M. J. Trlica

Sediment movement through riparian vegetation under simulated rainfall and overland flow.

A 2-year rainfall simulation study was conducted to evaluate the effectiveness of riparian vegetation to reduce sediment movement. Three vegetation height treatments [clipped to the soil surface, 10 cm height, and undisturbed (unclipped)] were evaluated in 2 montane riparian vegetation communities in northern Colorado. One community was a tufted hairgrass (Deschampsia caespitosa (L.) Beauv.), cinquefoil (Potentilla gracilis Dougl. ex Hook), Kentucky bluegrass (Poa pratensis L.), and sedge (Carex spp.) association. The other community was dominated by beaked sedge (Carex rostrata Stokes) and water sedge (Carex aquatilis Wahl.). Water was sprayed on plots (3 m X 10 cm) at a rate of 60 mm hour-1 with a rainfall simulator, while overland flow containing sediment was introduced at the upper end of the plots at a rate of 25 mm hour-1. Two sediment sources were evaluated. In the first years studies the sediment was derived from an upland soil. The second year a fine silica sediment was used. Thirty kg of sediment was added to each plot. The first of 2 experiments was concerned with movement of sand particles greater than 200 micrometers. The second experiment was designed to evaluate the quantity of 5 particle size classes (2-10, 10-30, 30-50, 50-100, and 100-200 micrometers) contained in sediment traps at 60, 120, and 180 cm downslope from the upslope border of the simulator plots. Results of the first experiment showed a significant increase in sand movement downslope when vegetation was clipped to the soil surface compared with undisturbed vegetation. In the second experiment, most significant differences in movement for finer particles occurred in the 2-10 micrometers and 10-30 micrometers particle ranges. A smaller percentage of particles in the 2-10 micrometers range was present in sediment traps at all 3 distances downslope when vegetation was clipped to the soil surface, as these smaller particles stayed in suspension. Increased vegetation height resulted in a significantly smaller percentage of the 10-30 micrometers particle size range present at 120 cm distance. This study showed that additional variables (% surface vegetation cover, aboveground biomass, % shrubs, surface roughness coefficient, soil texture of introduced sediment, % bare ground, distance downslope, vegetation density, grass spp., and sedge spp.), besides vegetation height, influenced sediment movement. Land managers should understand that when they manage ecosystems for a single factor, such as vegetation height, they cannot address complex issues such as sediment particle detachment, movement and filtration.

Sediment filtration in a montane riparian zone under simulated rainfall

A 2 year study was conducted to evaluate the effectiveness of riparian vegetation to filter sediment from overland water flow. Three vegetation height treatments: clipped to the soil surface, clipped to a 10 cm height, and undisturbed were evaluated in 2 montane riparian vegetation communities (grass and sedge) in northern Colorado. Water was sprayed on 2 macro-plots (3 m X 10 m) and 2 micro-plots (0.6 m X 2 m) simultaneously at a rate of 60 mm hr-1 with a rotating boom rainfall simulator. Overland flow containing sediment was introduced at the upper end of the plots at a rate of 25 mm hr-1 to simulate runoff and sediment transport from an upland area. Two sediment sources were used, a sandy loam soil and a ground silica sediment (loam). Thirty kg of sediment were added to each macro-plot and 1.2 kg of sediment were introduced to each micro-plot (10 Mg ha-1). Sediment yields, at the downslope end of the plot, were greater when the finer silica sediment was introduced into overland flow as compared with sediment derived from the sandy loam soil. As expected the small micro-plots yielded more sediment and were often more sensitive to community and treatment differences than larger plots. We believe this resulted from the shorter travel distance. However, sediment filtration treatment effects were usually similar for both plot sizes. Sediment yields, measured at the outlet of the plots, did not decrease, or increase, as vegetation heights increased. Accurate prediction of sediment filtration from shallow flow in riparian zones required consideration of a combination of vegetation and soil surface characteristics.

Ground based digital imagery for grassland biomass estimation

Aboveground biomass was estimated on the shortgrass prairie of eastern Colorado using ground based conventional (RGB) digital camera imagery. The accuracy and efficiency of image‐based estimates were compared with clipped biomass measurements. Field measurements of aboveground biomass were obtained on three grazing treatments and three sample dates (phenological status). Grazing treatments did not significantly affect (p>0.10) estimates of clipped green biomass taken from digital images. However, plant phenology, green biomass estimates from images, and the interaction of plant phenology and green biomass estimates from images significantly affected clipped green biomass measurements (p≤0.04). Analysed images provided fair estimates of total clipped green biomass (R2 = 0.55) and clipped green biomass without cactus (R2 = 0.73) when plant phenological status was included in the models. When plant phenology was removed from the models, the variability explained by green biomass estimates from images declined to 25% for clipped green biomass, and 32% for clipped green biomass without cactus. Thus, results showed that plant phenological status was the most important variable in the prediction of green vegetation biomass. Results indicated that the usefulness of RGB digital camera imagery for green biomass estimation is limited for the shortgrass prairie.

