Carbon Stocks and Fluxes in Rangelands of the Río de la Plata Basin

Grasslands are one of the most modified biomes on Earth. Land use changes had a large impact on carbon (C) stocks of grasslands. Understanding the impact of land use/land cover changes on C stocks and fluxes is critical to evaluate the potential of rangeland ecosystem as C sinks. In this article we analyze C stocks and fluxes across the environmental gradients of one of the most extensive temperate rangeland areas: the Río de la Plata Grasslands (RPG) in South America. The analysis summarizes information provided by field studies, remote sensing estimates, and modeling exercises. Average estimates of aboveground net primary production (ANPP) ranged from 240 to 316 g C· m^?2·yr^?1. Estimates of belowground NPP (BNPP) were more variable than ANPP and ranged from 264 to 568 g C· m^?2·yr^?1. Total Carbon ranged from 5 004 to 15 008 g C· m^?2. Plant biomass contribution to Total Carbon averaged 13% and varied from 9.5% to 27% among sites. The largest plant C stock corresponded to belowground biomass. Aboveground green biomass represented less than 7% of the plant C. Soil organic carbon (SOC) was concentrated in the slow and passive compartments of the organic matter. Active soil pool represented only 6.7% of the SOC. The understanding of C dynamics and stocks in the RPG grasslands is still partial and incomplete. Field estimates of ANPP and BNPP are scarce, and they are not based on a common measurement protocol. Remotely sensed techniques have the potential to generate a coherent and spatially explicit database on ANPP. However, more work is needed to improve estimates of the spatial and temporal variability of radiation use efficiency. The absence of a flux tower network restricts the ability to track seasonal changes in C uptake and to understand fine-scale controls of C dynamics.     

Interannual Herbaceous Biomass Response to Increasing Honey Mesquite Cover on Two Soils

This study quantified herbaceous biomass responses to increases in honey mesquite (Prosopis glandulosa Torr.) cover on two soils from 1995 to 2001 in north central Texas. Vegetation was sampled randomly with levels of mesquite ranging from 0% to 100%. With no mesquite covering the silt loam soils of bottomland sites, peak herbaceous biomass averaged (±SE) 3 300 ± 210 kg · ha⁻¹ vs. 2 560 ± 190 kg · ha⁻¹ on clay loam soils of upland sites (P = 0.001). A linear decline of 14 ± 2.5 kg · ha⁻¹ in herbaceous biomass occurred for each percent increase in mesquite cover (P = 0.001). The slope of this decline was similar between soils (P = 0.135). Herbaceous biomass with increasing mesquite cover varied between years (P = 0.001) as did the slope of decline (P = 0.001). Warm-season herbaceous biomass decreased linearly with increasing mesquite cover averaging a 73 ± 15% reduction at 100% mesquite cover (P = 0.001) compared to 0% mesquite cover. Cool-season herbaceous biomass was similar between soils with no mesquite, 1 070 ± 144 kg · ha⁻¹ for silt loam vs. 930 ± 140 kg · ha⁻¹ for clay loam soils, but averaged 340 ± 174 kg · ha⁻¹ more on silt loam than on clay loam soils at 100% mesquite cover (P = 0.004). Multiple regression analysis indicated that each centimeter of precipitation received from the previous October through the current September produced herbaceous biomass of 51 kg · ha⁻¹ on silt loam and 41 kg · ha⁻¹ on clay loam soils. Herbaceous biomass decreased proportionally with increasing mesquite cover up to 29 kg · ha⁻¹ at 100% mesquite cover for each centimeter of precipitation received from January through September. Increasing mesquite cover reduces livestock forage productivity and intensifies drought effects by increasing annual herbaceous biomass variability. From a forage production perspective there is little advantage to having mesquite present.     

