@article {50505, title = {Multi-scale temporal variation in water availability: implications for vegetation dynamics in arid and semi-arid ecosystems}, journal = {Journal of Arid Environments}, volume = {65}, number = {2}, year = {2006}, month = {04/2006}, pages = {219-234}, chapter = {219}, abstract = {

The objective of this paper is to illustrate the importance of variation in water availability to temporal variation in vegetation dynamics. We hypothesize that fine-scale pulses of precipitation interact with longer-scale variation in climate and weather to generate temporal variation in plant community composition. Arid and semiarid regions exhibit a high degree of temporal variability in water availability as a result of variation in climate and weather at multiple scales and vegetation-soil water feedbacks. The scales of variation include: shifting climate regimes over centuries and decades, inter-annual variation in weather patterns, seasonal differences in winter and summer precipitation, and within-season variability in precipitation frequency and magnitude. In arid and semiarid regions pulses of rainfall are separated by intervening dry periods of variable lengths. This results in fluctuating availability of water that limits plant production and controls other ecological processes, such as rates of nutrient cycling. In many semiarid and arid systems, temporal variation in water availability may create positive feedbacks that facilitate encroachment of C3 woody plant species into areas formerly dominated by C4 grasses. Our ability to predict these complex shifts in vegetation composition may be improved by including temporal variation in climate, weather, and ecosystem processes.

}, keywords = {variation, vegetation dynamics, water availabililty}, doi = {10.1016/j.jaridenv.2005.06.023}, url = {/files/bibliography/06-004.pdf}, author = {K. A. Snyder and S. Tartowski} } @inbook {50507, title = {Patterns and Controls of Soil Water in the Jornada Basin}, booktitle = {Structure and Function of a Chihuahuan Desert Ecosystem: The Jornada Basin Long-Term Ecological Research Site}, year = {2006}, month = {July 1, 2006}, pages = {107-132}, publisher = {Oxford University Press}, organization = {Oxford University Press}, chapter = {5}, address = {New York, NY}, abstract = {This chapter focuses on the controls and patterns of soil moisture in the Jornada Basin. First, we describe general properties that commonly contribute to soil water heterogeneity; secondly, we offer a brief overview of soil water research in the Jornada Basin; and lastly, we describe specific patterns of soil water content and availability observed in the Jornada Basin. Our goal is to describe general patterns of soil water that are likely to occur across the Chihuahuan Desert region.}, keywords = {Chihuahuan Desert, Jornada Basin, LTER, synthesis book}, url = {/files/bibliography/06-056.pdf}, author = {K. A. Snyder and K. A. Mitchell and Jeffrey E. Herrick}, editor = {Kris Havstad and L. Huenneke and W. H. Schlesinger} } @proceedings {50506, title = {Shrub-grass transitions and multiscale temporal variation in water availability}, year = {2006}, note = {ProCite field[28]: November 1, 2004ProCite field[29]: May 13, 2006}, month = {2006}, abstract = {Desert regions in the southwestern United States and northern Mexico exhibit a high degree of temporal variability in water availability with important consequences for transitions between grasslands and shrublands. This temporal variability is a result of: shifting climate regimes over centuries and decades, interannual variation in weather patterns, seasonal differences in the nature of winter and summer precipitation, within-season variability in precipitation frequency and magnitude, and feedbacks between vegetation and soil water.}, keywords = {multiscale, shrub-grass transitions, temporal variation, water availability}, author = {K. A. Snyder and S. Tartowski} } @conference {50603, title = {Multi-scale responses of carbon and water fluxes to magnitude and frequency of precipitation pulses in the Northern Chihuahuan desert}, booktitle = {Ecological Society of America, 90th Annual Meeting}, year = {2005}, month = {August 7-12, 200}, pages = {599}, address = {Montreal, Quebec, CANADA}, abstract = {Changes in the frequency and magnitude of precipitation may be as important as changes in total seasonal precipitation amounts, especially in water-limited environments. We manipulated both the frequency and magnitude of precipitation over two growing seasons in the northern part of the Chihuahuan desert at the Jornada Experimental Range. A 46\% increase relative to mean summer rainfall was applied at two different frequencies and magnitudes over twelve weeks during the summer growing season to plots containing both mesquite (Prosopis glandulosa) and black grama grass (Bouteloua eriopoda). Plots received either: ambient precipitation (controls), ambient plus frequent small (5-6 mm) rainfall events applied weekly, and ambient plus infrequent large (20-24 mm) events applied monthly. Fluxes of carbon and