Central Plains Experimental Range Research Articles

372 What we have learned and are still learning about enteric methane emission measurements in extensive grazing environments

Abstract Across the Western U.S., beef cattle spend considerable time grazing extensive rangelands. Enteric methane (CH4) production in grazing systems represents more than 80% of the total enteric CH4 emissions. However, more measured CH4 data from cattle grazing extensive rangelands is needed to inform measurement, reporting, modeled emissions, and to identify mitigation opportunities. The availability of pasture-ready automatic head-chamber systems (AHCS; C-Lock Inc, Rapid City, South Dakota) has allowed the determination of emissions in rangelands. In this regard, different methods to ensure an accurate determination of enteric CH4 using this measurement technique have been evaluated in recent years. An initial step to obtaining accurate enteric emissions data is ensuring efforts secure adequate AHCS visitation. Investigations in small (< 99 ac) pastures have shown that ≥ 15 visits to ≥ 55 visits in feedlot pens are adequate for robust enteric CH4 production data. Here, our goal is to examine AHCS visitation outcomes to inform the use of this technique in extensive rangeland. For two grazing seasons at the USDA ARS Central Plains Experimental Range (CPER), we have studied enteric CH4 emissions from yearling steer herds ranging in size from 26 to 60 animals in continuously stocked rangeland (Table 1). After 30 d of acclimation in 2022, 46% of individuals continued using an AHCS until the end of the drought-shortened grazing season, totaling 60 d of constant AHCS exposure. In 2023, individuals were measured first at post-weaning, and those with ≥ 15 visits to an AHCS were stocked at CPER. These steers were previously exposed to an AHCS on average 69 d earlier (range: 28 to 118). Of these 60 individuals, 68% used the two available AHCS ≥ 15 times when grazing rangeland for 76 d, totaling 137 d of discontinuous AHCS exposure. While grazing, individual visits per season were reduced in 2022 and substantially greater in 2023. At this point in our enteric CH4 emission investigation, we learned that either exposing an individual multiple times to an AHCS after weaning or doubling the days of exposure in the acclimation phase increases visitation during the measurement phase by about 20% at the herd level and 450% on an individual basis. Insights learned on rangeland have uncovered several considerations for measuring enteric CH4 emissions in extensive environments: lengthy past exposure to AHCS is ideal for adequate visitation during the measurement phase and visitation of unacclimated individuals is unlikely during the measurement phase. Further work should determine the true number of individual visits required for accurate methane measurement in extensive rangelands and evaluate 1) efficacy of different acclimation practices, 2) continued use of previously exposed animals and temporal visitation decay, and 3) alternative placement of AHCS, and use of palatable attractants to increase AHCS visitation may improve measurement of enteric CH4 emissions.

The LTAR Grazing Land Common Experiment at the Central Plains Experimental Range: Collaborative adaptive rangeland management.

Semiarid rangelands throughout the western Great Plains support livestock production and many other ecosystem services. The degree to which adaptive multi-paddock (AMP) grazing management approaches can help achieve desired ecosystem services remains unclear. At the Central Plains Experimental Range in northeastern Colorado, a management-science partnership with a diverse stakeholder group is comparing collaborative adaptive rangeland management (CARM), designed to incorporate AMP principles, to traditional rangeland management (TRM), consisting of season-long grazing during the growing season. Each treatment was implemented on a set of 10, 130-ha pastures paired by soils, topography, and plant communities to evaluate how CARM affects vegetation (composition and production), livestock production (steer weight gain), and wildlife habitat (vegetation structure for grassland birds). For the first 5 years of the experiment, CARM cattle were managed as a single herd using AMP grazing with planned year-long rest in 20% of the pastures. Relative to TRM, CARM enhanced heterogeneity in vegetation structure across the landscape, benefiting two grassland bird species. However, this came at the cost of 12%-16% lower steer weight gains in CARM versus TRM and declining populations of a third bird species of conservation concern in both treatments. Here we discuss how increased understanding of ecological and social processes during the experiment's first 5 years led to changes in the CARM treatment and management objectives during the next 5 years. We also discuss how innovations in remote sensing, environmental sensors, ecosystem modeling, social learning, and economic analyses are being integrated into and supported by the CARM experiment.

