Browsing by Author "Cooper, David, committee member"

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Open Access
Characterization of peat soil hydraulic conductivity and its dependence on vegetation type in mountain wetlands
(Colorado State University. Libraries, 2015) Crockett, Audrey, author; Ronayne, Michael, advisor; Cooper, David, committee member; Sanford, William, committee member
Peat-forming wetlands enhance biodiversity and provide carbon storage in mountain environments. Persistence of these wetlands requires sustained water inflows. Reduced or altered inflows associated with climate change could lower the water table, potentially resulting in peat oxidation and carbon release to the atmosphere, as well as the loss of wetland plant and animal species. An understanding of the hydrology and site hydraulic properties is necessary to manage mountain wetlands and assess their vulnerability to climate change. This study characterized the hydraulic conductivity of wetland peat soils in Rocky Mountain National Park (RMNP). Peat-forming wetlands in RMNP are classified as fens because their main source of water is groundwater. Fens in RMNP contain a broad range of vegetation. Dominant vegetation type is one factor that may influence peat hydraulic conductivity, so the fens in this study were divided based on dominant vegetation type. The three vegetation classifications used were “large sedge,” “small sedge,” and “heterogeneous,” indicating that the fens were dominated by large sedges (mainly Carex); small sedges (Eleocharis quinqueflora); or a mixture of woody plants, sedges, and moss; respectively. In this study, field measurements were combined with a numerical model and parameter estimation scheme to produce estimates of hydraulic conductivity with a high degree of confidence. Single-ring infiltration tests were performed in the field. A numerical model was constructed, and a parameter estimation scheme was used to find the hydraulic conductivity that best reproduced the results of the single-ring infiltration test. The fens dominated by small sedges have significantly lower hydraulic conductivity than the fens dominated by large sedges or heterogeneous vegetation. Fens which have relatively high hydraulic conductivity (those dominated by large sedges or a heterogeneous mixture of plants) may be especially at risk of draining under changing climate regimes. Small-sedge fens may be more likely to maintain a high water table due to their low hydraulic conductivity.
Open Access
Interactions between Bromus tectorum, grasshoppers, and native plants in sagebrush steppe communities
(Colorado State University. Libraries, 2014) Cumberland, Catherine, author; Paschke, Mark, advisor; Cooper, David, committee member; Jonas, Jayne, committee member; Pejchar, Liba, committee member
Invasion by the exotic grass Bromus tectorum L. (cheatgrass) has produced widespread, persistent changes in the Intermountain West. As a result of this and other disturbances, sagebrush shrubland is among North America's most imperiled ecosystems. Restoration of B. tectorum-infested areas has often been unsuccessful, and there is a need to understand the factors limiting revegetation success. There is evidence that B. tectorum is a superior competitor for space and resources. But it's also possible that B. tectorum dominance is enhanced by native herbivores through the mechanism of apparent competition: species competing indirectly through shared natural enemies. If an invasive exotic is a less preferred food source for herbivores than native plants, per capita feeding impacts may increase on natives, thus facilitating exotic proliferation. In sagebrush shrubland, grasshoppers are often the dominant herbivores. Their feeding patterns have been shown to limit and structure plant distributions. The objective of this study was to elucidate whether grasshoppers may be promoting B. tectorum spread through herbivore-mediated apparent competition. Using native plants commonly seeded in sagebrush restoration projects and the native generalist grasshopper Melanoplus bivittatus in laboratory trials and greenhouse experiments, I tested how grasshopper herbivory affects native plants in comparison to B. tectorum. Grasshoppers significantly increased mortality rates for most native plants but had minimal impact on B. tectorum mortality. Certain native species were much more highly preferred and / or impacted by herbivory, including the keystone shrub Artemisia tridentata (big sagebrush). However, several native species were either less preferred or more tolerant of herbivory than B. tectorum, and could therefore be good restoration candidates where grasshoppers are common. In addition, my results suggest B. tectorum could promote population increase in certain grasshopper species, particularly agricultural and rangeland pests. Study results could provide guidance regarding seed mixes and possible control of insect herbivores to improve restoration success in B. tectorum-invaded areas.