Do Introduced Grasses Improve Forage Production on the Northern Mixed Prairie

Relative benefit of introducing forage species to the Northern Great Plains have been examined with contradictory conclusions. In most cases, studies were either confounded by time of establishment or treatments were not randomized and lacked independence. We examined aboveground net primary production (ANPP) in northern mixed prairie using a randomized complete block design with four treatments: crested wheatgrass (Agropyron cristatum [L.] Gaertn.), Russian wildrye (Psathyrostachys juncea [Fisch.] Nevski), a native control that was not harvested, and a harvested native. The experiment was conducted in a Stipa–Agropyron–Bouteloua site and a Stipa–Bouteloua site over 13 yr and 12 yr, respectively. The data were analyzed by sampling period (Stipa–Agropyron–Bouteloua: 1, 1994 to 1997; 2, 1998 to 2001; 3, 2002 to 2006; and Stipa–Bouteloua: 1, 1995 to 1998; 2, 1999 to 2002; 3, 2003 to 2006). ANPP among treatments was influenced (P < 0.05) by site and its interaction with treatment and sampling period (1 to 3). ANPP from the native-control, harvested-native, crested wheatgrass, and Russian wildrye treatments was 220.9, 183.9, 300.8, and 189.6 g · m^–2 (SEM = 11.2), respectively, in the Stipa–Agropyron–Bouteloua site and 122.9, 98.2, 216.3, and 115.9 g · m^–2 (SEM = 12.0), respectively, in the Stipa–Bouteloua site. Mean ANPP (SEM) within each sampling period (1 to 3) was 186.4 (9.1), 135.4 (5.8), and 263.9 (8.8) g · m^–2 in the Stipa–Agropyron–Bouteloua site, respectively, and 124.5 (6.4), 138.6 (6.1), and 151.3 (10.5) g · m^–2 in the Stipa–Bouteloua site, respectively. Russian wildrye in the Stipa–Bouteloua site and crested wheatgrass in both sites was relatively more productive in the first period after establishment than in subsequent years. The study confirms the relative ANPP advantage of crested wheatgrass over native on the Stipa–Bouteloua site but not on the Stipa–Agropyron–Bouteloua site, whereas Russian wildrye exhibited no ANPP advantage over the native on either site.     

Vegetation Response to a One-Time Spent Drilling Mud Application to Semiarid,Mixed-Grass Prairie

Landspraying while drilling (LWD) is an approved disposal method for water-based drilling mud (WBM) systems in western Canada. The mud is applied either on cultivated land, where it is incorporated by cultivation, or on vegetated land where it is not incorporated. This study examined the effects of summer WBM application (0, 15, 20, 40, and 80 m³ · ha^?1) on native vegetation properties. Our results indicated that LWD increased bare ground but decreased lichen cover at the 80 m³ · ha^?1 rate relative to the untreated control. Nitrogen (N), sulfur (S), and magnesium (Mg) concentrations in aboveground plant tissue increased with increasing LWD rate in samples taken 45 d after WBM application, but these differences disappeared 1 yr after treatment. Increase in tissue concentration of phosphorus (P) with LWD rate, however, was only detected 3 yr after LWD. Nonetheless, these changes in tissue chemistry were not associated with significant changes in biomass yield or species composition. Overall, our results suggest that single WBM applications at rates (≤ 20 m³ · ha^?1) commonly used in western Canada, if properly managed, are unlikely to adversely affect native prairie vegetation.     

Productivity,Respiration, and Light-Response Parameters of World Grassland and Agroecosystems Derived From Flux-Tower Measurements

