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In many places along the lower Colorado River, saltcedar (Tamarix spp) has replaced the native shrubs and trees, including arrowweed, mesquite, cottonwood and willows. Some have advocated that by removing saltcedar, we could save water and create environments more favourable to these native species. To test these assumptions we compared sap flux measurements of water used by native species in contrast to saltcedar, and compared soil salinity, ground water depth and soil moisture across a gradient of 200?1500 m from the river's edge on a floodplain terrace at Cibola National Wildlife Refuge (CNWR). We found that the fraction of land covered (fc) with vegetation in 2005?2007 was similar to that occupied by native...
Groundwater is a key driver of riparian condition on dryland rivers but is in high demand for municipal, industrial, and agricultural uses. Approaches are needed to guide decisions that balance human water needs while conserving riparian ecosystems. We developed a space-for-time substitution model that links groundwater change scenarios implemented within a Decision Support System (DSS) with proportions of floodplain vegetation types and abundances of breeding and migratory birds along the upper San Pedro River, AZ, USA. We investigated nine scenarios ranging from groundwater depletion to recharge. In groundwater decline scenarios, relative proportions of tall-canopied obligate phreatophytes (Populus/Salix, cottonwood/willow)...
Increases in the abundance or density of woody plants in historically semiarid and arid grassland ecosystems have important ecological, hydrological, and socioeconomic implications. Using a simplified water-balance model, we propose a framework for conceptualizing how woody plant encroachment is likely to affect components of the water cycle within these ecosystems. We focus in particular on streamflow and the partitioning of evapotranspiration into evaporation and transpiration. On the basis of this framework, we suggest that streamflow and evaporation processes are affected by woody plant encroachment in different ways, depending on the degree and seasonality of aridity and the availability of subsurface water....
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We investigated the effects of winter and summer drought on a shrub/grass community of the Colorado Plateau in western North America, a winter-cold, summer-hot desert that receives both winter and summer precipitation. Summer, winter and yearlong drought treatments were imposed for 2 consecutive years using rainout shelters. We chose three perennial species for this study, representing different rooting patterns and responsiveness to precipitation pulses: Oryzopsis hymenoides, a perennial bunch grass with shallow roots; Gutierrezia sarothrae, a subshrub with dimorphic roots; and Ceratoides lanata, a predominantly deep-rooted woody shrub. Growth for all three species was far more sensitive to winter than to summer...
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The release of the saltcedar beetle (Diorhabda carinulata) has resulted in the periodic defoliation of tamarisk (Tamarix spp.) along more than 1000 river km in the upper Colorado River Basin and is expected to spread along many other river reaches throughout the upper basin, and possibly into the lower Colorado River Basin. Identifying the impacts of these release programs on tamarisk water use and subsequent water cycling in arid riparian systems are largely unknown, due in part to the difficulty of measuring water fluxes in these systems. We used lab-calibrated, modified heat-dissipation sap flux sensors to monitor tamarisk water use (n = 20 trees) before, during and after defoliation by the saltcedar leaf beetle...
The loss of snow cover and the initiation of streamflow are key triggers for both terrestrial and aquatic biota. Landscape-scale snowmelt and streamflow dynamics are difficult to estimate, however, because they integrate large spatial extents and can vary rapidly in time. Remotely sensed observations are often temporally discontinuous and point observations lack sufficient spatial density (e.g. point measures from data-logging piezometers). In this study, we employ inexpensive temperature/light sensors to monitor the distribution of snowmelt and headwater stream discharge as a proxy for hydrological state of the landscape with high spatial and temporal resolution. This study was conducted at the Redondo Peak, a...
The impact of climate variability on the water cycle in desert ecosystems is controlled by biospheric feedback at interannual to millennial timescales. This paper describes a unique field dataset from weighing lysimeters beneath nonvegetated and vegetated systems that unequivocally demonstrates the role of vegetation dynamics in controlling water cycle response to interannual climate variability related to El Nin˜ o southern oscillation in the Mojave Desert. Extreme El Nin˜ o winter precipitation (2.3–2.5 times normal) typical of the U.S. Southwest would be expected to increase groundwater recharge, which is critical for water resources in semiarid and arid regions. However, lysimeter data indicate that rapid...