water were measured at different scales using static chambers and an IRGA. Isotopic partitioning of evapotranspiration (ET) fluxes indicated that the proportion of ET accounted for by mesquite transpiration increased in large infrequent rainfall treatments relative to small frequent rainfall treatments; however, mesquite was able to use water from both large and small magnitude rainfall treatments. Evapotranspiration flux in large chambers containing mesquite, grass and soil was higher in large infrequent watering treatments than in the other two treatments in August and September, but not July. However, increased uptake of CO2 was observed only in large infrequent watering treatments in August. In contrast to this, smaller chambers containing only grass and soil showed increased uptake of CO2 in both small frequent rainfall and large infrequent rainfall treatments. These data suggest that changes in the depth and duration of soil wetting in conjunction with the degree of mesquite encroachment may have important implications for carbon and water cycling.}, keywords = {carbon, Chihuahuan, fluxes, frequency, multi-scale, northern, precipitation, water}, author = {K. A. Snyder and Connie J. Maxwell and Cable, J.M.} } @conference {50718, title = {Arid ecosystem responses to variations in the frequency and magnitude of growing season precipitation}, booktitle = {89th Annual Meeting, Ecological Society of America}, year = {2004}, month = {August 1, 2004}, pages = {Abstract p. 476}, address = {Portland, OR}, abstract = {Global climate change models predict regional changes in average seasonal precipitation, but few models or experiments address how changes in the delivery of precipitation within the growing season will affect ecosystem responses. Arid systems differ from mesic systems in that water availability is frequently low enough to severely restrict biological activity. Consequently, the frequency and magnitude of precipitation events, which dictate the length and severity of intervening dry periods, may have substantial effects on ecosystem function regardless of the total amount of seasonal rainfall. We examined how a 60 mm increase in summer precipitation (46\% increase over mean summer rainfall) applied with two different frequencies and magnitudes during the growing season affected carbon, water and nitrogen dynamics of a Chihuahuan Desert scrub ecosystem. Plots containing mesquite (Prosopis glandulosa) and black grama grass (Bouteloua eriopoda) were assigned to either: ambient precipitation (controls), ambient plus frequent small (5-6 mm) rainfall events applied weekly, and ambient plus infrequent large (20-24 mm) events applied monthly. Mesquite used rainfall from 5 mm and 24 mm storms, as evidenced by increased stem elongation, improved water status and uptake of deuterium-labeled rainfall, but mesquite response was greater under large infrequent rainfalls. Daytime measurements of net ecosystem carbon exchange on subplots containing soil, plant roots and biological soil crusts indicated biological soil crust photosynthesis was improved by frequent small rainfalls. Keeling plot analyses of d13C of respired CO2 during dark hours indicated that the contribution from different sources (roots, microbes and soil crusts) varied with rainfall treatment. Both infrequent large rainfalls and frequent small rainfalls significantly increased nitrogen availability, especially in the surface soil. We found that changes in the depth and duration of soil wetting, resulting from different precipitation patterns, have important implications for ecosystem carbon and nitrogen dynamics because of the differential sensitivity of ecosystem components.}, keywords = {arid systems, climate change, ecosystem responses, growing season precipitation}, author = {K. A. Snyder and Cable, J.M. and T. Huxman and S. Tartowski} } @conference {50719, title = {The effects of altered precipitation patterns on ecosystem responses in a desert rangeland}, booktitle = {4th International Conference on Applications of Stable Isotope Techniques to Ecological Studies}, year = {2004}, month = {April 19, 2004}, pages = {79}, abstract = {Many desert regions are characterized by seasonally dominated rainfall regimes. Global climate change models predict regional changes in average seasonal precipitation, but few models address how changes in the delivery of precipitation within a season will affect ecosystem responses. In the southwestern US, summer rainfall is highly unpredictable and punctuated by intervening dry periods of variable lengths. These patterns of precipitation pulses and subsequent dry periods may differentially affect water use and net ecosystem carbon exchange, depending on the ability of various ecosystem components (e.g., plants, soil microbes, and biological soil crusts) to respond to increases in water availability and maintain function during soil dry-down. The objective of this study was to examine how a 60-mm increase in summer precipitation (46\% increase relative to mean summer rainfall) applied in two contrasting treatments affected