Extending Conifer Removal and Landscape Protection Strategies from Sage-grouse to Songbirds,a Range-Wide Assessment

Recent and unprecedented scale of greater sage-grouse (Centrocercus urophasianus) conservation in the American West enables assessment of community-level benefits afforded to other sagebrush-obligate species. We use North American Breeding Bird Survey (BBS) count data and machine-learning to assess predictors influencing spatial distribution and abundance of three sagebrush-obligate songbirds (Brewer’s sparrow [Spizella breweri], sagebrush sparrow [Artemisiospiza nevadensis], and sage thrasher [Oreoscoptes montanus]). We quantified co-occurrence of songbird abundance with sage-grouse lek distributions using point pattern analyses and evaluated the concurrence of songbird abundance within sage-grouse habitat restoration and landscape protection. Sagebrush land-cover predictors were positively associated with the abundance of each songbird species in models that explained 16 ? 37% of variation in BBS route level counts. Individual songbird models identified an apparent 40% threshold in sagebrush land-cover, over which songbird abundances nearly doubled. Songbird abundances were positively associated with sage-grouse distributions (P < 0.01); range-wide, landscapes supporting > 50% of males on leks also harbored 13 ? 19% higher densities of songbirds compared with range-wide mean densities. Eighty-five percent of the conifer removal conducted through the Sage Grouse Initiative coincided with high to moderate Brewer’s sparrow abundance. Wyoming’s landscape protection (i.e., “core area”) strategy for sage-grouse encompasses half the high to moderate abundance sagebrush sparrow and sage thrasher populations. In the Great Basin half the high to moderate abundance sagebrush sparrow and sage thrasher populations coincide with sage-grouse Fire and Invasive Assessment Tool priorities, where conservation actions are being focused in an attempt to reduce the threat of wildfire and invasive plants. Our work illustrates spatially targeted actions being implemented ostensibly for sage-grouse largely overlap high abundance centers for three sagebrush obligate passerines and are likely providing significant conservation benefits for less well-known sagebrush songbirds and other sagebrush-associated wildlife.     

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.     

Assessment of Juniper Encroachment With The Use of Satellite Imagery and Geospatial Data

Juniper encroachment into otherwise treeless shrub lands and grasslands is one of the most pronounced environmental changes observed in rangelands of western North America in recent decades. Most studies on juniper change are conducted over small areas, although encroachment is occurring throughout regions. Whether changes in juniper cover can be assessed over large areas with the use of long-term satellite data is an important methodological question. A fundamental challenge in using satellite imagery to determine tree abundance in rangelands is that a mix of trees, sagebrush, and herbaceous cover types can occur within a given image pixel. Our objective was to determine if spectral mixture analysis could be used to estimate changes in Rocky Mountain juniper (Juniperus scopulorum Sarg) and Utah juniper (Juniperus osteosperma [Torr.] Little) cover over 20 yr and 20000 ha in southeast Idaho with the use of Landsat imagery. We also examined the spatial patterns and variation of encroachment within our study area using Geographic Information Systems–based data sets of grazing use, land-cover types, and topography. Juniper cover determined from 15-cm-resolution digital aerial orthophotography was used to train and validate juniper presence/absence classification in 1985 and 2005 Landsat images. The two classified images were then compared to detect changes in juniper cover. The estimated rate of juniper encroachment over our study area was 22–30% between 1985 and 2005, consistent with previous ground-based studies. Moran’s I analysis indicated that juniper encroachment pattern was spatially random rather than clustered or uniform. Juniper encroachment was significantly greater in grazed areas (P = 0.02), and in particular in grazed shrub land cover type (P = 0.06), compared to ungrazed areas. Juniper encroachment was also greater on intermediate slopes (10–35% slopes) compared to steeper or flatter terrain, and encroachment was somewhat less on north-facing (P = 0.03) and more on west-facing (P = 0.02) slopes compared to other aspects.     