192 Outcomes of Simulated Combinations of Rangeland Management Scenarios on Soil and Nutrient Processes

Abstract The relationships among rangeland management practices and ecosystem on soil and nutrient processes are complex. Simulation models can provide valuable insight into this complexity to guide future research and management decisions for sustainable grazing systems. The objective of this analysis was to evaluate the combinations of grazing management (continuous vs. rotational), stocking rate (low, moderate, or high), and burning (none vs. annual spring burn) on soil erosion, nutrient losses, and carbon sequestration. Soil, weather, and forage species data were collected for 3 locations in the Central Great Plains (Konza Prairie Biological Station, Kansas; Agricultural Research Center-Hays, Kansas; Central Plains Experimental Range, Wyoming). These data were used to simulate combinations of rangeland management scenarios using the Agricultural Policy/Environmental eXtender (APEX) model. A stocker cattle operation was simulated at Hays, Kansas and Wyoming locations, and a cow-calf operation at Konza, Kansas location. Burning increased soil loss from water erosion regardless of stocking rate or grazing management. Rotational grazing increased soil loss at the Kansas locations, but decreased soil loss at Wyoming compared with continuous grazing. Stocking rate had little effect on nitrogen losses, but rotational grazing decreased nitrogen loss compared with continuous grazing. Greater stocking rates slightly decreased phosphorus losses with continuous grazing, but not rotational grazing. Burning decreased phosphorus losses with continuous grazing at Konza, but not the other locations. Greater stocking rates decreased the deposition of soil organic carbon especially with continuous grazing, and burning resulted in losses of soil organic carbon at Konza and Wyoming. In conclusion, rangeland management practices differentially affect nutrient processes among ecosystems such that a one-size-fits-all management scheme will not maximize retention of all nutrients in all ecosystems.

Using Hyperspectral Imagery to Characterize Rangeland Vegetation Composition at Process-Relevant Scales

Rangelands are composed of patchy, highly dynamic herbaceous plant communities that are difficult to quantify across broad spatial extents at resolutions relevant to their characteristic spatial scales. Furthermore, differentiation of these plant communities using remotely sensed observations is complicated by their similar spectral absorption profiles. To better quantify the impacts of land management and weather variability on rangeland vegetation change, we analyzed high resolution hyperspectral data produced by the National Ecological Observatory Network (NEON) at a 6500-ha experimental station (Central Plains Experimental Range) to map vegetation composition and change over a 5-year timescale. The spatial resolution (1 m) of the data was able to resolve the plant community type at a suitable scale and the information-rich spectral resolution (426 bands) was able to differentiate closely related plant community classes. The resulting plant community class map showed strong accuracy results from both formal quantitative measurements (F1 75% and Kappa 0.83) and informal qualitative assessments. Over a 5-year period, we found that plant community composition was impacted more strongly by weather than by the rangeland management regime. Our work displays the potential to map plant community classes across extensive areas of herbaceous vegetation and use resultant maps to inform rangeland ecology and management. Critical to the success of the research was the development of computational methods that allowed us to implement efficient and flexible analyses on the large and complex data.

Vegetation characteristics and precipitation jointly influence grassland bird abundance beyond the effects of grazing management

AbstractGrassland birds have experienced some of the steepest population declines of any guild of birds in North America. The shortgrass steppe contains some of North America’s most intact grasslands, which makes the region particularly important for these species. It is well known that grassland birds differentially respond to variation in vegetation structure generated by spatiotemporally varying disturbance like grazing management. However, understanding how species respond to characteristics beyond vegetation structure or grazing could better inform management for these species in the shortgrass steppe. We analyzed point count data for 5 grassland bird species breeding on the Central Plains Experimental Range in northeastern Colorado from 2013 to 2017 to examine the predictive capacity of models representing fine-scale (~5 ha) vegetation attributes (vegetation structure and cover type) and topography, combined with interannual precipitation variability (i.e. vegetation-abiotic models). We then compared these models to models based on grazing management treatments (applied to whole pastures, ~130 ha) and edaphic conditions (ecological sites), which represented information more generally available to rangeland managers. Precipitation, vegetation structure, and vegetation cover type influenced all species in a manner consistent with, but more nuanced than, vegetation structure alone. These models also explained more variation in abundance for species that responded to grazing management. Thus, while grazing management can be applied adaptively to improve habitat for these species, our more detailed vegetation-abiotic models identified species-specific habitat components that could be targeted for management. For example, not grazing pastures with extensive, homogenous stands of mid-height grasses (e.g., Hesperostipa comata) for an entire growing season during wet years could be one strategy to enhance Grasshopper Sparrow (Ammodramus savannarum) abundance and stockpile residual forage for future utilization by livestock. Our models provide a better understanding of and reveal nuances in the suite of environmental conditions to which grassland birds respond in shortgrass steppe rangelands.