Open Access
Numerical modeling investigation of long-term hydrologic change due to surface water and groundwater withdrawals from a high Andean headwater basin, southern Peru
(Colorado State University. Libraries, 2024) Stansfield, William J., author; Ronayne, Michael J., advisor; Cooper, David, committee member; Sanford, William, committee member
Reliable surface water and groundwater resources are of critical importance in the arid central Andes of Southern Peru. Low elevation cities and towns in coastal regions are dependent on water derived from relatively humid mountains and high elevation basins on the Andean Altiplano. The Huaytire Basin, located on the Altiplano at approximately 4,450 meters above mean sea level, is a headwater basin that has experienced hydrologic change in recent years. Surface water diversions from the perennial Lake Suches began in the 1960s, followed by groundwater pumping starting in the 1980s. These water development activities have been accompanied by observations of declining surface water quantity in the downgradient Rio Callazas and deteriorating phreatophytic vegetation within the basin itself. A review of precipitation and pan evaporation data from weather stations in the region did not reveal any clear climate-related trends that would impact water resource availability. A numerical groundwater flow model was constructed using MODFLOW to estimate the impact of long-term pumping on local hydrology and to investigate the sustainability of the Huaytire Basin pumping regime. The model accounted for groundwater-surface water interaction, including lake-aquifer exchange and stream-aquifer exchange along three major streams that originate within the basin. Surface water levels and flow rates were computed as part of the numerical solution, dependent on the simulated hydraulic head in the aquifer, which controls the amount of groundwater-surface water exchange. Three different recharge scenarios were considered to acknowledge uncertainty associated with groundwater recharge rates on the Altiplano. All three scenarios resulted in the study area converting from an open basin with surface water outflow to a closed basin within 50 years of the start of development. Other simulated impacts of pumping include a reduction in the stage and areal extent of Lake Suches and significant reductions in head-dependent outflows for the aquifer system. Relative to predevelopment conditions, groundwater discharge to Lake Suches, groundwater discharge to gaining stream reaches, phreatophytic evapotranspiration, and underflow out of the basin were all lower at the end of the 63-year transient simulation (by ~ 9.5, 50, 18, and 51%, respectively, using base-case recharge rates). It was concluded that groundwater development in the Huaytire Basin is a key factor that explains the observed hydrologic changes and that current pumping rates may be unsustainable.
Open Access
Soil shear resistance and plant community recovery after disturbance in a montane riparian ecosystem
(Colorado State University. Libraries, 2011) Bliss, Shaan T., author; Trlica, Milton Joe, advisor; Leininger, Wayne, committee member; Cooper, David, committee member
Vegetation recovery after a severe cattle trampling disturbance was studied in a montane riparian community. The disturbance allowed for the successful establishment of cuttings of mountain willow (Salix monticola Bebb.). However, most of the planeleaf willow (S. planifolia) cuttings had very poor establishment. Weedy annual forbs were present after the first and second years following the grazing disturbance. But, after six years of recovery the weedy forbs had declined and the dominant sedge (Carex spp.) community returned. However, some introduced forbs still persisted after six years of protection from livestock grazing. Season of trampling had significant effects on herbaceous species composition and rate of recovery. Plots trampled in the fall had greater recovery of herbaceous cover (P = 0.05) and native species (P = 0.03) as compared with late spring and early summer trampling disturbances. A severe disturbance probably increased resource availability and weedy forbs quickly took advantage of these resources. Plots trampled in the late spring retained the greatest (P = 0.0011) cover of ruderal species (23%), even after six years of recovery. However, other trampled plots returned to a composition dominated by native grasses and sedges. Mountain willow had greater survival (P = 0.01) and stem growth (P = 0.01) compared with planeleaf willow (Salix planifoliaPursh). Planeleaf willow's poor performance may have resulted from a combination of a deep water table depth, competition with adjacent plants, and poor transplantablity. Mountain willow tolerated lower water table depths and some competition from adjacent herbaceous plants. Belowground root-soil cores for several