Infiltration,Runoff, and Sediment Yield in Response to Western Juniper Encroachment in Southeast Oregon

Infiltration was measured in a western juniper (Juniperus occidentalis Hook.) watershed to characterize the hydrologic processes associated with landscape position. Infiltration rate, runoff, and sediment content were measured with the use of a small-plot rainfall simulator. Study sites were located in each of the four primary aspects (north, south, east, and west). Research sites were located in two ecological sites—South Slopes 12–16 PZ and North Slopes 12–16 PZ. Within aspect, plots were located in three juniper cover levels: high (> 22%), moderate (13%–16%), and low (<3%) juniper canopy cover. During rainfall simulation, water was applied at a 10.2-cm · h⁻¹ rate, levels comparable to an infrequent, short-duration, high-intensity precipitation event. Runoff was measured at 5-min intervals for 60 min. Comparing canopy cover levels, steady-state infiltration rates on control plots (9.0 cm · h⁻¹) was 68% greater than high juniper cover sites (2.87 cm · h⁻¹) and 34% greater than moderate juniper cover sites (5.97 cm · h⁻¹) on south-facing slopes. On north-facing slopes, no differences in infiltration rates were observed between juniper cover levels, demonstrating differential hydrologic responses associated with ecological site. Generally, all water applied to control plots infiltrated. Highest infiltration rates were positively associated with increased surface litter and shrub cover. In addition, depth of water within the soil profile was lowest in high juniper cover plots. This suggests that less water is available to sustain understory and intercanopy plant growth in areas with high juniper cover. Accelerated runoff and erosion in juniper dominated sites (high level) across east-, west-, and south- facing slopes can lead to extensive degradation to the hydrology of those sites. These data suggest that sustained hydrologic processes are achieved with reduced western juniper canopy cover.     

Short-Term Response of Sage-Grouse Nesting to Conifer Removal in the Northern Great Basin

Conifer woodlands expanding into sage-steppe (Artemisia spp.) are a threat to sagebrush obligate species including the imperiled greater sage-grouse (Centrocercus urophasianus). Conifer removal is accelerating rapidly despite a lack of empirical evidence to assess outcomes to grouse. Using a before-after-control-impact design, we evaluated short-term effects of conifer removal on nesting habitat use by monitoring 262 sage-grouse nests in the northern Great Basin during 2010–2014. Tree removal made available for nesting an additional 28% of the treatment landscape by expanding habitat an estimated 9603 ha (3201 ha [± 480 SE] annually). Relative probability of nesting in newly restored sites increased by 22% annually, and females were 43% more likely to nest within 1000 m of treatments. From 2011 (pretreatment) to 2014 (3 yr after treatments began), 29% of the marked population (9.5% [± 1.2 SE] annually) had shifted its nesting activities into mountain big sagebrush habitats that were cleared of encroaching conifer. Grouping treatments likely contributed to beneficial outcomes for grouse as individual removal projects averaged just 87 ha in size but cumulatively covered a fifth of the study area. Collaboratively identifying future priority watersheds and implementing treatments across public and private ownerships is vital to effectively restore the sage-steppe ecosystem for nesting sage-grouse.     

Age and Body Condition of Goats Influence Consumption of Juniper and Monoterpene-Treated Feed

Redberry juniper (Juniperus pinchotii Sudworth) is an invasive, evergreen tree that is rapidly expanding throughout western and central Texas. Goats will consume some juniper on rangelands; however, intake is limited. The objective of our research was to determine how the age and body condition of goats influence their consumption of juniper and an artificial feed containing 4 monoterpenes. Two separate experiments were conducted. Experiment 1 examined the intake of redberry juniper foliage and used 39 goats either young (2 yr) or mature (> 6 yr). One-half of each age group was fed appropriate basal rations to reach either a high (HBC) or low body condition (LBC). Goats in LBC ate more (P < 0.01, 8.6 g · kg⁻¹ body weight [BW] ± 0.7 SE) juniper than those in HBC (2.3 g · kg⁻¹ BW ± 0.3 SE), and young animals consumed more (P < 0.05, 7.2 g · kg⁻¹ BW ± 0.7 SE) juniper than mature goats (3.9 g · kg⁻¹ BW ± 0.5 SE) across body condition treatments. In experiment 2, 36 goats, either young (2 yr) or mature (> 6 yr) and in either HBC or LBC, were offered a synthetic ration treated with 20.8 g · kg⁻¹ of 4 monoterpenes found in redberry juniper. Goats in LBC ate more (P < 0.01, 25.3 g · kg⁻¹ BW ± 1.0 SE) of the terpene-treated feed than those in HBC (17.5 g · kg⁻¹ BW ± 0.7 SE), and young animals ate more (P < 0.05, 22.5 g · kg⁻¹ BW ± 0.8 SE) than mature goats (20.3 g · kg⁻¹ BW ± 0.8 SE) across body condition treatments. Total intake as a proportion of body weight was also affected by body condition. Age and body condition are important factors that influence intake of chemically defended plants. A better understanding of how these attributes affect diet selection will aid livestock producers in improving grazing management.     