Vegetation Classification Exercise for the Pawnee National Grasslands, USA

Aims: Vegetation classifications are useful for a variety of management purposes as well as scientific exploration. Local classifications are common throughout the United States but only recently have been integrated into a national classification system, which is now expected for local classifications. Study Area: The Pawnee National Grasslands (PNG) in northeastern Colorado, USA, has not been classified using plot data, and is thus a gap on the baseline knowledge of the PNG plant communities that hinders impact assessment of various anthropogenic activities. Methods: Here, we use 128 plots to classify the vegetation of the PNG using a two-step process: first, classifying the PNG plots alone to characterize local uniqueness, and then employing a semi-supervised classification with an additional 64 plots from areas to the north and east of the PNG, using standard classification procedures. Results: We document on the PNG the occurrence of two Classes, three Subclasses, four Formations, five Divisions, six Macrogroups, seven Groups and eight Alliances and Associations already described in the USNVC. Conclusions: The PNG is dominated by the Bouteloua gracilis-Buchloe dactyloides Grassland Association, which we further subdivide and describe as three local subassociations. The mixed-grass concepts in the USNVC do not exist in the PNG. Taxonomic reference: Hazlett (1998). Syntaxonomic reference: USNVC (2016). Abbreviations: BLM = Bureau of Land Management; CPER = Central Plains Experimental Range; ESA = Ecological Society of America; EST = Ecological Site Type; GPS UTM = Global Positioning System Universal Transverse Mercator; NEON = National Ecological Observatory Network; PNG = Pawnee National Grasslands; USNVC = United States Vegetation Classification.

Quantifying characteristic growth dynamics in a semi-arid grassland ecosystem by predicting short-term NDVI phenology from daily rainfall: a simple four parameter coupled-reservoir model

Predicting impacts on phenology of the magnitude and seasonal timing of rainfall pulses in water-limited grassland ecosystems concerns ecologists, climate scientists, hydrologists, and a variety of stakeholders. This report describes a simple, effective procedure to emulate the seasonal response of grassland biomass, represented by the satellite-based normalized difference vegetation index (NDVI), to daily rainfall. The application is a straightforward adaptation of a staged linear reservoir that simulates the pulse-like entry of rainwater into the soil and its redistribution as soil moisture, the uptake of water by plant roots, short-term biomass development, followed by the subsequent transpiration of water through foliage. The algorithm precludes the need for detailed, site specific information on soil moisture dynamics, plant species, and the local hydroclimate, while providing a direct link between discrete rainfall events and consequential biomass responses throughout the growing season. We applied the algorithm using rainfall data from the Central Plains Experimental Range to predict vegetation growth dynamics in the semi-arid shortgrass steppe of North America. The mean annual rainfall is 342 mm, which is strongly bifurcated into a dominantly ‘wet’ season, where during the three wettest months (May, June and July) the mean monthly rainfall is approximately 55 mm month−1; and a ‘dry’ season, where during the three driest months (December, January and February), the mean monthly rainfall is approximately 7 mm month−1. NDVI data from the Moderate Resolution Imaging Spectroradiometer (MODIS) MOD13Q1 16 day, 250 m × 250 m product were used as a proxy for grassland phenology for the period-of-record 2000–2013. Allowing for temporal changes in basic parameters of the response function over the growing season, the predicted response of the model tracks the observed NDVI metric with correlation coefficients exceeding 0.92. A two-stage series reservoir is preferred, whereby the characteristic time for transfer of a rainfall event to the peak response of NDVI decreases from 24 days (early growing season) to 12 days (late growing season), while the efficiency of a given volume of rainfall to produce a correspondingly similar amount of aboveground biomass decreases by a factor of 40% from April to October. Behaviours of the characteristic time of greenup and loss of rainfall efficiency with progression of the growing season are consistent with physiological traits of cool-season C3 grasses versus warm-season C4 grasses, and with prior research suggesting that early season production by C3 grasses is more responsive to a given amount of precipitation than mid-summer growth of C4 shortgrasses. Our model explains >90% of seasonal biomass dynamics. We ascribe a systematic underprediction of observed early season greenup following drought years to a lagged or ‘legacy’ effect, as soil inorganic nitrogen, accumulated during drought, becomes available for future plant uptake.

