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Ecological processes of low-productivity ecosystems have long been considered to be driven by abiotic controls with biotic interactions playing an insignificant role. However, existing studies present conflicting evidence concerning the roles of these factors, in part due to the short temporal extent of most data sets and inability to test indirect effects of environmental variables modulated by biotic interactions. Using structural equation modeling to analyze 65 years of perennial vegetation change in the Sonoran Desert, we found that precipitation had a stronger positive effect on recruitment beneath existing canopies than in open microsites due to reduced evaporation rates. Variation in perennial canopy cover...
Projected increases in aridity throughout the southwestern United States due to anthropogenic climate change will likely cause reductions in perennial vegetation cover, which leaves soil surfaces exposed to erosion. Accelerated rates of dust emission from wind erosion have large implications for ecosystems and human well-being, yet there is poor understanding of the sources and magnitude of dust emission in a hotter and drier climate. Here we use a two-stage approach to compare the susceptibility of grasslands and three different shrublands to wind erosion on the Colorado Plateau and demonstrate how climate can indirectly moderate the magnitude of aeolian sediment flux through different responses of dominant plants...
Knowledge about the condition of vegetation cover and composition is critical for assessing the structure and function of ecosystems. To effectively quantify the impacts of a rapidly changing environment, methods to track long-term trends of vegetation must be precise, repeatable, and time- and cost-efficient. Measuring vegetation cover and composition in arid and semiarid regions is especially challenging because vegetation is typically sparse, discontinuous, and individual plants are widely spaced. To meet the goal of long-term vegetation monitoring in the Sonoran Desert and other arid and semiarid regions, we determined how estimates of plant species, total vegetation, and soil cover obtained using a widely-implemented...
Abstract (from http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0148795): Arid climates have unpredictable precipitation patterns, and wildlife managers often provide supplemental water to help desert ungulates endure the hottest, driest periods. When surface water is unavailable, the only source of water for ungulates comes from the forage they consume, and they must make resourceful foraging decisions to meet their requirements. We compared two desert bighorn sheep ( Ovis canadensis nelsoni) populations in Arizona, USA: a treatment population with supplemental water removed during treatment, and a control population. We examined whether sheep altered their seasonal diets without supplemental water....
1. Riparian vegetation in dry regions is influenced by low-flow and high-flow components of the surface and groundwater flow regimes. The duration of no-flow periods in the surface stream controls vegetation structure along the low-flow channel, while depth, magnitude and rate of groundwater decline influence phreatophytic vegetation in the floodplain. Flood flows influence vegetation along channels and floodplains by increasing water availability and by creating ecosystem disturbance. 2. On reference rivers in Arizona's Sonoran Desert region, the combination of perennial stream flows, shallow groundwater in the riparian (stream) aquifer, and frequent flooding results in high plant species diversity and landscape...
Biological soil crusts (BSCs) are the dominant living cover in many drylands of the world. They possess many features that can influence different aspects of local hydrologic cycles, including soil porosity, absorptivity, roughness, aggregate stability, texture, pore formation, and water retention. The influence of biological soil crusts on these factors depends on their internal and external structure, which varies with climate, soil, and disturbance history. This paper presents the different types of biological soil crusts, discusses how crust type likely influences various aspects of the hydrologic cycle, and reviews what is known and not known about the influence of biological crusts on sediment production and...
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Over the past several decades, the expansion and thickening of woodlands in the western United States has caused a range of ecological changes. Woody expansion often leads to increases in soil organic matter (SOM) pools with implications for both biogeochemical cycling and ecological responses to management strategies aimed at restoration of rangeland ecosystems. Here we directly measure C and N stocks and use simple non-steady-state models to quantify the dynamics of soil C accumulation under and around trees of varied ages in southern Utah woodlands. In the two pinyon-juniper forests of Grand Staircase Escalante National Monument studied here, we found approximately 3 kg C/m2 and approximately 0.12 kg N/m2 larger...
Defining site potential for an area establishes its possible long-term vegetation growth productivity in a relatively undisturbed state, providing a realistic reference point for ecosystem performance. Modeling and mapping site potential helps to measure and identify naturally occurring variations on the landscape as opposed to variations caused by land management activities or disturbances (Rigge et al. 2020). We integrated remotely sensed data (250-m enhanced Moderate Resolution Imaging Spectroradiometer (eMODIS) Normalized Difference Vegetation Index (NDVI) (https://earthexplorer.usgs.gov/)) with land cover, biogeophysical (i.e., soils, topography) and climate data into regression-tree software (Cubist®). We...
Ecological processes of low-productivity ecosystems have long been considered to be driven by abiotic controls with biotic interactions playing an insignificant role. However, existing studies present conflicting evidence concerning the roles of these factors, in part due to the short temporal extent of most data sets and inability to test indirect effects of environmental variables modulated by biotic interactions. Using structural equation modeling to analyze 65 years of perennial vegetation change in the Sonoran Desert, we found that precipitation had a stronger positive effect on recruitment beneath existing canopies than in open microsites due to reduced evaporation rates. Variation in perennial canopy cover...
A number of studies have shown that dust delivers essential nutrients that sustain terrestrial productivity over millennia. Dust, however, contains a range of elements ranging from base cations, N and P, to heavy metals. Some of these elements will stimulate primary productivity over the time scale of soil development while others, such as metals, could inhibit biological activity. As dust accumulates, it also influences water flux and availability by altering soil texture. We are examining the ecological impacts of dust accumulation in Canyonlands National Park in Utah. Using magnetic susceptibility as a proxy for far-traveled dust, we have established transects in sandy surficial deposits (50-90% sand) in which...