Hydrologic Response to Mechanical Shredding in a Juniper Woodland

We investigated soil compaction and hydrologic responses from mechanically shredding Utah juniper (Juniperus ostesperma [Torr.] Little) to control fuels in a sagebrush/bunchgrass plant community (Artemisia nova A. Nelson, Artemisia tridentata Nutt. subsp. wyomingensis Beetle & Young/Pseudoroegneria spicata [Pursh] A. Löve, Poa secunda J. Presl) on a gravelly loam soil with a 15% slope in the Onaqui Mountains of Utah. Rain simulations were applied on 0.5-m² runoff plots at 64 mm · h^?1 (dry run: soil initially dry) and 102 mm · h^?1 (wet run: soil initially wet). Runoff and sediment were collected from runoff plots placed in five blocks, each containing four microsites (juniper mound, shrub mound, vegetation-free or bare interspace, and grass interspace) with undisturbed or tracked treatments for each microsite type and a residue-covered treatment for grass and bare interspace microsites. Soil penetration resistance was measured at the hill slope scale, and canopy and ground cover were measured at the hill slope and runoff plot scale. Although shredding trees at a density of 453 trees · ha^?1 reduced perennial foliar cover by 20.5%, shredded tree residue covered 40% of the ground surface and reduced non–foliar-covered bare ground and rock by 17%. Tire tracks from the shredding operation covered 15% of the hill slope and increased penetration resistance. For the wet run, infiltration rates of grass interspaces were significantly decreased (39.8 vs. 66.1 mm · h^?1) by tire tracks, but infiltration rates on juniper mounds and bare interspaces were unchanged. Bare interspace plots covered with residue had significantly higher infiltration rates (81.9 vs. 26.7 mm · h^?1) and lower sediment yields (38.6 vs. 313 g · m^?2) than those without residue. Because hydrologic responses to treatments are site- and scale-dependent, determination of shredding effects on other sites and at hill slope or larger scales will best guide management actions.     

Intermountain Presettlement Juniper: Distribution,Abundance, and Influence on Postsettlement Expansion

Successful implementation of watershed restoration projects involving control of piñon and juniper requires understanding the spatial extent and role presettlement trees (> 140 yr) play in the ecology of Intermountain West landscapes. This study evaluated the extent, abundance, and spatial pattern of presettlement western juniper (Juniperus occidentalis Hook.) in four woodlands located in southeast Oregon and southwest Idaho. The potential for modeling presence/absence of presettlement juniper using site characteristics was tested with logistic regression and the influence presettlement trees had on postsettlement woodland (trees < 140 yr) expansion was evaluated with a Welch’s t-test. Pre- and postsettlement tree densities, tree ages, site characteristics, and understory vegetation were measured along four 14–27 km transects. Presettlement juniper occurred in 16%–67% of stands in the four woodlands and accounted for 1%–10% of the population of trees > 1 m tall. Presettlement trees were generally widely scattered and more common in lower elevation stands with greater surface rock cover and higher insolate exposure. Presettlement trees sparsely occupied productive sites on deeper soils in southwest Idaho, suggesting the area had sustained a different disturbance regime than southeast Oregon. Southwest Idaho might have experienced a high frequency of lower severity fire that afforded survival to widely distributed legacy trees. This supposition is in contrast to most reports of a disturbance regime including either stand replacement or frequent fire of sufficient intensity to preclude survival of trees to maturity. Stands sustaining presettlement trees initiated woodland expansion 24 yr earlier than stands lacking presettlement trees. Presettlement trees may serve as a seed source potentially reducing the longevity of juniper control treatments. For areas with greater abundances and spatial distribution of presettlement trees such as southwest Idaho, management maintaining low intensity fire or cutting treatments at frequencies of less than 50 yr should sustain relatively open stands.     