Cover, density, and demographics of shortgrass steppe plants mapped 1997–2010 in permanent grazed and ungrazed quadrats

AbstractThis data set was created to map and analyze demographic rates of many common plant species in the shortgrass steppe of North America under grazed and ungrazed conditions. Twenty‐four permanent 1‐m2 quadrats were established as part of the Shortgrass Steppe Long Term Ecological Research project in six grazed and ungrazed study sites on the Central Plains Experimental Range in Nunn, Colorado, USA. We used a pantograph to record the location and shape of individual plants annually from 1997 to 2010. These data make it possible to determine the fate of individual plants using a Geographic Information System and spatial analysis techniques. The data provide unique opportunities to test the interactive effects of grazing and climate variables on demographic rates, plant–plant interactions, and population and community dynamics. We provide the following data and data formats: (1) the digitized maps in shapefile format; (2) a tabular representation of centroid or point location (x, y coordinates), and basal cover (m2) for plants mapped as polygons; (3) quadrat information including location (longitude and latitude), elevation, and livestock grazing treatment and history; (4) detailed information on stocking rates during the period of quadrat sampling; (5) daily temperature and precipitation values; (6) a species list including the total records for each species, plant growth forms, and longevity; and (7) an inventory of the years each quadrat was sampled.

Impact of precipitation dynamics on net ecosystem productivity

AbstractNet ecosystem productivity (NEP) was measured on shortgrass steppe (SGS) vegetation at the USDA Central Plains Experimental Range in northeastern Colorado from 2001 to 2003. Large year‐to‐year differences were observed in annual NEP, with >95% of the net carbon uptake occurring during May and June. Low precipitation during the 2002 April to June time period greatly reduced annual net carbon uptake. Large precipitation events (>10 mm day−1) promoted carbon uptake, while small precipitation events (<10 mm day−1) enhanced heterotrophic respiration and resulted in a net loss of carbon from the system. Large precipitation event enhanced carbon uptake was attributed to increased soil water content (SWC), which promotes plant photosynthesis. The large precipitation events which occurred from July to October have lower increases in daytime net CO2 uptake (NEPd) due to the presence of low live plant biomass compared to earlier in the growing season. Live aboveground plant biomass (AGB), solar radiation, and SWC were the major variables that controlled NEPd, while AGB, SWC, and relative humidity control nighttime respiration losses (NEPn). Aboveground plant biomass is the most important variable for controlling both NEPd and NEPn dynamics. These results suggest that the major factor controlling growing season NEPn is the amount of carbon fixed via photosynthesis during the day. Heterotrophic soil respiration is greatly enhanced for one to 2 days following rainfall events with daily rainfall events >5 mm having a similar increase in respiration (>3.00 g m Cm−2 day−1). In addition, the size of the heterotrophic respiration pulse is independent of both the amount of time since the last rainfall event and the time of occurrence during the growing season.

Seasonal and interannual variability in surface energy partitioning and vegetation cover with grazing at shortgrass steppe

We evaluated shortgrass steppe energy budgets based on the Bowen Ratio Energy Balance method for three different grazing intensity treatments at the Central Plains Experimental Range Long-Term Ecological Research (CPER-LTER) site. We tested the correlations between aboveground biomass and surface energy fluxes for three different precipitation years based on continuously measured 20 min interval data.Grazing has a potential impact on energy partitioning under conditions of higher water availability, but not during dry conditions. Our study confirms that precipitation, not grazing treatment, explains the majority of variation in aboveground biomass at the CPER-LTER site. In addition, we are suggesting effective temperature, not air temperature, as a superior metric to evaluate surface heat change. Effective temperature takes into account humidity as well as air temperature.