Grasslands and agroecosystems occupy one-third of the terrestrial area, but their contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of CO₂ exchange measurements to quantify gross primary productivity, respiration, and light-response parameters of grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. We analyzed data from 72 global flux-tower sites partitioned into gross photosynthesis and ecosystem respiration with the use of the light-response method (Gilmanov, T. G., D. A. Johnson, and N. Z. Saliendra. 2003. Growing season CO₂ fluxes in a sagebrush-steppe ecosystem in Idaho: Bowen ratio/energy balance measurements and modeling. Basic and Applied Ecology 4:167–183) from the RANGEFLUX and WORLDGRASSAGRIFLUX data sets supplemented by 46 sites from the FLUXNET La Thuile data set partitioned with the use of the temperature-response method (Reichstein, M., E. Falge, D. Baldocchi, D. Papale, R. Valentini, M. Aubinet, P. Berbigier, C. Bernhofer, N. Buchmann, M. Falk, T. Gilmanov, A. Granier, T. Grünwald, K. Havránková, D. Janous, A. Knohl, T. Laurela, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J. M. Ourcival, D. Perrin, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. Seufert, F. Vaccari, T. Vesala, and D. Yakir. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biology 11:1424–1439). Maximum values of the quantum yield (α=75 mmol · mol^?1), photosynthetic capacity (A_max=3.4 mg CO₂ · m^?2 · s^?1), gross photosynthesis (P_g,max=116 g CO₂ · m^?2 · d^?1), and ecological light-use efficiency (ε_ecol=59 mmol · mol^?1) of managed grasslands and high-production croplands exceeded those of most forest ecosystems, indicating the potential of nonforest ecosystems for uptake of atmospheric CO₂. Maximum values of gross primary production (8 600 g CO₂ · m^?2 · yr^?1), total ecosystem respiration (7 900 g CO₂ · m^?2 · yr^?1), and net CO₂ exchange (2 400 g CO₂ · m^?2 · yr^?1) were observed for intensively managed grasslands and high-yield crops, and are comparable to or higher than those for forest ecosystems, excluding some tropical forests. On average, 80% of the nonforest sites were apparent sinks for atmospheric CO₂, with mean net uptake of 700 g CO₂ · m^?2 · yr^?1 for intensive grasslands and 933 g CO₂ · m^?2 · d^?1 for croplands. However, part of these apparent sinks is accumulated in crops and forage, which are carbon pools that are harvested, transported, and decomposed off site. Therefore, although agricultural fields may be predominantly sinks for atmospheric CO₂, this does not imply that they are necessarily increasing their carbon stock.     

Fire and Nitrogen Effects on Purple Threeawn (Aristida purpurea) Abundance in Northern Mixed-Grass Prairie Old Fields

Purple threeawn (Aristida purpurea Nutt. varieties) is a native grass capable of increasing on rangelands, forming near monocultures, and creating a stable state. Productive rangelands throughout the Great Plains and Intermountain West have experienced increases in purple threeawn abundance, reducing overall forage quality. Our objectives were to 1) reveal the effects of prescribed fire and nitrogen amendments on purple threeawn abundance and 2) assess nontarget plant response posttreatment. Season of fire (no fire, summer fire, fall fire) and nitrogen addition (0 kg N · ha^?1, 46 kg N · ha^?1, and 80 kg N · ha^?1) were factorially arranged in a completely randomized design and applied to two similar sites in southeastern Montana. We evaluated fire and nitrogen effects on purple threeawn basal cover, relative composition, and current-year biomass one growing season postfire at two sites treated during different years. Spring weather following fire treatments was very different between years and subsequently impacted community response. Initial purple threeawn biomass at both sites was 1 214 ± 46 kg · ha^?1 SEc. When postfire growing conditions were wet, current-year biomass of purple threeawn was reduced 90% and 73% with summer and fall fire, respectively. Under dry postfire growing conditions, purple threeawn current-year biomass was reduced 73% and 58% with summer and fall fire, respectively. Nitrogen additions had no effect on purple threeawn current-year biomass at either site. Current-year biomass of C₃ perennial grass doubled with nitrogen additions and was not impacted by fire during a wet spring. Nitrogen additions and fire had no effect on C₃ perennial grass current-year biomass following a dry spring. Prescribed fire appears to be a highly effective tool for reducing purple threeawn abundance on semiarid rangelands, with limited detrimental impacts to nontarget species.     

Net Carbon Fluxes Over Burned and Unburned Native Tallgrass Prairie

Prescribed burning of aboveground biomass in tallgrass prairie is common and may influence dynamics and magnitudes of carbon (C) movement between the surface and atmosphere. Carbon dioxide (CO₂) fluxes were measured for 2 yr using conditional sampling systems on two adjacent watersheds in an ungrazed tallgrass prairie near Manhattan, Kansas. One watershed was burned annually (BA) and the other biennially (BB). Leaf and soil CO₂ fluxes were measured in the source area. Net ecosystem exchange (NEE) of CO₂ reached a maximum daily gain of 26.4 g CO₂·m^?2·d^?1 (flux toward surface is positive) in July 1998 (year when both sites were burned and precipitation was above normal); gains were similar between sites in 1998. The maximum daily NEE loss of CO₂ was ?21.8 g CO₂·m^?2·d^?1 from BA in September 1997 (year when only BA was burned and precipitation was below normal). When data were integrated over the two years, both sites were net sources of atmospheric CO₂; NEE was ?389 g C·m^?2·2 yr^?1 on BA and ?195 g C·m^?2·2 yr^?1 on BB. Burning increased canopy size and photosynthesis, but the greater photosynthesis was offset by corresponding increases in respiration (from canopy and soil). Carbon losses from fire represented 6–10% of annual CO₂ emissions (bulk came from soil and canopy respiration). Data suggest that annual burning promotes C loss compared to less-frequently burned tallgrass prairie where prairie is not grazed by ungulates. Greater precipitation in 1998 caused large increases in biomass and a more positive growing season NEE, indicating that C sequestration appears more likely when precipitation is high. Because C inputs (photosynthesis) and losses (canopy and soil respiration) were large, small measurement or modeling errors could confound attempts to determine if the ecosystems are long-term CO₂ sources or sinks.     