Growth of Chickasaw Plum in Oklahoma

Management of rangelands for wildlife and livestock entails understanding growth of clonal shrubs such as Chickasaw plum (Prunus angustifolia Marsh.). We studied growth of this species in one county in north-central (Payne) and two counties in northwestern Oklahoma (Ellis, Harper) during 2006 and 2007. We estimated age of stems and roots by growth rings and area of stands with the use of a handheld GPS unit. Based on zero-intercept regression models, stands grew at similar rates (overlapping 95% confidence intervals [CIs]) among counties with a pooled estimate of 31.0 m² · yr⁻¹ (95% CI = 26.5–35.6 m² · yr⁻¹; n = 95). This rate showed considerable variability within and among study sites (r = 0.52). Stem diameter increased (zero-intercept models) more rapidly in north-central Oklahoma (5.27 mm · yr⁻¹; 95% CI = 5.01–5.53 mm · yr⁻¹; r = 0.90; n = 53) than in northwestern Oklahoma (3.68 mm · yr⁻¹; 95% CI = 3.55–3.81 mm · yr⁻¹; r = 0.91; n = 102); data were pooled because of similar rates in Ellis and Harper counties. Stem height was a power function of stem age (y = 0.97x^0.28; r = 0.56), indicating rate of growth in height (m · yr⁻¹) declined with age according to dy/dx = 0.27x^−0.72. Knowledge of the area expansion rate of Chickasaw plum clones aids in management planning to increase or decrease canopy coverage by this shrub.     

Hydrologic and Erosion Responses of Sagebrush Steppe Following Juniper Encroachment,Wildfire, and Tree Cutting

Extensive woodland expansion in the Great Basin has generated concern regarding ecological impacts of tree encroachment on sagebrush rangelands and strategies for restoring sagebrush steppe. This study used rainfall (0.5 m² and 13 m² scales) and concentrated flow simulations and measures of vegetation, ground cover, and soils to investigate hydrologic and erosion impacts of western juniper (Juniperus occidentalis Hook.) encroachment into sagebrush steppe and to evaluate short-term effects of burning and tree cutting on runoff and erosion responses. The overall effects of tree encroachment were a reduction in understory vegetation and formation of highly erodible, bare intercanopy between trees. Runoff and erosion from high-intensity rainfall (102 mm · h^?1, 13 m² plots) were generally low from unburned areas underneath tree canopies (13 mm and 48 g · m^?2) and were higher from the unburned intercanopy (43 mm and 272 g · m^?2). Intercanopy erosion increased linearly with runoff and exponentially where bare ground exceeded 60%. Erosion from simulated concentrated flow was 15- to 25-fold greater from the unburned intercanopy than unburned tree canopy areas. Severe burning amplified erosion from tree canopy plots by a factor of 20 but had a favorable effect on concentrated flow erosion from the intercanopy. Two years postfire, erosion remained 20-fold greater on burned than unburned tree plots, but concentrated flow erosion from the intercanopy (76% of study area) was reduced by herbaceous recruitment. The results indicate burning may amplify runoff and erosion immediately postfire. However, we infer burning that sustains residual understory cover and stimulates vegetation productivity may provide long-term reduction of soil loss relative to woodland persistence. Simply placing cut-downed trees into the unburned intercanopy had minimal immediate impact on infiltration and soil loss. Results suggest cut-tree treatments should focus on establishing tree debris contact with the soil surface if treatments are expected to reduce short-term soil loss during the postcut understory recruitment period.     

Longer-Term Evaluation of Sagebrush Restoration After Juniper Control and Herbaceous Vegetation Trade-offs