carbon exchange and water use. Eighteen 4x6-m plots containing both a mesquite shrub (Prosopis glandulosa) and scattered black grama grass (Bouteloua eriopoda) were assigned to one of three watering treatments (n=6): ambient precipitation (controls), ambient plus frequent, small (5-6 mm) rainfall events applied weekly, and ambient plus infrequent, large (20-24 mm) events applied monthly. Treatments were applied for 12 weeks during summer using deuterium-enriched water. Stable isotope ratios of hydrogen (dD) in plant xylem water and carbon (d13C) in respired CO2 were used to partition water source use and respiratory CO2 flux. The dD of mesquite xylem sap indicated that mesquite was able to use water from both small and large precipitation events, while control plants accessed water from deep within the soil profile. Changes in soil moisture, mesquite predawn water potentials, and mesquite stem elongation were highest in monthly watered plots, intermediate in weekly plots, and lowest in control plots. Although large events appeared to favor carbon gain of C3 shrubs, biological soil crust communities may have taken advantage of smaller more frequent events. Daytime measurements of net ecosystem carbon exchange on subplots containing soil, plant roots, and biological soil crusts indicated control and monthly plots were losing CO2 to the atmosphere, while weekly plots were acquiring CO2. After application of 2 mm of water to all plots, net uptake of CO2 was observed in all plots, but the magnitude of crust photosynthetic response was greatest in weekly watered plots. Keeling plot analyses of the d13C of respired CO2 during dark hours indicated that the contribution from different sources (roots, microbes, and soil crusts) varied with watering treatment. Small frequent rainfall appeared to promote the proportional contribution of microbial respiration or C4 roots to total respiratory flux, while infrequent large rainfall resulted in a greater proportion of CO2 originating from biological soil crusts or C3 root activity. Control plots that had received little ambient rainfall had more negative d13C values, indicating another potentially deeper C3 dominated source of respired CO2.}, keywords = {altered, carbon exchange, desert rangeland, ecosystem responses, precipitation patterns, precipitation pulses, water use}, author = {K. A. Snyder and Cable, J.M. and T. Huxman and Connie J. Maxwell} } @article {50722, title = {Extensive summer water pulses do not necessarily lead to canopy growth of Great Basin and northern Mojave Desert shrubs}, journal = {Oecologia}, volume = {141}, number = {2}, year = {2004}, month = {October 1, 2004}, pages = {325-334}, abstract = {Plant species and functionally related species groups from arid and semiarid habitats vary in their capacity to take up summer precipitation, acquire nitrogen quickly after summer precipitation, and subsequently respond with ecophysiological changes (e.g., water and nitrogen relations, gas exchange). For species that respond ecophysiologically, the use of summer precipitation is generally assumed to affect long-term plant growth and, thus, alter competitive interactions that structure plant communities and determine potential responses to climate change. We assessed ecophysiological and growth responses to large, short-term irrigation pulses over one to three growing seasons for several widespread Great Basin and northern Mojave Desert shrub species: Chrysothamnus nauseosus, Sarcobatus vermiculatus, Atriplex confertifolia, and A. parryi. We compared control and watered plants in nine case studies that encompassed adults of all four species, juveniles for three of the species, and two sites for two of the species. In every comparison, plants used summer water pulses to improve plant water status or increase rates of functioning as indicated by other ecophysiological characters. Species and life history stage responses of ecophysiological parameters were consistent with several previous short-term studies. However, use of summer water pulses did not affect canopy growth in eight out of nine comparisons despite the range of species, growth stages, and site conditions. Summer water pulses affected canopy growth only for C. nauseosus adults. The general lack of growth effects for these species might be due to close proximity of groundwater at these sites, colimitation by nutrients, or inability to respond due to phenological canalization. An understanding of the connections between short-term ecophysiological responses and growth for different habitats and species is critical for determining the significance of summer precipitation for desert community dynamics.