The Experts below are selected from a list of 44766 Experts worldwide ranked by ideXlab platform
David D Breshears - One of the best experts on this subject based on the ideXlab platform.
-
partitioning evapotranspiration across gradients of Woody Plant cover assessment of a stable isotope technique
Geophysical Research Letters, 2010Co-Authors: Lixin Wang, Juan Camilo Villegas, David D Breshears, Greg A Barrongafford, Kelly K Caylor, Travis E HuxmanAbstract:in the stable isotopic composition of water vapor (d 2 H). Our technique employs a newly‐developed laser‐based isotope analyzer and the Keeling plot approach for surface flux partitioning. The applicability of the technique was verified by comparison to separate, simultaneous lysimeter and sap flow estimates of ET partitioning. The results showed an expected increase in fractional contribution of transpiration to evapotranspiration as Woody cover increased—from T/ET =0 .61 at 25% Woody cover toT/ET = 0.83 at 100% cover. Further development of this technique may enable field characterization of evapotranspiration partitioning across diverse Woody cover gradients, a central issue in addressing dryland ecohydrological responses to land use and climate change. Citation: Wang, L., K. K. Caylor, J. C. Villegas, G. A. Barron‐ Gafford, D. D. Breshears, and T. E. Huxman (2010), Partitioning evapotranspiration across gradients of Woody Plant cover: Assessment of a stable isotope technique, Geophys. Res. Lett., 37, L09401, doi:10.1029/2010GL043228.
-
soil carbon heterogeneity in pinon juniper woodland patches effect of Woody Plant variation on neighboring intercanopies is not detectable
Journal of Arid Environments, 2010Co-Authors: David K Reiley, David D Breshears, Paul H Zedler, Michael H Ebinger, Clifton W MeyerAbstract:Soil carbon often varies significantly among vegetation patch types, but less known is how the size and species of Plants in the tree canopy patches and the cover types of the intercanopy patches affect the carbon storage, and whether vegetation characteristics affect storage in adjacent patches. To assess this, we measured fine-fraction soil carbon in a semiarid woodland in New Mexico USA for canopy patches of two co-dominant Woody species, Pinus edulis and Juniperus monosperma that were paired with intercanopy patch locations covered by herbaceous grass (Bouteloua gracilis) or bare ground. Soil carbon at shallow depths was greater in canopy than intercanopy patches by a factor of 2 or more, whereas within intercanopy patches soil carbon in grass locations exceeded that in bare locations only after accounting for coarse-fraction carbon. Hypothesized differences among canopy patches associated with species or size were not detected (although some size-depth interactions consistent with expectations were detected), nor, importantly, were effects of species or size of Woody Plant on intercanopy soil carbon. The results are notable because where applicable they justify estimates of soil carbon inventories based on readily observable heterogeneity in above-ground Plant cover without considering the size and species of the Woody Plants.
-
a conceptual framework for dryland aeolian sediment transport along the grassland forest continuum effects of Woody Plant canopy cover and disturbance
Geomorphology, 2009Co-Authors: David D Breshears, Jeffrey J Whicker, Jason P Field, Craig D AllenAbstract:ARTICLE I NFO Aeolianprocesses are of particular importance in dryland ecosystems where ground cover is inherently sparse because of limited precipitation. Dryland ecosystems include grassland, shrubland, savanna, woodland, and forest, and can be viewed collectively as a continuum of Woody Plant cover spanning from grasslands with no Woody Plant cover up to forests with nearly complete Woody Plant cover. Along this continuum, the spacing and shape of Woody Plants determine the spatial density of roughness elements, which directly affects aeolian sediment transport. Despite the extensiveness of dryland ecosystems, studies of aeolian sediment transport have generally focused on agricultural fields, deserts, or highly disturbed sites where rates of transport are likely to be greatest. Until recently, few measurements have been made of aeolian sediment transport over multiple wind events and across a variety of types of dryland ecosystems. To evaluate potential trends in aeolian sediment transport as a function of Woody Plant cover, estimates of aeolian sediment transport from recently published studies, in concert with rates from four additional locations (two grassland and two woodlandsites),arereportedhere.Thesynthesisofthesereportsleadstothedevelopmentofanewconceptual framework for aeolian sediment transport in dryland ecosystems along the grassland-forest continuum. The findings suggest that: (1) for relatively undisturbed ecosystems, shrublands have inherently greater aeolian sediment transport because of wake interference flow associated with intermediate levels of density andspacingofWoodyPlants;and(2)fordisturbedecosystems,theupperboundforaeoliansedimenttransport decreases as a function of increasing amounts of Woody Plant cover because of the effects of the height and density of the canopy on airflow patterns and ground cover associated with Woody Plant cover. Consequently, aeolian sediment transport following disturbance spans the largest range of rates in grasslands and associated systems with no Woody Plants (e.g., agricultural fields), an intermediate range in shrublands, and a relatively small range in woodlands and forests. These trends are consistent with previous observations relating large rates of wind erosion to intermediate values for spatial density of roughness elements. The framework for aeolian sediment transport,which isalso relevanttodust fluxes,wind erosion,andrelatedaeolianprocesses,is applicable to a diverse suite of environmental challenges, including land degradation and desertification, dust storms, contaminant transport, and alterations of the hydrological cycle.