Degradation of shrublands around the world from altered fire regimes, overutilization, and anthropogenic disturbance has resulted in a widespread need for shrub restoration. In western North America, reestablishment of mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana [Rydb.] Beetle) is needed to restore ecosystem services and function. Western juniper (Juniperus occidentalis ssp. occidentalis Hook) encroachment is a serious threat to mountain big sagebrush communities in the northern Great Basin and Columbia Plateau. Juniper trees can be controlled with fire; however, sagebrush recovery may be slow, especially if encroachment largely eliminated sagebrush before juniper control. Short-term studies have suggested that seeding mountain big sagebrush after juniper control may accelerate sagebrush recovery. Longer-term information is lacking on how sagebrush recovery progresses and if there are trade-offs with herbaceous vegetation. We compared seeding and not seeding mountain big sagebrush after juniper control (partial cutting followed with burning) in fully developed juniper woodlands (i.e., sagebrush had been largely excluded) at five sites, 7 and 8 yr after seeding. Sagebrush cover averaged ~ 30% in sagebrush seeded plots compared with ~ 1% in unseeded plots 8 yr after seeding, thus suggesting that sagebrush recovery may be slow without seeding after juniper control. Total herbaceous vegetation, perennial grass, and annual forb cover was less where sagebrush was seeded. Thus, there is a trade-off with herbaceous vegetation with seeding sagebrush. Our results suggest that seeding sagebrush after juniper control can accelerate the recovery of sagebrush habitat characteristics, which is important for sagebrush-associated wildlife. We suggest land manager and restoration practitioners consider seeding sagebrush and possibly other shrubs after controlling encroaching trees where residual shrubs are lacking after control.     

One-Seed Juniper Sapling Use by Goats in Relation to Stocking Density and Mixed Grazing With Sheep

Suppression of one-seed juniper (Juniper monosperma [Englem.] Sarg.) reinvasion with goats requires achieving levels of defoliation of newly established saplings that eventually kill or suppress plant growth. We tested the effects of stocking density and mixed grazing with sheep on the level of use of one-seed juniper saplings by goats. In summer and spring, groups of 10 does (goats alone, GA) or 5 does and 4 ewes (mixed grazing, MG), grazed 20 × 30 m cells infested with saplings (500–533 · ha^?1; mean: 0.8 m tall), either continuously for 6 d (low stocking density, LD) or with daily rotation through 10 × 10 m cells during the 6-d period (high stocking density, HD) in a block design. Feeding activity; juniper in feces; utilization of herbaceous vegetation; frequency of saplings with light, moderate, and heavy foliage and bark use; and branch utilization were determined. Goats in HD spent more time feeding on saplings, less time feeding on herbaceous forages, and tended to consume more juniper than goats in LD. Utilization of herbaceous vegetation ranged from 52% to 73% and was higher for MG than GA and for LD than HD. The MG–HD treatment resulted in the highest frequency of short saplings (< 0.5 m) with heavy defoliation in summer and spring, and lowest frequency of saplings with light debarking in spring. Heavy defoliation was more frequent in short saplings, whereas heavy debarking was more frequent in tall (> 1 m) saplings. Sapling mortality was not affected by treatments (P > 0.05) and averaged 5% across treatments. Branch debarking was greater in spring (P = 0.02) and explained approximately 80% of branch mortality and 62% and 52% of the reduction in sapling live crown height and volume. Branch utilization (percent length) was not affected by grazing treatments (range: 45–48%), but was influenced by the length and diameter of branches. This study suggests that high stocking density and mixed grazing stimulate feeding behaviors that increase utilization of juniper saplings by goats. Susceptibility of saplings to defoliation and debarking varies with sapling size, branch structure, and season. Targeted grazing in spring appears to have a greater impact on sapling suppression and branch mortality due to higher debarking frequency.     

Feeding Redberry Juniper (Juniperus pinchotii) at Weaning Increases Juniper Consumption by Goats on Pasture

Redberry (Juniperus pinchotii Sudw.) and ashe (Juniperus ashei Buchh.) juniper dominate rangelands throughout central Texas. Our objective was to attempt to improve the efficacy of goats as a biological control mechanism for juniper through behavioral training. Conditioning sheep and goats to increase the palatability of chemically defended plants can be a useful tool in brush control. Previous research illustrated that goats can be conditioned to consume more juniper while in individual pens when foraging choices are limited. To test whether this creates a longer-lasting increase in juniper preference, we determined if goats would continue to consume juniper on pasture for one year after being fed juniper in individual pens for 14 d. Female Boer-cross goats (n = 40) were randomly divided into two treatments: conditioned and naive to juniper. At approximately 12 mo of age, conditioned goats were placed in individual pens and fed redberry juniper 1 h daily for 14 d, while naive goats received only alfalfa pellets to meet maintenance requirements. After the pen-feeding phase of the study, goats were placed in one of four pastures (10 goats · pasture^?1) for 12 mo. Two pastures housed conditioned goats, and two pastures housed naive goats at a moderate stocking rate (1 animal unit · yr^?1 · 8 ha^?1). Bite count surveys were conducted twice per month, while herbaceous standing crop and monoterpene levels were measured once per month. Juniper preference varied monthly; however, conditioned goats consistently ate more (P < 0.05) juniper than naive goats except for April, when the study began, and March, when the study ended. When selection of herbaceous forages decreased, conditioned goats increased selection of juniper, while naive goats increased selection of other palatable shrubs. Seasonal changes of monoterpene levels in juniper had no apparent effect on juniper preference. We contend that feeding juniper at weaning will increase use of the plant in grazing situations.     