As the earth becomes a quilt of managed patches, ecohydrologists need to move from describing to predicting the consequences of human activities, using knowledge to improve human well-being. We highlight three current opportunities in ecohydrology. The first is the need for stronger research in arid and semi-arid ecosystems, where water is scarce and a tight coupling exists between hydrology and ecology. The second is to build better predictive frameworks for understanding the consequences of vegetation change. The new framework we propose here combines landscape connectivity, through recharge and discharge dynamics, with global climate. In systems where annual precipitation and evapotranspiration are similar, the...
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We investigated the effects of winter and summer drought on plants of the Colorado Plateau in western North America. This winter-cold, summer-hot desert region receives both winter and summer precipitation. Droughts were imposed for two consecutive years using rainout shelters. Here, we examine drought effects on the hydrologic interactions between plants and soil. We chose three perennial species for this study, representing different rooting patterns and responsiveness to precipitation pulses: Oryzopsis hymenoides, a perennial bunch grass with shallow roots; Gutierrezia sarothrae, a subshrub with dimorphic roots; and Ceratoides lanata, a predominantly deep-rooted woody shrub. Drought effects on plant water status...
The North American Monsoon (NAM) is characterized by seasonal precipitation pulses which produce a considerable impact on land surface conditions. Changes in land surface processes have received less attention among the factors that concurrently influence the NAM. In this study, we utilize remotely-sensed and ground-based measurements to infer land surface changes that can influence the NAM through a vegetation?rainfall feedback mechanism. Our study is focused over the period 2004?2007 in northwest Mexico. Results from the analysis of remote sensing data indicate that the ground observations at an eddy covariance tower are representative of the land surface dynamics in subtropical scrublands. The extensive subtropical...
The North American Monsoon (NAM) is characterized by seasonal precipitation pulses which produce a considerable impact on land surface conditions. Changes in land surface processes have received less attention among the factors that concurrently influence the NAM. In this study, we utilize remotely-sensed and ground-based measurements to infer land surface changes that can influence the NAM through a vegetation–rainfall feedback mechanism. Our study is focused over the period 2004–2007 in northwest Mexico. Results from the analysis of remote sensing data indicate that the ground observations at an eddy covariance tower are representative of the land surface dynamics in subtropical scrublands. The extensive subtropical...
The amount and spatial distribution of deep drainage (downward movement of water across the bottom of the root zone) and groundwater recharge affect the quantity and quality of increasingly limited groundwater in arid and semiarid regions. We synthesize research from the fields of ecology and hydrology to address the issue of deep drainage in arid and semiarid regions. We start with a recently developed hydrological model that accurately simulates soil water potential and geochemical profiles measured in thick (.50 m), unconsolidated vadose zones. Model results indicate that, since the climate change that marked the onset of the Holocene period 10 000–15 000 years ago, there has been no deep drainage in vegetated...
Water controls the dynamics of terrestrial ecosystems directly, as a resource for the biota, and indirectly, as a driver for abiotic processes on the Earth's surface, in the atmosphere, and belowground. The biota, in turn, modulate several hydrological processes and the rate of the water cycle. Here we review recent advances related to fundamental processes and feedbacks emerging from the interactions among hydrologic processes and ecosystems, with a particular focus on soil moisture dynamics and river flow. Most terrestrial vegetation interacts with hydrological processes through the soil-water balance, which is affected by soil properties, random climate drivers, and feedbacks with the biota. River flow enhances...
Vegetation, particularly its dynamics, is the often-ignored linchpin of the land-surface hydrology. This work emphasizes the coupled nature of vegetation-water-energy dynamics by considering linkages at timescales that vary from hourly to interannual. A series of two papers is presented. A dynamic ecohydrological model [tRIBS + VEGGIE] is described in this paper. It reproduces essential water and energy processes over the complex topography of a river basin and links them to the basic plant life regulatory processes. The framework focuses on ecohydrology of semiarid environments exhibiting abundant input of solar energy but limiting soil water that correspondingly affects vegetation structure and organization. The...
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Drainage channels are among the most conspicuous surficial features of deserts, but little quantitative analysis of their influence on plant distributions is available. We analysed the effects of desert stream channels (?washes?) on Larrea tridentata and Ambrosia dumosa density and cover on an alluvial piedmont in the Mojave Desert, based on a spatial analysis of transect data encompassing a total length of 2775 m surveyed in 5 cm increments. Significant deviations from average transect properties were identified by bootstrapping. Predictably, shrub cover and density were much reduced inside washes, and elevated above average levels adjacent to washes. Average Larrea and Ambrosia cover and density peaked 1�2?1�6...