-
foliar absorption of intercepted rainfall improves Woody Plant water status most during drought
Ecology, 2008Co-Authors: David D Breshears, Nate G Mcdowell, Kelly L Goddard, Katherine E Dayem, Scott N Martens, Clifton W Meyer, Karen M BrownAbstract:A large proportion of rainfall in dryland ecosystems is intercepted by Plant foliage and is generally assumed to evaporate to the atmosphere or drip onto the soil surface without being absorbed. We demonstrate foliar absorption of intercepted rainfall in a widely distributed, continental dryland, Woody-Plant genus: Juniperus. We observed substantial improvement in Plant water status, exceeding 1.0 MPa water potential for drought-stressed Plants, following precipitation on an experimental plot that excluded soil water infiltration. Experiments that wetted shoots with unlabeled and with isotopically labeled water confirmed that water potential responded substantially to foliar wetting, that these responses were not attributable to re-equilibration with other portions of the xylem, and that magnitude of response increased with water stress. Foliar absorption is not included in most ecological, hydrological, and atmospheric models; has implications for interpreting Plant isotopic signatures; and not only supplements water acquisition associated with increases in soil moisture that follow large or repeated precipitation events, but also enables Plants to bypass soil water uptake and benefit from the majority of precipitation events, which wet foliage but do not increase soil moisture substantially. Foliar absorption of intercepted water could be more important than previously appreciated, especially during drought when water stress is greatest.
-
effects of topography and Woody Plant canopy cover on near ground solar radiation relevant energy inputs for ecohydrology and hydropedology
Geophysical Research Letters, 2007Co-Authors: Greg A Barrongafford, David D BreshearsAbstract:[1] The emerging interdisciplinary approaches of ecohydrology and hydropedology are sensitive to variation in soil-surface energy inputs, which are primarily modified by topography and Woody Plant canopies. Yet a synthesis of the interactive effects of these two modification types is lacking. We systematically estimated near-ground surface solar radiation inputs as modified by key attributes of topography (aspect and slope) and tree cover (degree of openness) using solar radiation modeling based on hemispherical photographs. For south aspects, reductions in annual transmission were dominated by canopy cover rather than topography, even when canopy cover was low, whereas for north aspects, canopy effects dominated the reduction in annual transmission for slopes of up to 10° at low canopy cover and up to 30° at high canopy cover. Our results provide a synthetic perspective of the nonlinear, interactive, and temporally dependent effects of slope, aspect, and amount of canopy cover on near-ground solar radiation.
Eileen M Obrien - One of the best experts on this subject based on the ideXlab platform.