Spatial versus temporal variation in precipitation in a semiarid ecosystem

Spatial and temporal variations in precip- itation are central features of semiarid ecosystems, influencing patterns of plant productivity and the distribution of native fauna. Although temporal variation in precipitation has been studied exten- sively, far less is known about the spatial scale and pattern of precipitation variability in semiarid regions. I used long-term precipitation records to examine spatial variation across the 63 km 2 Central Plains Experimental Range in northeastern Colorado, and across the 117,000 km 2 region of shortgrass steppe in eastern Colorado. Relative to temporal variation, spatial variation was low at scales\10 km, increased linearly across scales of 40-120 km, and was nearly equal in magnitude to temporal variation across distances of 120-160 km. Although I hypoth- esized that most spatial variation would be generated by early-summer convective thunderstorms in June, I found that the magnitude and spatial pattern of variation was similar for precipitation received in June compared to cumulative precipitation received during the full growing season. The degree of spatial autocorrelation in precipitation across all distances that I evaluated was similar for drought, dry, above- average and wet years. Across distances of 10- 120 km, spatial variation within a single growing season was approximately two times greater than spatial variation in long-term mean growing-season precipitation, indicating spatial shifting in the loca- tions of patches of high and low precipitation over multiple years. Overall, these findings suggest spatial variation at scales of 10-160 km may have been an important factor influencing vegetation patterns and migratory fauna of the shortgrass steppe, and have implications for livestock producers and future assessments of climate change.

Concerning the Measurement and Magnitude of Heat, Water Vapor, and Carbon Dioxide Exchange from a Semiarid Grassland

AbstractGrassland environments constitute approximately 40% of the earth’s vegetated surface, and they play a key role in a number of processes linking the land surface with the atmosphere. To investigate these linkages, a variety of techniques, including field and modeling studies, are required. Using data collected at the Central Plains Experimental Range (CPER) in northeastern Colorado from 25 March to 10 November 2004, this study compares two common ways of measuring turbulent fluxes of latent heat, sensible heat, and carbon dioxide in the field: the eddy covariance (EC) and Bowen ratio energy balance (BREB) methods. The turbulent fluxes measured by each of these methods were compared in terms of magnitude and seasonal behavior and were combined to calculate eddy diffusivities and examine turbulent transport. Relative to the EC method, the BREB method tended to overestimate the magnitude of the sensible heat, latent heat, and carbon dioxide fluxes. As a result, substantial differences in both the diurnal pattern and long-term magnitudes of the water and carbon budgets were apparent depending on which method was used. These differences arise from (i) the forced closure of the surface energy balance and (ii) the assumption of similarity between the eddy diffusivities required by the BREB method. An empirical method was developed that allows the BREB and EC datasets to be reconciled; this method was tested successfully using data collected at the CPER site during 2005. Ultimately, however, the BREB and EC methods show important differences that must be recognized and taken into account when analyzing issues related to the energy, water, or carbon cycles.

Overwintering Site Selection by Short-Horned Lizards (Phrynosoma Hernandesi) in Northeastern Colorado

Radio telemetry was used to track 16 adult Short-Horned Lizards (Phrynosoma hernandesi) to their individual overwintering sites on the Central Plains Experimental Range (CPER) in Weld County, Colorado. Above-ground habitat characteristics of overwintering sites and randomly selected points within the study area were assessed. All individuals entered overwintering between 29 August and 19 September with a mean estimated entrance date of 7 September. Arrival of the first subzero nighttime air temperatures occurred shortly thereafter on 16 September. No lizard left its normal area of use to overwinter, and there was no tendency to aggregate. Lizards did not necessarily overwinter on warmer south-facing slopes; the proportion of overwintering sites oriented southward (0.62) was not different from random. Most lizards (75%) selected overwintering sites in the banks of washes that had relatively steep slopes and at specific locations where substrate was relatively bare and penetrable. Overwintering sites also tended to have a greater coverage of Yucca glauca (0.02%) than the general study area (0.01%). Analyses of historical soil temperature data from the CPER revealed that lizards would have to overwinter at a subsoil depth of about 1 m to avoid freezing temperatures. Banks that contain suitable hibernation sites that are located within an individual's normal area of use may be the habitat feature most important to successful overwintering.