Does Shrub Removal Increase Groundwater Recharge in Southwestern Texas Semiarid Rangelands?

Evapotranspiration (ET) is a key component limiting groundwater recharge past the root zone in semiarid regions. Vegetation management may alter groundwater recharge if ET is altered due to changes in vegetation type or cover. This study quantifies changes in groundwater recharge following vegetation cover change from native woodland to pasture in a semiarid region of southwest Texas. The Carrizo–Wilcox aquifer is a valuable groundwater resource in this area, where overuse by dependent farming practices has lowered aquifer levels significantly in the last 85 yr. Combining data from short-term (30 mo) monitoring of the changes in soil moisture and long-term (5–30 yr) changes in total soil chloride indicated deep drainage increased slightly where land had been cleared of vegetation. Annual recharge rates below rooting depths (standardized to 155 cm) averaged only 0.72 ±  mm · yr^-1 (mean ± SE) in areas not cleared of woody vegetation, as estimated by chloride mass balance. Upon clearing, 72% of the total chloride naturally occurring in the soil profile was flushed away within 30 yr, leading to an estimated 2.59 ±  mm · yr^-1 additional recharge. Deep soil moisture in recently cleared land increased by up to 17% during the growing season of wet years (double the average rainfall) but did not increase in dry or normal precipitation years, providing supporting evidence that more water penetrated below the roots under certain environmental conditions. These results demonstrate that brush management can increase recharge by modest, but measurable, amounts depending on site-specific soil characteristics and degree of reduction in vegetation.     

Effects of 34 Years of Experimentally Manipulated Burn Seasons and Frequencies on Prairie Plant Composition

Historically, tallgrass prairie burns occurred at many seasons and frequencies. Currently, tallgrass prescribed burns often occur annually in the spring, usually for cattle forage production. Altering burning season and frequency is known to affect plant composition and biomass production, but researchers are still uncertain how burning season and frequency interact. We present the long-term effects of a factorial combination of different burn seasons (spring, summer, autumn, or variable [rotated through seasons]) and frequencies (annual or quadrennial) on the plant composition and biomass production of an ungrazed, restored tallgrass prairie in eastern Nebraska, United States. The experimental plots were established in 1978 and visually surveyed for baseline data in 1979 and 1981. Experimental burn treatments were begun in 1982. Plots were visually surveyed until 2011 with the following results: 1) annual spring and summer burns increased C₄ graminoid abundance; 2) annual autumn burns increased forb abundance; 3) burn season had little effect on plant composition for quadrennial burns; and 4) variable season burns generally led to plant composition that was intermediate between annual spring/summer and annual autumn burns. We also clipped biomass to estimate aboveground annual net primary production (ANPP) in 2015, a year in which both annual and quadrennial burns occurred. Total ANPP did not differ significantly between burn frequencies nor between spring and autumn burns (772 g m^? 2 average) but was lower in summer burns (541 g m^? 2). ANPP results were similar to visual surveys, with significantly higher C₄ graminoid ANPP in spring than autumn burns and significantly lower forb and C₃ graminoid ANPP in spring than autumn burns. Overall, these results suggest autumn burns can increase forb and C₃ graminoid abundance, without strongly affecting total ANPP relative to spring burns. Future studies should compare plant and livestock production between spring and autumn burns in grazed fields.     