-
global models for predicting Woody Plant richness from climate development and evaluation
Ecology, 2005Co-Authors: Richard Field, Eileen M Obrien, Robert J WhittakerAbstract:There have been few attempts to generate global models of climate-richness relationships, and fewer still that aim to predict richness rather than fitting a model to data. One such model, grounded on theory (biological relativity to water-energy dynamics) is the interim general model (IGM1) of the climatic potential for Woody Plant richness. Here we present a second-generation model (IGM2), and genus and family versions of both models. IGM1 describes horizontal climate-richness relationships based on climate station data and systematic species range maps, with IGM2 additionally incorporating vertical changes in climate due to topographic relief. The IGMs are mathematical transformations of empirical relationships obtained for the southern subcontinent of Africa, whereby the re-described regression models apply to the full range of global variation in all independent climate parameters. We undertake preliminary validation of the new IGMs, first by mapping the distribution and relative spatial variation in forecasted richness (per 25 000 km 2 ) across the continent of Africa, then by evaluating the precision of forecasted values (actual vs. predicted) for an independent study system, the Woody Plants of Kenya. We also compare the IGMs with a recent example of purely statistical regression models of climate-richness relationships; namely, the ''global'' model of A. P. Francis and D. J. Currie for angiosperm family richness. We conclude that the IGMs are globally applicable and can provide a fundamental baseline for systematically estimating differences in (Woody) Plant richness and for exploring the hierarchy of subordinate relationships that should also contribute to differences in realized richness (mostly at more discrete scales of analysis). Further, we found that the model of Francis and Currie is useful for predicting angiosperm richness in Africa, on a conditional basis (somewhere, sometime); we examined the relationship that it describes between climate and richness. Lastly, we found that indices of available soil water used in ''water-budget'' or ''water-balance'' analyses are not proxies for available liquid water as a function of climatological dynamics.
-
water energy dynamics climate and prediction of Woody Plant species richness an interim general model
Journal of Biogeography, 1998Co-Authors: Eileen M ObrienAbstract:Predictable geographic patterns in the distribution of species richness, especially the latitudinal gradient, are intriguing because they suggest that if we knew what the controlling factors were we could predict species richness where empirical data is lacking (e.g. tropics). Based on analyses of the macro-scale distribution of Woody Plant species richness in Southern Africa, one controlling factor appears to be climate-based water-energy dynamics. Using the regression models of climate's relationship to species richness in Southern Africa, I was able to describe an Interim General Model (IGM) and to predict first-order macro-scale geographic variations in Woody Plant species richness for the continent of Africa, as well as elsewhere in the world—exemplified using South America, the United States and China. In all cases, the geographic pattern of variation in species richness is in accord with geographic variations in vegetation (visual comparison with vegetation maps) and net primary productivity. What validation was possible (Africa and U.S.A.) suggests that the IGM provides ‘reasonable’ estimates for actual Woody Plant species richness where species richness is in relative equilibrium with climate. Areas of over- or under-prediction support the contention of earlier workers that edaphic, topographic, historical, and dispersal factors need to be considered in a more complete explanation for spatio-temporal variations in species richness. In addition to providing a means for systematically estimating Woody Plant species richness where present-day empirical data is lacking, the Interim General Model may prove useful for modelling the effects of climate change (past/future) on species richness (and, by association, the vegetation).
-
climatic gradients in Woody Plant species richness towards an explanation based on an analysis of southern africa s Woody flora
Journal of Biogeography, 1993Co-Authors: Eileen M ObrienAbstract:The distribution of southern Africa's Woody flora (N=1372 species) describes a west-to-east pattern of increasing species richness, being lowest in arid to semi- arid areas and highest in mesic to humid areas. Climate accounts for 77.8% (R2; P
Steven R Archer - One of the best experts on this subject based on the ideXlab platform.