Near-Ground Remote Sensing of Green Area Index on the Shortgrass Prairie

Accurate and efficient leaf area measurements of shortgrass prairie vegetation are difficult to obtain. Few studies have considered the green area index (GAI) as an approximation of the total area of photosynthetically active tissue per unit of ground area. The main objective of this study was to evaluate several near-ground remote sensing methods as reliable and cost efficient measures of GAI on the shortgrass prairie. GAI measured with a standard leaf area meter was compared to 1) spectral vegetation indices calculated from multispectral radiometer data, 2) GAI obtained from laser point-frame measurements, and 3) green cover estimates derived from digital camera images. All methods were assessed for accuracy, time, and cost efficiency. Data were collected in 2001 at the Central Plains Experimental Range in northern Colorado. The standard leaf area meter method was neither time nor cost efficient in comparison with the other methods evaluated in this study. The cost of GAI measurement with the traditional leaf area meter method ($225 per plot) was 20 times greater than GAI estimation with the multispectral radiometer ($11 per plot). Comparison of GAI obtained with the standard leaf area meter method with red-band reflectance index values (0.63‐0.69 lm) obtained with a portable multispectral radiometer resulted in the best model predictions (R 2 ¼ 0.76, Akaike’s information corrected criterion [AICC] ¼ 182.9) and the most cost efficient method for GAI estimation. Green cover estimates from digital image analysis resulted in a good correlation with the leaf area meter GAI (R 2 ¼ 0.72, AICC ¼ 178.1). However, classification accuracies of digital images were decreased by limited spectral separability between green vegetation, brown vegetation, and soil background. Further calibration and refinement of near-ground remote sensing techniques for vegetation might establish these methods as efficient ground-truth alternatives to satellite-based remote-sensing applications of rangelands such as the shortgrass prairie.

CO2ENHANCES PRODUCTIVITY, ALTERS SPECIES COMPOSITION, AND REDUCES DIGESTIBILITY OF SHORTGRASS STEPPE VEGETATION

The impact of increasing atmospheric CO2 concentrations has been studied in a number of field experiments, but little information exists on the response of semiarid rangelands to CO2, or on the consequences for forage quality. This study was initiated to study the CO2 response of the shortgrass steppe, an important semiarid grassland on the western edge of the North American Great Plains, used extensively for livestock grazing. The experiment was conducted for five years on native vegetation at the USDA-ARS Central Plains Experimental Range in northeastern Colorado, USA. Three perennial grasses dominate the study site, Bouteloua gracilis, a C4 grass, and two C3 grasses, Pascopyrum smithii and Stipa comata. The three species comprise 88% of the aboveground phytomass. To evaluate responses to rising atmospheric CO2, we utilized six open-top chambers, three with ambient air and three with air CO2 enriched to 720 μmol/mol, as well as three unchambered controls. We found that elevated CO2 enhanced production of the shortgrass steppe throughout the study, with 41% greater aboveground phytomass harvested annually in elevated compared to ambient plots. The CO2-induced production response was driven by a single species, S. comata, and was due in part to greater seedling recruitment. The result was species movement toward a composition more typical of the mixed-grass prairie. Growth under elevated CO2 reduced the digestibility of all three dominant grass species. Digestibility was also lowest in the only species to exhibit a CO2-induced production enhancement, S. comata. The results suggest that rising atmospheric CO2 may enhance production of lower quality forage and a species composition shift toward a greater C3 component.