Evaluation of Low-Stress Herding and Supplement Placement for Managing Cattle Grazing in Riparian and Upland Areas

Management practices are often needed to ensure that riparian areas are not heavily grazed by livestock. A study was conducted in Montana during midsummer to evaluate the efficacy of low-stress herding and supplement placement to manage cattle grazing in riparian areas. Three treatments were evaluated in three pastures over a 3-yr period in a Latin-square design (n = 9). Each year, naïve 2-yr-old cows with calves were randomly assigned to the three treatments: 1) free-roaming control, 2) herding from perennial streams to upland target areas, and 3) herding to upland sites with low-moisture block supplements. Stubble heights along the focal stream were higher (P = 0.07) in pastures when cattle were herded (mean ± SE, 23 ± 2 cm) than in controls (15 ± 3 cm). Global positioning system telemetry data showed that herding reduced the time cows spent near (< 100 m) perennial streams (P = 0.01) and increased the use of higher elevations (P = 0.07) compared with controls. Evening visual observations provided some evidence that free-roaming cows (44% ± 19%) were in riparian areas more frequently (P = 0.11) than herded cows (23% ± 6%). Fecal abundance along the focal stream was less (P = 0.07) with herding (61.9 ±  kg · ha⁻¹) than in controls (113.2 ±  kg · ha⁻¹). Forage utilization within 600 m of supplement sites was greater (P = 0.06) when cows were herded to low-moisture blocks (18% ± 6%) compared with controls and herding alone (8% ± 2%). Moving cattle to uplands at midday using low-stress herding is an effective tool to reduce use of riparian areas. Herding cattle to low-moisture blocks can increase grazing of nearby upland forage but may not provide additional reduction in cattle use of riparian areas compared with herding alone.     

Spatial Redistribution of Nitrogen by Cattle in Semiarid Rangeland

Nitrogen (N) availability can strongly influence forage quality and the capacity for semiarid rangelands to respond to increasing atmospheric CO₂. Although many pathways of nitrogen input and loss from rangelands have been carefully quantified, cattle-mediated N losses are often poorly understood. We used measurements of cattle N consumption rate, weight gains, and spatial distribution in shortgrass rangeland of northeastern Colorado to evaluate the influence of cattle on rangeland N balance. Specifically, we estimated annual rates of N loss via cattle weight gains and spatial redistribution of N into pasture corners and areas near water tanks, and used previous studies to calculate ammonia volatilization from urine patches. Using measurements of plant biomass and N content inside and outside grazing cages over 13 yr, we estimate that cattle stocked at 0.65 animal unit months (AUM) · ha^?1 consumed 3.34 kg N · ha^?1 · yr^?1. Using an independent animal-based method, we estimate that cattle consumed 3.58 kg N · ha^?1 · yr^?1 for the same stocking rate and years. A global positioning system tracking study revealed that cattle spent an average of 27% of their time in pasture corners or adjacent to water tanks, even though these areas represented only 2.5% of pasture area. Based on these measurements, we estimate that cattle stocked at 0.65 AUM · ha^?1 during the summer can remove 0.60 kg N · ha^?1 in cattle biomass gain and spatially redistribute 0.73 kg N · ha^?1 to areas near corners and water tanks. An additional 0.17 kg N · ha^?1 can be lost as NH₃ volatilization from urine patches. Cumulatively, these cattle-mediated pathways (1.50 kg N · ha^?1) may explain the imbalance between current estimates of atmospheric inputs and trace gas losses. While NO_x emission remains the largest pathway of N loss, spatial N redistribution by cattle and N removed in cattle biomass are the second and third largest losses, respectively. Management of cattle-mediated N fluxes should be recognized as one means to influence long-term sustainability of semiarid rangelands.     

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.     