-
black tailed prairie dog cynomys ludovicianus reintroduction can limit Woody Plant proliferation in grasslands
Frontiers in Ecology and Evolution, 2020Co-Authors: Sarah L Hale, John L Koprowski, Steven R ArcherAbstract:Tree and shrub proliferation has been widespread in grasslands worldwide, and has altered ecosystem function and wildlife habitat. Several causes have been proposed for the Woody Plant encroachment phenomenon. The widespread eradication of a native keystone herbivore in North American grasslands, the prairie dog (Cynomys spp.), is one potential contributing factor that has received relatively little attention. We hypothesized prairie dogs would have historically suppressed Woody Plants by creating “browse traps” through their systematic clipping of vegetation. We tested this hypothesis by conducting surveys and experimentally manipulating shrub accessibility via exclosures and artificial saplings on and around recently reestablished black-tailed prairie dog (C. ludovicianus) colonies in southeastern Arizona, USA. Shrubs were common on the nascent colonies (mean ± SE = 132 ± 32.7 Plants ha-1), but at substantially reduced densities compared to off colonies (305 ± 94.9 Plants ha-1). Among branches placed on colonies to simulate ‘saplings’, 89% were damaged within three days of ‘Planting’, whereas those placed off colonies were virtually untouched. This was true for both a deciduous, N2-fixing shrub (velvet mesquite, Prosopis velutina) and an evergreen non-N2-fixing shrub (creosote bush, Larrea tridentata). Prairie dogs on newly established colonies did not extirpate Woody Plants over the time-frame of our study, but reduced their abundance and suppressed their growth, which would ostensibly prevent them from achieving dominance. Implications for extending the longevity of widely practiced ‘brush management’ grassland restoration treatments are discussed in the context of perceptions of prairie dogs as rangeland pests. Prairie dogs represent an enigma in keystone conservation. Whereas the reintroduction of large or charismatic keystone species (e.g. sea otters [Enhydra lutris]) are conducted to restore critical ecological function, reintroductions of other keystone species, such as gray wolves [Canis lupus] and in our case, prairie dogs, are highly controversial. Our findings suggest reintroductions of this negatively perceived small herbivore could function as a tool to locally suppress Woody Plant proliferation that is widely regarded as an impediment to livestock production. Accordingly, prairie dogs could promote restoration efforts to re-establish and maintain habitat for grassland endemics while promoting biological diversity and other ecosystem services.
-
Woody Plant encroachment causes and consequences
2017Co-Authors: Steven R Archer, Erik M Andersen, Katharine I Predick, Susanne Schwinning, Robert J Steidl, Steven R WoodsAbstract:Woody vegetation in grasslands and savannas has increased worldwide over the past 100–200 years. This phenomenon of “Woody Plant encroachment” (WPE) has been documented to occur at different times but at comparable rates in rangelands of the Americas, Australia, and southern Africa. The objectives of this chapter are to review (1) the process of WPE and its causes, (2) consequences for ecosystem function and the provision of services, and (3) the effectiveness of management interventions aimed at reducing Woody cover. Explanations for WPE require consideration of multiple interacting drivers and constraints and their variation through time at a given site. Mean annual precipitation sets an upper limit to Woody Plant cover, but local patterns of disturbance (fire, browsing) and soil properties (texture, depth) prevent the realization of this potential. In the absence of these constraints, seasonality, interannual variation, and intensity of precipitation events determine the rate and extent of Woody Plant expansion. Although probably not a triggering factor, rising atmospheric CO2 levels may have favored C3 Woody Plant growth. WPE coincided with the global intensification of livestock grazing that by reducing fine fuels, hence fire frequency and intensity, facilitated WPE. From a conservation perspective, WPE threatens the maintenance of grassland and savanna ecosystems and its endemic biodiversity. Traditional management goals aimed at restoring forage and livestock production after WPE have broadened to support a more diverse portfolio of ecosystem services. Accordingly, we focus on how WPE and management actions aimed at reducing Woody Plant cover influence carbon sequestration, water yield, and biodiversity, and discuss the trade-offs involved when balancing competing management objectives.
-
Woody Plant proliferation in north american drylands a synthesis of impacts on ecosystem carbon balance
Journal of Geophysical Research, 2011Co-Authors: Nichole N Barger, Steven R Archer, John L Campbell, Choying Huang, Jeffery A Morton, Alan K KnappAbstract:[1] Changes in the magnitude and direction of ecosystem carbon (C) balance accompanying Woody Plant encroachment are among the largest contributors to the uncertainty in the North American C budget. In this synthesis we identify the important species contributing to Woody encroachment, summarize our current knowledge of aboveground and belowground C storage change with Woody encroachment, and evaluate the range of human and natural disturbance factors that alter the course of C gains and losses within ecosystems experiencing Woody encroachment. Available data indicate that relative to the historic vegetation, aboveground net primary production (ANPP) decreases with Woody Plant encroachment in arid regions (mean annual precipitation (MAP) < 336 mm), but increases in semiarid and subhumid regions (on the order of 0.7 g C m−2 yr−1 per mm of MAP over 336 mm). Soil organic carbon response to Woody Plant encroachment ranged from losses of 6200 g C m−2 to gains of 2700 g C m−2 with an average accumulation of 385 g C m−2 across all studies and did not appear to be closely coupled to ANPP. Taken together, in the absence of disturbance, Woody encroachment appears to result in a net ecosystem C gain across most species and ecoregions. However, disturbance associated with wildfire, land management practices, and drought may quickly and significantly offset these gains and should be explicitly factored into regional-scale C balance estimates. Our findings may be used to better constrain future estimates of Woody Plant encroachment influences on the North American C budget.