The influence of woody vegetation and biological soil crusts on infiltration capacity is one of the several uncertainties associated with the ecohydrologic effects of woody plant encroachment into arid and semi-arid land systems. The objective of this study was to quantify the effects of Utah juniper (Juniperus osteosperma) and pinyon pine (Pinus edulis) on subcanopy and intercanopy ecohydrologic properties. We measured soil sorptivity, unsaturated hydraulic conductivity [K(h)], soil water content (SWC), and water repellency along radial line transects from under Juniperus and Pinus trees into the centre of the intercanopy space between trees. In the subcanopy, litter mounds, hydrophobic soils, and roots all appear...
In this study, we investigate changes in ecosystem structure that occur over a gradient of land-degradation in the southwestern USA, where shrubs are encroaching into native grassland. We evaluate a conceptual model which posits that the development of biotic and abiotic structural connectivity is due to ecogeomorphic feedbacks. Three hypotheses are evaluated: 1. Over the shrub-encroachment gradient, the difference in soil properties under each surface-cover type will change non-linearly, becoming increasingly different; 2. There will be a reduction in vegetation cover and an increase in vegetation-patch size that is concurrent with an increase in the spatial heterogeneity of soil properties over the shrub-encroachment...
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Populus fremontii (Fremont cottonwood) was once a dominant species in desert riparian forests but has been increasingly replaced by the exotic invasive Tamarix ramosissima (saltcedar). Interspecific competition, reduced flooding frequency, and increased salinity have been implicated in the widespread decline of P. fremontii. To elucidate some of the multiple and interacting mechanisms of this decline, we examined ecological processes in a control stand of P. fremontii along the Colorado River in Utah, USA, as well as a disturbed stand characterized by high groundwater salinity and invasion of T. ramosissima. Sap flux data showed that P. fremontii at the saline site experienced large reductions in afternoon canopy...
In semiarid landscapes, the linkage between runoff and vegetation is a particularly close one. In this paper we report on the results of a long-term and multiple-scale study of interactions between runoff, erosion, and vegetation in a piñon–juniper woodland in New Mexico. We use our results to address three knowledge gaps: (1) the temporal scaling relationships between precipitation and runoff; (2) the effects of spatial scale on runoff and erosion, as influenced by vegetation; and (3) the influence of disturbance on these relationships. On the basis of our results, we tested three assumptions that represent current thinking in these areas (as evidenced, for example, by explicit or implicit assumptions embedded...
Soil evaporation, a critical ecohydrological process in drylands, can exhibit substantial spatio-temporal variation. Spatially, ecohydrological controls of soil evaporation may generally depend on a hierarchical structure spanning from the presence or absence of litter, through canopy patches of woody plants and intercanopy patches separating them, up to the overall vegetation mosaic characterized by density of woody plant cover in the landscape, although assessment of these factors in concert is generally lacking. Temporally, ecohydrological controls can be further complicated by not only seasonal climate, but also phenology, particularly in seasonally deciduous drylands. We experimentally assessed the interactive...


map background search result map search result map The influence of stream channels on distributions of Larrea tridentata and Ambrosia dumosa in the Mojave Desert, CA, USA: patterns, mechanisms and effects of stream redistribution Summer and winter drought in a cold desert ecosystem (Colorado Plateau) part I: effects on soil water and plant water uptake Ecohydrology in a Colorado River riparian forest: implications for the decline of Populus fremontii Sap flux-scaled transpiration by tamarisk (Tamarix spp.) before, during and after episodic defoliation by the saltcedar leaf beetle (Diorhabda carinulata) Summer and winter drought in a cold desert ecosystem (Colorado Plateau) part II: effects on plant carbon assimilation and growth Ecohydrology in a Colorado River riparian forest: implications for the decline of Populus fremontii Summer and winter drought in a cold desert ecosystem (Colorado Plateau) part I: effects on soil water and plant water uptake Summer and winter drought in a cold desert ecosystem (Colorado Plateau) part II: effects on plant carbon assimilation and growth Sap flux-scaled transpiration by tamarisk (Tamarix spp.) before, during and after episodic defoliation by the saltcedar leaf beetle (Diorhabda carinulata) The influence of stream channels on distributions of Larrea tridentata and Ambrosia dumosa in the Mojave Desert, CA, USA: patterns, mechanisms and effects of stream redistribution