Leafy Spurge Suppression by Flea Beetles in The Little Missouri Drainage Basin,USA

The Ecological Area-wide Management Leafy Spurge, or TEAM Leafy Spurge, began collecting and redistributing flea beetles (Aphthona spp.) to research/demonstration sites and landowners throughout the Little Missouri River drainage basin to control leafy spurge in 1998. A study to evaluate the change over time of leafy spurge (Euphorbia esula L.) phytosociological characteristics following release of flea beetles was initiated in 2002 on leafy spurge–infested pasture and rangeland in the Little Missouri River drainage of Montana, North Dakota, South Dakota, and Wyoming. A total of 292 flea beetle release sites were analyzed in June and July 2002 and 2003 for leafy spurge stem density, foliar cover, flea beetle density, and vegetation composition. Leafy spurge stem density suppression was evident at 91% of the study sites. On two-thirds of the study sites stem density was reduced from greater than 100 stems·m⁻¹ to less than 25 stems·m⁻¹. Leafy spurge foliar cover was less than 5% on approximately two-thirds of the flea beetle release sites and less than 25% on over 90% of the release sites. Area of observed leafy spurge suppression ranged from 0 m² to 30000 m². Approximately 40% of the release sites had leafy spurge suppression ranging from 1000 m² to 5000 m², and 14% of the release sites had greater than 10000 m² of leafy spurge control. Plant community composition following leafy spurge suppression was characteristic of native plant communities that had not been burned or grazed. Flea beetles effectively reduced leafy spurge stem density and cover in 4–5 yr across a variety of locations and corresponding environmental conditions, both within the Little Missouri River drainage and in selected nearby locations.     

Spatial and Temporal Variability in Aboveground Net Primary Production of Uruguayan Grasslands

Aboveground net primary production (ANPP) is a variable that integrates many aspects of ecosystem functioning. Variability in ANPP is a key control for carbon input and accumulation in grasslands systems. In this study, we analyzed the spatial and temporal variability of ANPP of Uruguayan grasslands during 2000–2010. We used enhanced vegetation index (EVI) data provided by the MODIS-Terra sensor to estimate ANPP according to Monteith's (1972) model as the product of total incident photosynthetically active radiation, the fraction of the radiation absorbed by green vegetation, and the radiation use efficiency. Results showed that ANPP varied spatially among geomorphological units, increasing from the north and midwest of Uruguay to the east and southeast. Hence, Cuesta Basáltica grasslands were the least productive (399 g DM · m^-2 · yr^-1), while grasslands of the Sierras del Este and Colinas y Lomas del Este displayed the highest productivity (463 and 465 g DM · m^-2 · yr^-1, respectively). This pattern is likely related to differences in soil depth and associated variation in water availability among geomorphological units. Seasonal variability in ANPP indicated peak productivity in the spring in all units, but differences in annual trends over the 10-yr study period suggested that ANPP drivers are operating spatially distinct. Understanding the spatial and temporal variability of ANPP of grasslands are prerequisites for sustainable management of grazing systems.     

Differences in Net Primary Productivity Among Contrasting Habitats in Artemisia ordosica Rangeland of Northern China

Artemisiaordosica Krasch. is a semishrub native to the Ordos Plateau of Inner Mongolia, northern China, and forms a unique and dominant vegetation type in the sandland of the region. To determine the variation of productivity in A. ordosica rangeland, we investigated net primary production (NPP), fine root turnover, soil microbial C (C_mic), and soil organic carbon density (SOC_d) on sand dunes differing in mobility (i.e., fixed, semifixed, and shifting sand dunes) in Mu Us sandland. We found that, on an area basis, the NPP, SOC_d, C_mic, and fine root turnover rates all increased with increasing vegetation cover. However, the ratios of root NPP to total NPP (RMR_N) increased with declining vegetation cover. Total NPP varied markedly among habitats and ranged from 18.3 g · m^-2 · yr^-1 for communities on the shifting sand dunes to 293.8 g · m^-2 · yr^-1 for communities on the fixed sand dunes; whereas the rates of fine root turnover varied from 0.16 · yr^-1 to 0.54 · yr^-1. Our study demonstrated that habitat change in sandland has significant impacts on ecosystem productivity by affecting many related aspects of NPP. From the perspective of biomass production, protection of the semifixed dunes from degradation should be taken as a higher priority than trying to convert shifting sand dunes to semifixed sand dunes; whereas conversion of semifixed sand dunes to fixed sand dunes would appear to be a much easier task than restoring shifting sand dunes.     