-
changes in aboveground primary production and carbon and nitrogen pools accompanying Woody Plant encroachment in a temperate savanna
Global Change Biology, 2006Co-Authors: Flint R Hughes, Steven R Archer, Gregory P Asner, Carol A Wessman, C R Mcmurtry, Jim A Nelson, James R AnsleyAbstract:When Woody Plant abundance increases in grasslands and savannas, a phenomenon widely observed worldwide, there is considerable uncertainty as to whether aboveground net primary productivity (ANPP) and ecosystem carbon (C) and nitrogen (N) pools increase, decrease, or remain the same. We estimated ANPP and C and N pools in aboveground vegetation and surface soils on shallow clay and clay loam soils undergoing encroachment by Prosopis glandulosa in the Southern Great Plains of the United States. Aboveground Prosopis C and N mass increased linearly, and ANPP increased logarithmically, with stand age on clay loam soils; on shallow clays, Prosopis C and N mass and ANPP all increased linearly with stand age. We found no evidence of an asymptote in trajectories of C and N accumulation or ANPP on either soil type even following 68 years of stand development. Production and accumulation rates were lower on shallow clay sites relative to clay loam sites, suggesting strong edaphic control of C and N accumulation associated with Woody Plant encroachment. Response of herbaceous C mass to Prosopis stand development also differed between soil types. Herbaceous C declined with increasing aboveground Prosopis C on clay loams, but increased with increasing Prosopis C on shallow clays. Total ANPP (Prosopis+herbaceous) of sites with the highest Prosopis basal area were 1.2 × and 4.0 × greater than those with the lowest Prosopis basal area on clay loam and shallow clay soils, respectively. Prosopis ANPP more than offset declines in herbaceous ANPP on clay loams and added to increased herbaceous ANPP on shallow clays. Although aboveground C and N pools increased substantially with Prosopis stand development, we found no corresponding change in surface soil C and N pools (0–10 cm). Overall, our findings indicate that Prosopis stand development significantly increases ecosystem C and N storage/cycling, and the magnitude of these impacts varied with stand age, soil type and functional Plant traits
-
biogeochemical changes accompanying Woody Plant encroachment in a subtropical savanna
Ecology, 2001Co-Authors: K A Hibbard, Steven R Archer, David S Schimel, D W ValentineAbstract:Ecosystem properties of surficial (0-10 cm) soils in remnant herbaceous patches were compared to those of contrasting Woody Plant patch types (upland discrete cluster, upland grove, and lowland woodland) where shifting land cover is known to have occurred over the past 50-77 yr. The purpose of this study was to evaluate and quantify the biogeochemical consequences and subsequent developmental rates of Woody Plant for- mation on sites formerly dominated by grasses. Clay and water content of woodland soil patches was higher than that of soils associated with upland discrete cluster and grove patches. Even so, lowland Woody patches were generally comparable to upland grove and discrete shrub cluster patches with respect to soil organic carbon (SOC), soil N, the ratio of annual N mineralization:total N, annual litterfall, and root biomass. The fact that finer soil texture, enhanced soil moisture, and the more advanced age of lowland Woody patches did not translate into greater accumulations of SOC and N relative to upland grove and discrete cluster patches suggests that C and N losses might be higher in recently developed lowland woodland communities. Fluctuations in monthly root biomass standing crop (0-10 cm) far exceeded annual foliar litterfall in upland and lowland Woody patch types, suggesting that belowground inputs of organic matter may drive changes in soil physical and chemical properties that occur subsequent to Woody Plant establishment. The estimated annual mean rates of soil C accretion in the ''islands of fertility'' that developed subsequent to tree/shrub encroachment were variable and ranged from 8 to 23 g/m 2 (in groves and discrete clusters, respectively); N accretion ranged from 0.9 to 2.0 g/ m 2 (in groves and discrete clusters, respectively), even though mean annual N mineralization rates were three- to fivefold greater than those measured in remnant herbaceous patches. Woody Plant proliferation in grasslands and savannas in recent history has been widely reported around the world. The causes for this shift in vegetation are controversial and center around changes in livestock grazing, fire, climate, and atmospheric CO2. Our data, which are conservative in that they examine only the upper 10 cm of the soil profile, indicate that the rate and extent of soil C and N accumulation associated with this phe- nomenon can be rapid, substantial, and accompanied by increased N turnover. This geo- graphically extensive vegetation change thus has important implications for understanding how the global carbon and nitrogen cycles may have been altered since Anglo-European settlement of arid and semiarid regions.