Fire risk in deserts is increased by high production of annual forbs and invasive grasses that create a continuous fine fuel bed in the interspaces between shrubs. Interspace production is influenced by water, nitrogen (N) availability, and soil texture, and in some areas N availability is increasing due to anthropogenic N deposition. The DayCent model was used to investigate how production of herbaceous annuals changes along gradients of precipitation, N availability, and soil texture, and to develop risk-based critical N loads. DayCent was parameterized for two vegetation types within Joshua Tree National Park, California, USA: creosote bush (CB) and pi´┐Żon?juniper (PJ). The model was successfully calibrated in...
We investigated the relationship between plant nitrogen limitation and water availability in dryland ecosystems. We tested the hypothesis that at lower levels of annual precipitation, aboveground net primary productivity (ANPP) is limited primarily by water whereas at higher levels of precipitation, it is limited primarily by nitrogen. Using a literature survey of fertilization experiments in arid, semi-arid, and subhumid ecosystems, we investigated the response of ANPP to nitrogen addition as a function of variation in precipitation across geographic gradients, as well as across year-to-year variation in precipitation within sites. We used four different indices to assess the degree of N limitation: (1) Absolute...
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Many soils in southeastern Utah are protected from surface disturbance by biological soil crusts that stabilize soils and reduce erosion by wind and water. When these crusts are disturbed by land use, soils become susceptible to erosion. In this study, we compare a never-grazed grassland in Canyonlands National Park with two historically grazed sites with similar geologic, geomorphic, and geochemical characteristics that were grazed from the late 1800s until 1974. We show that, despite almost 30 years without livestock grazing, surface soils in the historically grazed sites have 38-43% less silt, as well as 14-51% less total elemental soil Mg, Na, P, and Mn content relative to soils never exposed to livestock disturbances....
Native grasses of semi-arid rangelands of the southwestern USA are more extensively colonized by dark septate endophytes (DSE) than by traditional mycorrhizal fungi. Roots of dominant grasses (Bouteloua sp.) native to arid southwestern USA rangelands were prepared and stained using stains specific for fungi (trypan blue) and for lipids (sudan IV). This revealed extensive internal colonization of physiologically active roots by atypical fungal structures that appear to function as protoplasts, without a distinguishable wall or with very thin hyaline walls that escape detection by methods staining specifically for fungal chitin. These structures were presumed to be active fungal stages that progressed to form stained...
Distribution of precipitation event sizes and interval lengths between events are important characteristics of arid and semi-arid climates. Understanding their importance will contribute to our ability to understand ecosystem dynamics in these regions. Our objective for this paper was to present a comprehensive analysis of the daily precipitation regimes of arid and semi-arid locations of the United States. We collected 30 years of daily precipitation and temperature data from 289 sites in the intermountain zone and 240 sites in the Great Plains for our analysis. The daily precipitation regimes at all sites were dominated by the smallest event sizes and the shortest intervals between events. Great Plains sites on...
Hotter and drier conditions projected for the southwestern United States can have a large impact on the abundance and composition of long-lived desert plant species. We used long-term vegetation monitoring results from 39 large plots across four protected sites in the Sonoran Desert region to determine how plant species have responded to past climate variability. This cross-site analysis identified the plant species and functional types susceptible to climate change, the magnitude of their responses, and potential climate thresholds. In the relatively mesic mesquite savanna communities, perennial grasses declined with a decrease in annual precipitation, cacti increased, and there was a reversal of the Prosopis velutina...
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Eolian dust constitutes most of the pedogenic material in latePleistocene and Holocene soils of many arid regions. Comparison ofthe compositions and influx rates of modern dust with the eoliancomponent of dated soils at 24 sites in southern Nevada andCalifornia yields information on: (1) the composition and influxrate of dust in late Pleistocene and Holocene soils, (2) paleoclimateand its effects on the genesis of aridic soils, especially withregard to "dust events", (3) the timing and relative contribution ofdust from playa sources versus alluvial sources, and (4) the effectsof accumulation of fines in soil horizons. The A and B horizons ofsoils formed on gravelly alluvial-fan deposits in the study area aresimilar...
Hazard-zone delineation for extreme events is essential for floodplain management near mountain fronts in arid and semiarid regions. On 31 July 2006, unprecedented debris flows occurred in the Santa Catalina Mountains of southeastern Arizona following extreme multiday precipitation (recurrence interval > 1000 years for 4-day precipitation). Most mobilized sediment contributing to debris flows was derived from shallow-seated failures of colluvium on steep slopes. A total of 435 slope failures in the southern Santa Catalina Mountains released 1.34 million Mg of sediment into the channels of 10 drainage basins. Five drainages produced debris flows that moved to the apices of alluvial fans on the southern edge of the...


    map background search result map search result map Soil carbon storage responses to expanding pinyon–juniper populations in southern Utah Multi-Decadal Impacts of Grazing on Soil Physical and Biogeochemical Properties in Southeast Utah Quaternary Soils and Dust Deposition in Southern Nevada and California Using Targeted Training Data to Develop Site Potential for the Upper Colorado River Basin from 2000 - 2018 Multi-Decadal Impacts of Grazing on Soil Physical and Biogeochemical Properties in Southeast Utah Soil carbon storage responses to expanding pinyon–juniper populations in southern Utah Quaternary Soils and Dust Deposition in Southern Nevada and California Using Targeted Training Data to Develop Site Potential for the Upper Colorado River Basin from 2000 - 2018