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.     

Models for Predicting Fuel Consumption in Sagebrush-Dominated Ecosystems

Fuel consumption predictions are necessary to accurately estimate or model fire effects, including pollutant emissions during wildland fires. Fuel and environmental measurements on a series of operational prescribed fires were used to develop empirical models for predicting fuel consumption in big sagebrush (Artemisia tridentata Nutt.) ecosystems. Models are proposed for predicting fuel consumption during prescribed fires in the fall and the spring. Total prefire fuel loading ranged from 5.3–23.6 Mg · ha^?1; between 32% and 92% of the total loading was composed of live and dead big sagebrush. Fuel consumption ranged from 0.8–22.3 Mg · ha^?1, which equates to 11–99% of prefire loading (mean = 59%). Model predictors include prefire shrub loading, proportion of area burned, and season of burn for shrub fuels (R² = 0.91). Models for predicting proportion of area burned for spring and fall fires were also developed (R² = 0.64 and 0.77 for spring and fall fire models, respectively). Proportion of area burned, an indicator of the patchiness of the fire, was best predicted from the coverage of the herbaceous vegetation layer, wind speed, and slope; for spring fires, day-of-burn 10-h woody fuel moisture content was also an important predictor variable. Models predicted independent shrub consumption measurements within 8.1% (fall) and 12.6% (spring) for sagebrush fires.     

Aspen Recovery Since Wolf Reintroduction on the Northern Yellowstone Winter Range

Quaking aspen (Populus tremuloides Michx.) recruitment and overstory stem densities were sampled in 315 clones in 1991 and 2006 on 560 km² of the Northern Yellowstone Winter Range (NYWR). A primary objective was to observe if aspen status had improved from 1991 to 2006: evidence of a wolf (Canis lupus) caused trophic cascade. Recruitment stems (height > 2 m and diameter at breast height < 5 cm) represent recent growth of aspen sprouts above elk (Cervus elaphus) browsing height, whereas overstory stems (all stems > 2 m) represent the cohort of stems, which will insure the sustainability of the clone. Overstory stem densities declined by 12% (P = 0.04) on the landscape scale when compared with paired t-tests. Overstory stems declined in 58% of individual clones and in 63% of the 24 drainages of the study area. The second objective was to determine which factors influenced changes in aspen density. Winter ungulate browsing (P = 0.0001), conifer establishment (P = 0.0001), and cattle (Bos spp.) grazing (P = 0.016) contributed to the decline in overstory stem densities when analyzed using a mixed effects model of log transformed medians. Eighty percent of the clones were classified as having medium to high browsing levels in 1991, whereas 65% of the clones received a similar rating in 2006, possibly due to the reduced NYWR elk population. Aspen recruitment has increased in some 2–10 km² areas, but not consistently. Our study found that a trophic cascade of wolves, elk, and aspen, resulting in a landscape-level recovery of aspen, is not occurring at this time.     