Grazing History Affects Willow Communities in a Montane Riparian Ecosystem

Abstract This study was conducted to compare data from 12 grazed and ungrazed areas and to examine the impacts of grazing treatments on a montane willow community during an 11-year period. Data were collected on willow canopy cover, species diversity, height, and stem density in a montane riparian ecosystem between 1988 and 1999 from 4 grazing treatments: long-term grazing (since the early 1900s), long-term grazing exclusion (exclosures built in the 1950s), recent grazing (sections of exclosures opened in 1988), and recent grazing exclusion (exclosures built in 1988). Willow canopy cover increased significantly for all treatments through time, with the recent grazing exclusion treatment becoming similar to that of the long-term exclusion treatment within 5 years. Species diversity was greatest in the long-term grazed treatment. Willow height averaged over treatments increased from 1988 to 1997 (P = 0.0001), but did not increase significantly after that. Height in the long-term exclosure averaged over time from 1988 to 1997 was 1.5 times greater than in the long-term grazing treatment. Stem density of willows was significantly greater in the recent exclosure than in the long-term exclosure (P = 0.008, 180%) and recent grazing treatments (P = 0.02, 120%). Recent grazing exclusion resulted in the greatest increase in canopy cover, height growth, and stem density during the 11 years of study, indicating that these variables respond positively to removal of livestock grazing. Results suggest that continued long-term grazing exclusion may lead to a closed canopy, lower willow species diversity, reduction in new stem height growth, and reduced stem recruitment. Information on the dynamics of willow growth under different grazing treatments should help resource managers determine appropriate livestock utilization levels in similar riparian areas, and develop management plans for these important ecosystems.

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

Abstract 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 (

Cattle use affects forage quality in a montane riparian ecosystem.

225 per plot) was 20 times greater than GAI estimation with the multispectral radiometer (

Steer diets in a montane riparian community

11 per plot). Comparison of GAI obtained with the standard leaf area meter method with red-band reflectance index values (0.63–0.69 μm) obtained with a portable multispectral radiometer resulted in the best model predictions (R2 = 0.76, Akaikes information corrected criterion [AICC] = 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 (R2 = 0.72, AICC = 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.

Seasonal diet selection of cattle grazing a montane riparian community

Forage nitrogen (N) and phosphorous (P) concentrations and in-vitro dry-matter digestibility (IVDMD) were measured in 2 important riparian species the year following short-term, high-intensity cattle grazing treatments in a montane riparian ecosystem in northcentral Colorado. Current years growth of water sedge (Carex aquatilus Wahlenb.) and planeleaf willow (Salix planifolia Pursh.) was collected monthly from May to September 1996. The effects of grazing and season of grazing in 1995 on forage quality the following growing season was determined. Season of grazing (i.e., late-spring, early-summer, late-summer, and fall) the previous year did not differentially affect forage quality in either species. However, grazing by cattle the previous year did increase forage quality of water sedge as compared with plants that were not previously grazed. Grazed water sedge plants had higher concentrations of N and P and greater IVDMD than ungrazed controls. Nitrogen and P concentrations of browsed planeleaf willow were not different from controls, but current years growth collected in the fall from previously browsed plants was 11% more digestible than current years growth from non-browsed willow. The 2 species responded uniquely to cattle use, which suggested that these 2 life forms differ in response to herbivory. This study supported the hypothesis that grazing by cattle would improve forage quality in a riparian ecosystem, although results varied with life form.

A Laser Point Frame to Measure Cover

Abstract Diets of fistulated steers that grazed a montane riparian community were determined throughout a growing season. The objective was to determine if willow (Salix spp.) consumption by steers was related to the season of use and the residual stubble heights of herbaceous forage species. Diet samples were collected at 4 levels of herbaceous utilization throughout 4 grazing periods that were based on willow phenology. Results suggested that spring grazing of riparian pastures was preferable to late-season use to minimize browsing on willows. Willow consumption increased substantially as herbaceous stubble height approached 10 and 18 cm during the spring and early-summer grazing periods, repectively. Stubble heights greater than 20 cm were needed to reduce willow consumption when they were most preferred during the late-summer and fall grazing seasons in this tall sedge (Carex spp.)/willow riparian community. Sedge and rush (Juncus spp.) composition in steer diets declined, while willow composition increased in steer diets, in response to decreasing stubble heights. Grass consumption by steers was little affected by stubble height, while forb consumption was directly related to forb availability. Although grazing activity has the potential to negatively impact riparian willows, these results suggested that timing of use and carefully controlled levels of herbaceous utilization could be used to minimize the consumption of willows by cattle in a montane riparian community.

Microchannels Affect Runoff and Sediment Yield From a Shortgrass Prairie

Abstract Cattle-grazing in riparian areas has become increasingly controversial. More information is needed about cattle use of these areas to develop Best Management Practices. This study was designed to determine seasonal changes in diet selection of cattle in a montane riparian community in northern Colorado. Previous cattle diet studies in riparian zones have not separated the interaction between seasonal preference and biomass utilization. The experiment was conducted within large exclosures that had not been grazed by livestock since 1956. Vegetation biomass estimates and diet samples from 5 esophageally fistulated steers were taken during 4 grazing periods (spring, early-summer, late- summer, and fall) in 1995. Vegetation measurements and diet samples were also collected during the late summer and fall of 1994. One of the 3 paddocks in each grazing period of 1995 had been grazed in 1994. Steer diets in 1995 were found to contain 15% more Carex spp. from paddocks that had been ungrazed until 1995 as...