}, keywords = {canopy growth, desert community, Great Basin, irrigation, Mojave Desert, summer water pulses}, url = {http://www.springerlink.com/media/eflppvlyxlhh19vhhl5x/contributions/j/u/l/f/julfeguq25yfl0cd.pdf}, author = {K. A. Snyder and L.A. Donovan and J. James and Tiller, R.L. and J. H. Richards} } @conference {50717, title = {Temporal variation in water availability}, booktitle = {Sixth Symposium on the Natural Resources of the Chihuahuan Desert Region}, year = {2004}, month = {October 15, 2004}, pages = {42}, address = {Alpine, TX}, abstract = {Desert regions in the southwestern United States and northern Mexico exhibit a high degree of temporal variability in water availability. This temporal variability is a result of shifting climate regimes over centuries and decades, interannual variation in weather patterns, seasonal differences n the nature of winter and summer precipitation, within-season variability of precipitation frequency and magnitude, soil textural properties, differences in vegetation structure, and soil water. Pulses of rainfall during the summer growing season are highly unpredictable and punctuated by intervening dry periods of variable lengths. These dry periods are frequently severe enough to limit biotic activity and modify abiotic processes. Therefore, within a landscape type the relationship between aboveground net primary productivity and annual rainfall lacks predictive power due to the pulse-driven nature of these systems. In the Chihuahuan Desert encroachment of C3 shrub species into areas formerly dominated by C4 grasses appears to interact with water variability and create positive feedbacks that facilitate further shrub encroachment and loss of grass cover. In order to understand these systems, we need to develop knowledge of how temporal patterns in water availability affect ecosystem water use, nutrient availability, productivity, and species composition. Data from rainfall manipulation experiments, Bowen-Ration towers, and long-term soil moisture illustrate that mesquite shrubs respond to both small and large rainfall events, respiratory losses of carbon dioxide often offset photosynthetic uptake of carbon dioxide, and the capacity for carbon sequestration during summer months is largely driven by rainfall patterns.}, keywords = {Chihuahuan Desert, pulse-driven nature, shifting climate regimes, temporal variation, water availability, weather patterns}, author = {K. A. Snyder and S. Tartowski and V. P. Gutschick} } @article {50829, title = {Defoliation alters water uptake by deep and shallow roots of Prosopis velutina (velvet mesquite)}, journal = {Functional Ecology}, volume = {17}, number = {3}, year = {2003}, month = {June 1, 2003}, pages = {363-374}, abstract = {1. Prosopis velutina Woot. (Velvet Mesquite) at a site with limited groundwater availability derived a greater percentage of water from shallow soil at the onset of the summer rainy season than did trees at a site with greater availability of groundwater. Predawn leaf water potentials were not a strong indicator of shallow water use for this species with roots in multiple soil layers.2. We experimentally defoliated P. velutina plants to determine if reduced-canopy photosynthesis would alter vertical patterns of root activity. After natural rain events, hydrogen isotope ratios of xylem sap indicated that defoliated P. velutina took up a greater percentage of its water from shallow soils than did undefoliated plants.3. Irrigation with deuterium-labelled water further demonstrated that undefoliated plants were able to use shallow soil water. Defoliation appeared to affect the ability of trees to use deep-water sources.4. Reduced carbon assimilation limited water uptake from deep soil layers. These data highlight that there are internal physiological controls on carbon allocation that may limit water uptake from different soil layers. During periods of high vapour pressure deficit or soil drought, when leaf gas exchange and carbon assimilation decline, this may create positive feedbacks where plants are unable to forage for deep water due to carbon limitations.}, url = {/files/bibliography/03-034.pdf}, author = {K. A. Snyder and Williams, D.G.} } @conference {50826, title = {An ecohydrological framework for determining the landscape effects of woody plant encroachment}, booktitle = {88th Annual Meeting, Ecological Society of America}, year = {2003}, month = {August 3-8, 2003}, pages = {314}, address = {Savannah, GA}, abstract = {Woody plant encroachment into semiarid and arid systems is a global phenomenon with large potential hydrological and biogeochemical consequences. We considered systems in the southwestern United States (pinyon-juniper, mesquite and creosote) where dramatic changes in physiognomy create potential feedbacks between woody plants and hydrology at the landscape scale. The outcome is a new conceptual framework useful in directing future research and determining broad-scale potential ecological and biogeochemical consequences across a range of ecosystems. Shrub removal has been proposed as a mechanism for increasing water yields, but increased water yields are likely only in systems with shallow bedrock, riparian systems, or mesic systems where precipitation exceeds potential evapotranspiration. In semiarid and arid systems, abiotic controls on evaporation overwhelm biotic control of streamflow. In these systems, the predominant effects of woody plant encroachment that promote ecohydrological feedbacks