approach for managing livestock and its relationship to biodiversity values in this region.     

Influence of 90 Years of Protection From Grazing on Plant and Soil Processes in the Subalpine of the Wasatch Plateau,USA

Human communities in the Intermountain West depend heavily on subalpine rangelands because of their importance in providing water for irrigation and forage for wildlife and livestock. In addition, many constituencies are looking to managed ecosystems to sequester carbon in plant biomass and soil C to reduce the impact of anthropogenic CO₂ on climate. This work builds on a 90-year-old grazing experiment in mountain meadows on the Wasatch Plateau in central Utah. The purpose of this study was to evaluate the influence of 90 years of protection from grazing on processes controlling the input, output, and storage of C in subalpine rangelands. Long-term grazing significantly reduced maximum biomass in all years compared with plots within grazing exclosures. For grazed plots, interannual variability in aboveground biomass was correlated with July precipitation and temperature (R² = 0.51), while there was a weak correlation between July precipitation and biomass in ungrazed plots (R² = 0.24). Livestock grazing had no statistically significant impacts on total soil C or particulate organic matter (POM), although grazing did increase active soil C and decrease soil moisture. Grazing significantly increased the proportion of total soil C pools that were potentially mineralizable in the laboratory, with soils from grazed plots evolving 4.6% of total soil C in 1 year while ungrazed plots lost 3.3% of total soil C. Volumetric soil moisture was consistently higher in ungrazed plots than grazed plots. The changes in soil C chemistry may have implications for how these ecosystems will respond to forecast climate change. Because grazing has resulted in an accumulation of easily decomposable organic material, if temperatures warm and summer precipitation increases as is anticipated, these soils may become net sources of CO₂ to the atmosphere creating a positive feedback between climate change and atmospheric CO₂.     

Pathways of Grazing Effects on Soil Organic Carbon and Nitrogen

Grazing modifies the structure and function of ecosystems, affecting soil organic carbon (SOC) storage. Although grazing effects on some ecosystem attributes have been thoroughly reviewed, current literature on grazing effects on SOC needs to be synthesized. Our objective was to synthesize the effects of grazing on SOC stocks in grasslands, establishing the major mechanistic pathways involved. Additionally, and because of its importance for carbon (C) biogeochemistry, we discuss the controls of soil organic nitrogen (N) stocks. We reviewed articles analyzing grazing effects on soil organic matter (SOM) stocks by comparing grazed vs. ungrazed sites, including 67 paired comparisons. SOC increased, decreased, or remained unchanged under contrasting grazing conditions across temperature and precipitation gradients, which suggests that grazing influences the factors that control SOC accumulation in a complex way. However, our review also revealed some general patterns such as 1) root contents (a primary control of SOC formation) were higher in grazed than in their ungrazed counterparts at the driest and wettest sites, but were lower at sites with intermediate precipitation (~400 mm to 850 mm); 2) SOM C:N ratios frequently increased under grazing conditions, which suggests potential N limitations for SOM formation under grazing; and 3) bulk density either increased or did not change in grazed sites. Nearly all sites located in the intermediate precipitation range showed decreases or no changes in SOC. We grouped previously proposed mechanisms of grazing control over SOC into three major pathways that can operate simultaneously: 1) changes in net primary production (NPP pathway), 2) changes in nitrogen stocks (nitrogen pathway), and 3) changes in organic matter decomposition (decomposition pathway). The relative importance of the three pathways may generate variable responses of SOC to grazing. Our conceptual model suggests that rangeland productivity and soil carbon sequestration can be simultaneously increased by management practices aimed at increasing N retention at the landscape level.