Alan K Knapp - One of the best experts on this subject based on the ideXlab platform.
-
Woody Plant proliferation in north american drylands a synthesis of impacts on ecosystem carbon balance
Journal of Geophysical Research, 2011Co-Authors: Nichole N Barger, Steven R Archer, John L Campbell, Choying Huang, Jeffery A Morton, Alan K KnappAbstract:[1] Changes in the magnitude and direction of ecosystem carbon (C) balance accompanying Woody Plant encroachment are among the largest contributors to the uncertainty in the North American C budget. In this synthesis we identify the important species contributing to Woody encroachment, summarize our current knowledge of aboveground and belowground C storage change with Woody encroachment, and evaluate the range of human and natural disturbance factors that alter the course of C gains and losses within ecosystems experiencing Woody encroachment. Available data indicate that relative to the historic vegetation, aboveground net primary production (ANPP) decreases with Woody Plant encroachment in arid regions (mean annual precipitation (MAP) < 336 mm), but increases in semiarid and subhumid regions (on the order of 0.7 g C m−2 yr−1 per mm of MAP over 336 mm). Soil organic carbon response to Woody Plant encroachment ranged from losses of 6200 g C m−2 to gains of 2700 g C m−2 with an average accumulation of 385 g C m−2 across all studies and did not appear to be closely coupled to ANPP. Taken together, in the absence of disturbance, Woody encroachment appears to result in a net ecosystem C gain across most species and ecoregions. However, disturbance associated with wildfire, land management practices, and drought may quickly and significantly offset these gains and should be explicitly factored into regional-scale C balance estimates. Our findings may be used to better constrain future estimates of Woody Plant encroachment influences on the North American C budget.
Zacheus Mahlangu - One of the best experts on this subject based on the ideXlab platform.
-
termite mounds as islands Woody Plant assemblages relative to termitarium size and soil properties
Journal of Vegetation Science, 2013Co-Authors: Grant S Joseph, Colleen L Seymour, Graeme S Cumming, David H M Cumming, Zacheus MahlanguAbstract:Questions: We investigated whether soils of small mounds resembled large mound or matrix soils, whether changes in Plant composition reflected changes in soils, and the sequence in which Plants colonize and disappear from mounds of increasing size. Location: Miombo woodland in northwest Zimbabwe. Methods: Macrotermitinae termitaria vary in size and soil nutrient concentrations, harbouring distinct Woody Plant assemblages, making them foci for Plant and animal diversity, and also influencing primary, secondary and tertiary productivity. In spite of the importance of termitaria to heterogeneity and diversity, no studies have investigated changes in Plant species assemblages as mound surface area increases to the point where mound vegetation is distinct from that of the matrix. We compared Woody Plant assemblages on 43 matrix plots with 95 Macrotermes termitaria across a range of surface areas, using ANOSIM, cluster analysis and MDS ordination. We compared soil nutrients, pH and clay, from ten large and ten small termitaria, and ten matrix sites. We also assessed how relative representation of large mound or matrix indicator species changed with mound area. Results: Change was apparent even at mound sizes of >10 m2, where both soils and Plant assemblages on mounds were significantly different to those of the matrix. Plant assemblages fell into two main groups at 20% similarity; the first comprised of matrix plots, mounds 30 m2. At 40% similarity, four groups emerged: matrix, mounds 30 m2. Woody Plant composition changed gradually as mound area increased. On termitaria 30 m2 in surface area, only mound indicator species were found. Conclusions: Through termite activities in concentrating nutrients and clay, termitaria provide habitat for species usually excluded from the matrix. The process of mound building and the nature of the Plants that establish on them seem to establish a positive feedback for establishment of other non-woodland matrix species.