are changes in: infiltration, depth of plant water removal, generation of transient overland flow and near-ground energy budgets that affect the ratio of evaporation (E) to transpiration (T). In these more xeric areas, the ratio of E:T is critical because it indicates the biological use of water which influences (through feed-forward or feed-back effects) landscape structure. Increasing woody plant cover is associated with larger open interspaces; at the landscape scale, the differential contribution of canopy versus interspaces may change landscape E:T. The alterations of spatial structure and ecohydrology likely influence carbon cycling. At the landscape scale, total respiration depends on the magnitude of canopy/interspace respiration flux scaled by the spatial extent of interspaces and canopy space. Canopy respiration is correlated to T, while interspace respiration is largely driven by soil water availability and, therefore, is driven by factors that control E. Resolving the partitioning of E:T should help resolve the net effect of woody plant encroachment on other biogeochemical cycles at the landscape scale.}, author = {K. A. Snyder and T. Huxman and David D. Breshears and Wilcox, B.P. and Small, E.E. and R.L. Scott and R. B. Jackson and Hultine, K.R.} } @article {50827, title = {Night-time conductance in C3 and C4 species: Do plants lose water at night?}, journal = {Journal of Experimental Botany}, volume = {54}, number = {383}, year = {2003}, month = {February 1, 2003}, pages = {861-865}, abstract = {Significant night-time stomatal conductance and transpiration were found for 11 out of 17 species with a range of life histories (herbaceous annual, perennial grass, shrub, tree), photosynthetic pathways (C3, C4), and habitats in the western United States. Across species and habitats, higher night-time conductance and transpiration were associated with higher daytime values. The prevalence, mechanisms and ecological implications of substantial night-time water loss deserve further investigation.}, keywords = {conductance, night-time, transpiration}, url = {http://jxb.oxfordjournals.org/cgi/reprint/54/383/861?maxtoshow=\&HITS=10\&hits=10\&RESULTFORMAT=\&fulltext=Night-time+conductance\&searchid=1\&FIRSTINDEX=0\&resourcetype=HWCIT}, author = {K. A. Snyder and J. H. Richards and L.A. Donovan} } @conference {50828, title = {Plant ecophysiological processes and soil water availability in semiarid and arid regions}, booktitle = {Society for Range Management, New Mexico Section, Winter Meeting}, year = {2003}, month = {January 21-23, 2}, pages = {2}, address = {Albuquerque, NM}, abstract = {The goal of this contributed paper was to demonstrate how determination of plant water source use through use of natural abundance stable isotopic tracer techniques has applications in riparian areas and on rangelands. Research was presented that determined what sources of water different plant species are using and how this varies spatially, due to differences in soil texture and geomorphology, and how this varies temporally, due to seasonally climate patterns. This knowledge is applicable to remediation and monitoring efforts because it provides information to land managers on what sources of water are available on different landscapes and what plant species will be capable of exploiting these water sources for biomass production. This research is also applicable to understanding how changes in the management of vegetation and hydrology interact on the landscape and result in different patterns of community structure.}, author = {K. A. Snyder} } @article {50946, title = {Riparian plant community patterns: A case study from southeastern Arizona}, journal = {Arizona-Nevada Academy of Science Journal}, volume = {34}, number = {2}, year = {2002}, month = {December 1, 2002}, pages = {106-111}, abstract = {There is concern about the preservation and, when necessary, restoration of riparian areas in semiarid regions. Understanding patterns of plant species composition and distribution within riparian corridors is helpful in planning for the management and protection of buffer strips that reduce the delivery of sediments to stream systems, maintain streambank stability, and filter excess nutrients that occur in runoff water. To determine how plant community patterns varied along environmental gradients in a semiarid riparian ecosystem, 192 quadrats located on transects radiating away from a mixed perennial and intermittent stream system were sampled for woody and herbaceous plants. Principal Components Analysis (PCA) was used to display community patterns based on species composition and distribution. Three communities of woody plants were observed across hydrologic and elevational gradients: a shrub-dominated Mimosa biuncifera community, a woodland community dominated by Prosopis velutina, and a riparian community dominated by Platanus wrightii. Patterns of herbaceous plants were not related to these gradients.}, url = {/files/bibliography/02-073.pdf}, author = {K. A. Snyder and Guertin, D. P. and Jemison, R.L. and Ffolliott, P.F.} }