BIOGEOCHEMISTRY IN A SHORTGRASS LANDSCAPE: CONTROL BY TOPOGRAPHY, SOIL TEXTURE, AND MICROCLIMATE

Biogeochemistry of terrestrial ecosystems is controlled by interactions among factors operating at several spatial and temporal scales. The purpose of this study was to evaluate the relative importance and interaction of relatively static landscape factors and more dynamic factors in a shortgrass steppe landscape. The landscape factors examined were topographic position, and soil texture. The dynamic factors studied were seasonal climate and the localized effects of individual plants on soils. Patterns were evaluated by sampling soil between and under individual Bouteloua gracilis plants in paired upland (erosional) and lowland (depositional) plots at eight locations at the Central Plains Experimental Range (CPER), Colorado. We quantified five organic C and N pools (total, fine and coarse particulate organic matter [POM], mineral-associated organic matter [MAOM], and potentially mineralizable C and N), and we estimated seasonal patterns of in situ N dynamics with three methods (extractable inorganic N, net N mineralization in uncovered cores, and N adsorbed on ion exchange resin [IER] bags). Topographic position and soil texture each explained much of the landscape-scale variation of C and N pools and vegetation structure. Most lowland plots were enriched in silt, clay, C, and N relative to adjacent upland plots, and topographic position affected most pools significantly. Most vegetation and biogeochemical variables were strongly correlated with soil sand content. Across the range of sand content encountered (40–83%), the fraction of area in bare soil openings >5 cm across increased sevenfold, and most C and N fractions increased by 2–4.5 times. Plant-induced, microscale heterogeneity of soil C and N was comparable in magnitude to landscape-scale heterogeneity for pools with more rapid turnover (POM and mineralizable C and N), but presence or absence of plants did not affect more stable, mineral-associated organic matter. Plant-induced heterogeneity was significant in all locations, but its importance likely decreases with decreasing sand content as cover becomes more continuous, particularly in lowlands. Total extractable inorganic N and nitrate, N adsorbed on resin bags, and the proportion of mineralized N that was nitrified during incubations increased with increasing soil water content or precipitation, but net N mineralization did not vary systematically with precipitation. Inorganic N availability was greatest during relatively moist spring periods, and these were the only times when indices of in situ N availability followed the spatial patterns expected from laboratory assays of C and N pool distribution. The relatively weak spatial patterns for N dynamics contrast with the substantial landscape and individual-plant scale variation in C and N pools. Landscape patterns of N mineralization and availability may be tied to N pools, such as POM N, that are not strongly related to topography or texture. Particulate organic matter appears to be especially important to N retention and availability on sandy soils, which typify most sites at the CPER; the proportion of total N residing in POM was high and POM C:N ratios were low in sandy upland soils. We suggest that soil texture is a key proximal control over biogeochemical processes and is largely responsible for observed landscape-scale patterns, including topographic differences. Models that integrate effects of texture and topography have the potential to link landscape biogeochemical patterns to short-term processes that directly influence SOM dynamics and to long-term geomorphic processes that influence soil distribution, including eolian redistribution of soil materials, which is important in many dry regions.

Habitats of native and exotic plants in Colorado shortgrass steppe: a comparative approach

Invading species often are close relatives, and therefore share many characteristics as a consequence of their common ancestry. This tends to confound studies of invasions, since many irrelevant characteristics are likely to be correlated with a species' geographic origin (alien or native). We address this problem by using phylogenetically independent comparisons to investigate the habitat characteristics of plants of the Central Plains Experimental Range (Colorado, U.S.A.). We initially show that exotic species are more likely than natives to occur in riparian zones, roadsides, and disturbed sites, and less likely to occur in grassland. The relationship between exotic origins and disturbed sites disappears when phylogenetic dependence is removed from the analyses; in contrast, the other associations persist following phylogenetic detrending, indicating that aliens and their native relatives consistently differ in their ability to exploit riparian, roadside, and grassland habitats. Our results indicate that disturbed sites currently are dominated by only a few groups of related exotic ruderals, while the ability to exploit roadsides and riparian zones has been and may continue to be important for the success of many taxa of invaders.Key words: biological invasions, comparative methods, disturbance, grasslands, phylogeny.

Controls on annual emissions of nitric oxide from soils of the Colorado shortgrass steppe

Estimates of NOx (NO+NO2) emissions from temperate grasslands range from 0.003 to 101 ng NO‐N m−2 s−1(average 4.17 ng NO‐N m−2 s−1). As a result of this uncertainty, the potential contribution of soil NOx emissions to the global budget and to nitrogen loss for this ecosystem is unclear. The few existing measurements are short‐term observations of growing‐season fluxes. We measured soil NOx emissions from six sites within the shortgrass steppe at the Central Plains Experimental Range in Colorado from June 1994 through October 1995. The soils at these sites provided a range of texture (from a sandy loam to a clay loam) and soil moisture. Mean NOx emissions over the sampling period ranged from 2.6 to 5.7 ng NO‐N m−2 s−1 from the four unfertilized sites. Temperature was the dominant control on seasonal variations in NOx fluxes. Seasonal fluxes were highest in the summers (5.4 to 10.5 ng NO‐N m−2 s−1) and lowest in the winter (0.2 to 1.5 ng NO‐N m−2 s−1). The winter NOx emissions contribute up to 25% to the mean annual flux. Water‐filled pore space (WFPS) alone was a poor predictor of NOx emissions; however, peak NOx emissions were found near the field capacities for these soils (32–35% WFPS for coarse soils and 66% WFPS for fine‐textured soils). Water additions produced large (22–51 ng NO‐N m−2 s−1) but short‐lived (24 hour) pulses Of NOx emissions that were independent of both the amount of water added and the number of antecedent dry days. Short‐term increases in NOx flux stimulated by wetting are significant, and increase the summer estimate of NOx emissions 8 times estimates calculated from periodic sampling. Nitrogen applied in previous studies, 5 to 12 years earlier, increased the average annual NOx emissions approximately 1.5 times. Extrapolating our estimate to similar systems around the world, we estimate that grassland ecosystems, globally, emit 1.0 Tg N yr−1 as NOx. A current estimate of NOx emissions from grassland soils is 0.6 Tg N yr−1 [Davidson, 1991].