Effects of Grazing Intensity,Precipitation, and Temperature on Forage Production

Questions have been raised about whether herbaceous productivity declines linearly with grazing or whether low levels of grazing can increase productivity. This paper reports the response of forage production to cattle grazing on prairie dominated by Kentucky bluegrass (Poa pratensis L.) in south-central North Dakota through the growing season at 5 grazing intensities: no grazing, light grazing (1.3 ±  animal unit months [AUM] · ha^-1), moderate grazing (2.7 ±  AUM · ha^-1), heavy grazing (4.4 ±  AUM · ha^-1), and extreme grazing (6.9 ±  AUM · ha^-1; mean ± SD). Annual herbage production data were collected on silty and overflow range sites from 1989 to 2005. Precipitation and sod temperature were used as covariates in the analysis. On silty range sites, the light treatment produced the most herbage (3 410 kg · ha^-1), and production was reduced as the grazing intensity increased. Average total production for the season was 545 kg · ha^-1 less on the ungrazed treatment and 909 kg · ha^-1 less on the extreme treatment than on the light treatment. On overflow range sites, there were no significant differences between the light (4 131 kg · ha^-1), moderate (4 360 kg · ha^-1), and heavy treatments (4 362 kg · ha^-1; P &spigt; 0.05). Total production on overflow range sites interacted with precipitation, and production on the grazed treatments was greater than on the ungrazed treatment when precipitation (from the end of the growing season in the previous year to the end of the grazing season in the current year) was greater than 267.0, 248.4, 262.4, or 531.5 mm on the light, moderate, heavy, and extreme treatments, respectively. However, production on the extreme treatment was less than on the ungrazed treatment if precipitation was less than 315.2 mm. We conclude that low to moderate levels of grazing can increase production over no grazing, but that the level of grazing that maximizes production depends upon the growing conditions of the current year.     

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.     

Land Management History of Canadian Grasslands and the Impact on Soil Carbon Storage

Grasslands represent a large potential reservoir in storing carbon (C) in plant biomass and soil organic matter via C sequestration, but the potential greatly depends on how grasslands are managed, especially for livestock and wild animal grazing. Positive and negative grazing effects on soil organic carbon have been reported by various studies globally, but it is not known if Canadian grasslands function as a source or a sink for atmospheric C under current management practices. This article examines the effect of grassland management on carbon storage by compiling historical range management facts and measurements from multiple experiments. Results indicate that grazing on grasslands has contributed to a net C sink in the top 15-cm depth under current utilization regimes with a removal rate of CO₂ at 0.19 ±  Mg · C · ha^-1 · yr^-1 from the atmosphere during recent decades, and net C sequestration was estimated at 5.64 ±  Mg · C · ha^-1 on average. Naturalization of 2.3 M ha of previously cultivated grasslands in the 1930s has also led to C sequestration in the Canadian prairies but has likely abated as the pool has saturated. Efforts made by researchers, policymakers, and the public has successfully led to the restoration of the Canadian prairies to a healthier state and to achieve considerable C sequestration in soils since their severe deterioration in the 1930s. In-depth analysis of management, legislation, and agricultural programs is urgently needed to place the focus on maintaining range health and achieving more C storage in soils, particularly when facing the reduced potential for further C sequestration.     

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.     

Consumption of Low Larkspur (Delphinium nuttallianum) by Grazing Sheep

Low larkspur (Delphinium nuttallianum Pritz.) poisoning causes serious economic loss to livestock producers that graze cattle on foothill and mountain ranges in western North America. In general, all Delphinium spp. are five times less toxic to sheep than to cattle. Because low larkspurs are less toxic to sheep than cattle, grazing sheep before cattle on rangelands with dense populations of low larkspur can reduce larkspur density and risk of poisoning to grazing cattle. All previous published work on sheep and larkspur interactions has involved tall larkspurs. This series of studies was conducted to determine if sheep would consume sufficient low larkspur to reduce subsequent risk to cattle. Four summer trials were conducted in Collbran, Colorado, and Soda Springs, Idaho on pastures with dense (> 9 plants · m^?2) low larkspur populations. In all trials, sheep ate very little low larkspur (< 0.5% of bites). During one final trial using high sheep density (two sheep · 0.015 ha^?1 for 9 d), sheep consumed little low larkspur, but animals appeared to trample much of the low larkspur. Toxic alkaloid concentrations in low larkspur ranged from 1.1 mg · g^?1 to 1.6 mg · g^?1 in all trials. The use of sheep to graze low larkspurs to reduce subsequent consumption by grazing cattle does not appear to be a viable option.