Fire History in a Chaparral Ecosystem: Implications for Conservation of a Native Ungulate

Mature chaparral vegetation in the San Gabriel Mountains, California, resulting from long fire-return intervals (50–70 yr), has resulted in reduced visibility and availability and quality of forage, all of which are important attributes of mountain sheep (Ovis canadensis) habitat. Concomitantly, vegetation changes have decreased availability and quality of forage. We developed a resource-selection model to determine the effect of fire history on habitat use by mountain sheep, examined the hypotheses that habitat selection was associated with fire occurrence, and determined whether fire occurrence influenced the amount of potential habitat available to mountain sheep. The best model indicated that mountain sheep selected vegetation that had burned within 15 yr and avoided areas that had not burned within that time frame. We then used our model to quantify potential changes in mountain sheep habitat that have occurred over time based on fire conditions. We identified 390 km² of mountain sheep habitat that existed in 2002 (when only 63 mountain sheep were tallied), 486 km² in 1980 (when the mountain sheep population was at its highest), and 422 km² in 2004 (just after a series of large wildfires). We also estimated that 615 km² of suitable habitat would be available in a hypothetical situation in which the entire study area burned. Our results suggest that restoration of mountain sheep to their historical distribution in chaparral ecosystems will depend upon more frequent fires in areas formerly occupied by those specialized herbivores.     

Rainfall Interception by Singleleaf Piñon and Utah Juniper: Implications for Stand-Level Effective Precipitation

The expansion of piñon and juniper trees into sagebrush steppe and the infilling of historic woodlands has caused a reduction in the cover and density of the understory vegetation. Water is the limiting factor in these systems; therefore, quantifying redistribution of water resources by tree species is critical to understanding the dynamics of these formerly sagebrush-dominated rangelands. Tree canopy interception may have a significant role in reducing the amount of rainfall that reaches the ground beneath the tree, thereby reducing the amount of available soil moisture. We measured canopy interception of rainfall by singleleaf piñon (Pinus monophylla Torr. & Frém.) and Utah juniper (Juniperus osteosperma [Torr.] Little) across a gradient of storm sizes. Simulated rainfall was used to quantify interception and effective precipitation during 130 rainfall events ranging in size from 2.2 to 25.9 mm hr^? 1 on 19 trees of each species. Effective precipitation was defined as the sum of throughfall and stemflow beneath tree canopies. Canopy interception averaged 44.6% (± 27.0%) with no significant difference between the two species. Tree allometrics including height, diameter at breast height, stump diameter, canopy area, live crown height, and width were measured and used as predictor variables. The best fit predictive model of effective precipitation under canopy was described by stump diameter and gross precipitation (R² = 0.744, P < 0.0001). An alternative management model based on canopy area and gross precipitation predicted effective precipitation with similar accuracy (R² = 0.741, P < 0.0001). Canopy area can be derived from various remote sensing techniques, allowing these results to be extrapolated to larger spatial scales to quantify the effect of increasing tree canopy cover on rainfall interception loss and potential implications for the water budget.     

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.     

Simulating Current Successional Trajectories in Sagebrush Ecosystems With Multiple Disturbances Using a State-and-Transition Modeling Framework

Disturbances and their interactions play major roles in sagebrush (Artemisia spp. L.) community dynamics. Although impacts of some disturbances, most notably fire, have been quantified at the landscape level, some have been ignored and rarely are interactions between disturbances evaluated. We developed conceptual state-and-transition models for each of two broad sagebrush groups—a warm-dry group characterized by Wyoming big sagebrush (Artemisia tridentata Nutt. subsp. wyomingensis Beetle & Young) communities and a cool-moist group characterized by mountain big sagebrush (Artemisia tridentata Nutt. subsp. vaseyana [Rydb.] Beetle) communities. We used the Vegetation Dynamics Development Tool to explore how the abundance of community phases and states in each conceptual model might be affected by fire, insect outbreak, drought, snow mold, voles, sudden drops in winter temperatures (freeze-kill), livestock grazing, juniper (Juniperus occidentalis var. occidentalis Hook.) expansion, nonnative annual grasses such as cheatgrass (Bromus tectorum L.), and vegetation treatments. Changes in fuel continuity and loading resulted in average fire rotations of 12 yr in the warm-dry sagebrush group and 81 yr in the cool-moist sagebrush group. Model results in the warm-dry sagebrush group indicated postfire seeding success alone was not sufficient to limit the area of cheatgrass domination. The frequency of episodes of very high utilization by domestic livestock during severe drought was a key influence on community phase abundance in our models. In the cool-moist sagebrush group, model results indicated at least 10% of the juniper expansion area should be treated annually to keep juniper in check. Regardless, juniper seedlings and saplings would remain abundant.