Contingent effects of plant species on soils along a regional moisture gradient in the Great Plains.

The central grassland region of the United States encompasses major gradients in temperature and precipitation that determine the distribution of plant life forms, which in turn may influence key ecosystem processes such as nutrient cycling and soil organic matter dynamics. One such gradient is the threefold increase in precipitation from the eastern Colorado shortgrass-steppe, in the rain shadow of the Rocky Mountains, to the tallgrass prairie in eastern Kansas. We investigated the relative roles of plant species and plant cover in influencing soil C and N cycling in three sites along this gradient. Plant cover (i.e., the presence or absence of an individual plant) was relatively more important than plant species in explaining variability in soil properties at the dry site, the Central Plains Experimental Range in␣northeastern Colorado. However, plant species explained relatively more of the variability in soil properties than did plant cover at the two wetter sites, Hays and Konza, in central and eastern Kansas. The wetter sites had more continuous plant cover, resulting in less plant-cover-induced variation in soil C and N, than did the dry site, which had distinct patches of bare ground. Plant species at the wetter sites had higher and more variable levels of tissue C:N than plant species at the dry site, due to both within species changes and changes in species composition. Aboveground tissue C:N was better correlated with net nitrogen mineralization rates at the wet sites than the dry site. Thus, tissue chemistry appears to exert more control on nitrogen dynamics at the wet than the dry sites. The results suggest that plant species traits that are relevant to nutrient cycling (e.g., tissue C:N ratios, spatial patterns, productivity) reflect environmental limitations as well as species' physiological potentials. Furthermore, a dominant environmental driver such as precipitation may ameliorate or exaggerate the importance of individual species traits for nutrient cycling.

CH4 and N2O fluxes in the Colorado shortgrass steppe: 1. Impact of landscape and nitrogen addition

A weekly, year‐round nitrous oxide (N2O) and methane (CH4) flux measurement program was initiated in nine sites within the Central Plains Experimental Range in the Colorado shortgrass steppe in 1990 and continued through 1994. This paper reports the observed intersite, interannual, and seasonal variation of these fluxes along with the measured variation in soil and air temperature and soil water and mineral nitrogen content. We found that wintertime fluxes contribute 20–40% of the annual N2O emissions and 15–30% of CH4 consumption at all of the measurement sites. Nitrous oxide emission maxima were frequently observed during the winter and appeared to result from denitrification when surface soils thawed. Interannual variation of N2O maximum annual mean fluxes was 2.5 times the minimum during the 4‐year measurement period, while maximum annual mean CH4 uptake rates were 2.1 times the minimum annual mean uptake rates observed within sites. Generally, site mean annual flux maxima for CH4 uptake corresponded to minimum N2O fluxes and vice versa, which supports the general concept of water control of diffusion of gases in the soil and limitations of soil water content on microbial activity. We also observed that pastures that have similar use history and soil texture show similar N2O and CH4 fluxes, as well as similar seasonal and annual variations. Sandy loam soils fertilized with nitrogen 5–13 years earlier consumed 30–40% less CH4 and produced more N2O than unfertilized soils. In contrast, the N addition 13 years ago does not affect CH4 uptake but continues to increase N2O emissions in a finer‐textured soil. Our long‐term data also show that soil mineral N concentration is not a reliable predictor of observed changes, or lack of changes, in either N2O efflux or CH4 uptake. Finally, from our data we estimate that annual global N2O emission rates for native, temperate grasslands are about 0.16 Tg N2O‐N yr−1, while CH4 consumption totals about 3.2 Tg CH4‐C yr−1.