herbaceous species from the montane riparian community were studied using shear resistance measurements as an indicator for erosion protection. Three important grasses, a rush (Juncus spp.), and two sedge species were compared at two depths from 0 to 10 and 10 to 20 cm. In addition, tests for bulk density, texture, belowground biomass, and organic matter content were made to establish relationships of these variables with vegetation type and soil shear resistance. Soil cores from vegetated areas were compared with unvegetated areas. These comparisons reveled a three-fold greater (P = 0.003) shear resistance (greater protection from erosion) for vegetated sites in the upper soil depth (0 to 10 cm). There was no significant difference between vegetated and unvegetated sites for the lower soil depth (10 to 20 cm). Sedges and Juncus had greater (P < 0.0001) belowground shear resistance than grasses in the top 10 cm soil layer. Both categories of species had little effect on shear resistance below the 10 cm soil layer (P = 0.1246) where belowground biomass was much less. Sedges had three times greater (P < 0.001) shear resistance in the top 0 to 10 cm compared to the bottom 10 to 20 cm soil depth. Covariate analysis showed that belowground plant biomass was the most influential component affecting shear resistance. Soil bulk density was also an important covariate. The top 10 cm of the soil layer had the largest concentration of root and rhizome biomass and lower bulk density compared to the 10 to 20 cm depth.
Open Access
The ecology of an irrigation system: wetland creation in an agricultural landscape
(Colorado State University. Libraries, 2012) Sueltenfuss, Jeremy, author; Knight, Rick, advisor; Waskom, Reagan, committee member; Cooper, David, committee member; Wallace, George, committee member
Irrigation has increased the agricultural productivity of the arid American West, but has also greatly altered the natural landscape. Irrigation canals transport water to 17 million ha of currently irrigated land. Because water is a limited resource in the west, and irrigated agriculture uses approximately 90% of all the water diverted from rivers, much attention has been paid to the efficiency of irrigation systems. Irrigation canals have been shown to leak up to 50% of the water they transport, affecting both groundwater recharge and return flows to rivers, though little work has been done documenting the ecological effects of irrigation canal seepage on wetland ecosystems. This study sought to identify the hydrologic processes linking canals and reservoirs to wetlands, identify the types of wetlands supported by irrigation canal seepage, and document the area of wetlands supported by irrigation within the service area of an irrigation company. All wetlands within the North Poudre Irrigation Company service area in Larimer County were mapped and their hydrologic source determined from visual clues. Groundwater monitoring wells were installed in wetlands adjacent to canals and reservoirs to identify the hydrologic influence of canal seepage on wetland hydrologic regime. To further demonstrate the hydrologic source of wetlands, stable oxygen isotopes were analyzed within wetlands and possible adjacent water sources. Vegetation characteristics and species percent composition was related to environmental variables to highlight the types of wetlands supported by an irrigation infrastructure. A total of 176 wetlands covering 652 ha were mapped, 92% of which were visually connected to the irrigation infrastructure. Wetland water tables fluctuated with adjacent canal flow, with increases in the water table when canals started transporting water, and decreases in water table depth during times when canals did not carry water. Isotopic data indicate that canal leakage is the hydrologic source for adjacent wetlands within the study area. The isotopic signature of canal water matched that of wetlands closer to canals, with evaporatively enriched isotopic signatures in wetlands further from canals. Wetland vegetation composition was related to both salinity and groundwater depth, with salt flats dominated by Atriplex spp. forming in areas with high salinity, marsh communities dominated by Typha latifolia and Schoenoplectus acutus forming in areas with low salinity and deeper standing water, and meadow communities dominated by Carex nebrascensis and Schoenoplectus pungens forming in areas with low salinity and water tables closer to the ground surface. Though land conversion and water diversions have led to dramatic reductions in historic wetland area in some places, it is clear from this study that current agricultural landscapes create wetlands that rely on excess irrigation water for their hydrologic maintenance. Any future changes in irrigation practices or water distribution may have negative consequences on wetland ecosystems.