@article {50720, title = {Plant water relations and photosynthesis during and after drought in a Chihuahuan Desert arroyo}, journal = {Journal of Arid Environments}, volume = {59}, number = {1}, year = {2004}, month = {October 1, 2004}, pages = {27-39}, abstract = {

In order to understand the physiology and ecology of perennial shrub species occupying the banks of ephemeral streams we studied the plant water relations and gas-exchange characteristics of six perennial shrub species growing along an ephemeral stream (arroyo) in the northern Chihuahuan Desert. Two of the species are restricted to riparian habitats (Brickellia laciniata and Chilopsis linearis), three are classified as semi-riparian (Fallugia paradoxa, Prosopis glandulosa, and Rhus microphylla) and one is considered to be non-riparian (Flourensia cernua). Differences in gas-exchange patterns and water relations emerged among these six species, yet differences among obligate, semi-riparian, and non-riparian classes of species were evident, but not universal. Chilopsis and Brickellia did not develop the low plant water potentials that characterized some semi-riparian (Rhus) and non-riparian species (Flourensia) during periods of drought. Rates of photosynthesis and transpiration were highest in Prosopis throughout the study, and were relatively constant in Chilopsis during and after drought. Whether water became available as channel flow or direct rainfall appeared to play a role in the physiology of these arroyo shrubs.

}, keywords = {channel flow, gas exchange, phreatophytic, plant water potential, rainfall, riparian}, doi = {10.1016/j.jaridenv.2004.01.011}, url = {files/bibliography/JRN00415.pdf}, author = {A.G. de Soyza and K.T. Killingbeck and Walter G. Whitford} } @article {JRN54840, title = {Assessing and Monitoring the Health of Western Rangeland Watersheds}, journal = {Environmental Monitoring and Assessment}, volume = {64}, year = {2000}, month = {September 2000}, pages = {153{\textendash}166}, chapter = {153}, abstract = {

The most important function of watersheds in the western U.S. is the capacity to retain soil and water, thereby providing stability in hydrologic head and minimizing stream sediment loads. Long-term soil and water retention varies directly with vegetation cover. Data on ground cover and plant species composition were collected from 129 sites in the Rio Grande drainage of south-central New Mexico. This area was previously assessed by classification of Advanced Very High Resolution Radiometry (AVHRR) imagery. The classification of irreversibly degraded sites failed to identify most of the severely degraded sites based on size of bare patches and 35\% of the sites classified as degraded were healthy based on mean bare patch size and vegetation cover. Previous research showed that an index of unvegetated soil (bare patch size and percent of ground without vegetative cover) was the most robust indicator of the soil and water retention function. Although the regression of mean bare patch size on percent bare ground was significant (p \< 0.001), percent bare ground accounted for only 11\% of the variability in bare patch size. Therefore bare patch size cannot be estimated from data on percent bare ground derived from remote sensing. At sites with less than 25\% grass cover, and on sites with more than 15\% shrub cover, there were significant relationships between percent bare soil and mean bare patch size (p \< 0.05). Several other indicators of ecosystem health were related to mean bare patch size: perennial plant species richness (r = 0.6, p \< 0.0001), percent cover of increaser species (r = 0.5, p \< 0.0001) and percent cover of forage useable by livestock (r = 0.62, p \< 0.0001). There was no relationship between bare patch size and cover of species that are toxic to livestock. In order to assess the ability of western rangeland watersheds to retain soil and water using remote sensing, it will be necessary to detect and estimate sizes of bare patches ranging between at least 0.5 m in diameter to several meters in diameter.

}, keywords = {bare patch size, grass, percent bare soil, remote sensing, shrub, Soil and water retention}, doi = {10.1007/978-94-011-4343-1_14}, url = {/files/bibliography/00-765.pdf}, author = {A.G. de Soyza and Walter G. Whitford and S.J. Turner and Justin Van Zee and A. R. Johnson} } @article {51121, title = {Indicators of Great Basin rangeland health}, journal = {Journal of Arid Environments}, volume = {45}, number = {4}, year = {2000}, month = {August 1, 2000}, pages = {289-304}, abstract = {

Early-warning indicators of rangeland health can be used to estimate the functional integrity of a site and may allow sustainable management of desert rangelands. The utility of several vegetation canopy-based indicators of rangeland health at 32 Great Basin rangeland locations was investigated. The indicators were originally developed in rangelands of the Chihuahuan Desert. Soil resources are lost through wind and water-driven erosion mainly from areas unprotected by plant canopies (i.e. bare soil). Study sites in Idaho had the smallest bare patches, followed by sites in Oregon. The more arid Great Basin Sagebrush Zone sites in Utah had the largest bare patches. Several vegetational indicators including percent cover by vegetation, percent cover by life-form, percent cover by sagebrush, and percent cover by resilient species were negatively related to mean bare patch size and are potential indicators of Great Basin rangeland condition. Plant community composition and the range of bare patch sizes were different at sites in the three locations in Idaho, Oregon and Utah. Therefore, expected indicator values are location specific and should not be extrapolated to other locations. The condition of study sites were often ranked differently by different indicators. Therefore, the condition of rangeland sites should be evaluated using several indicators.

}, keywords = {Great Basin, indicators, management, rangeland health}, doi = {10.1006/jare.2000.0645}, url = {/files/bibliography/00-014.pdf}, author = {A.G. de Soyza and Justin Van Zee and Walter G. Whitford and A.C. Neale and Tallent-Hallsel, N. and Jeffrey E. Herrick and Kris Havstad} } @proceedings {51122, title = {Soil surface indicators of desertification from the northern Chihuahuan Desert and the Great Basin}, volume = {1}, year = {2000}, month = {July 19-23, 1999}, pages = {137}, address = {Townsville, Australia}, abstract = {We measured vegetation canopy and litter cover in ungrazed exclosures and adjacent pastures at three locations in the Chihuahuan Desert, five locations near Burns, Oregon, seven locations near Milford, Utah, and six near Idaho Falls, Idaho, all of which are in the Great Basin. We found that vegetation cover is not a good surrogate for estimating litter cover. Refinements that take into account vegetation species composition may improve the ability to predict litter cover from vegetation data.}, keywords = {Chihuahuan Desert, desertification, Great Basin, indicators, soil surface}, url = {/files/bibliography/00-002.pdf}, author = {A.G. de Soyza and Walter G. Whitford and Justin Van Zee and Jeffrey E. Herrick and Kris Havstad} } @article {51249, title = {Early warning indicators of desertification: Examples of tests in the Chihuahuan Desert}, journal = {Journal of Arid Environments}, volume = {39}, number = {2}, year = {1998}, month = {June 1, 1998}, pages = {101-112}, abstract = {

Perennial grasslands that dominated the south-west United States during the past 10,000 years have been desertified to varying extents. Desertification is a temporal phenomenon, defined in this paper as a change in the scale of the spatial distribution of resources. We report here on several indicators of desertification, using bare patch indices as surrogates for direct measures. A bare patch index based on canopy cover, which is relevant for precipitation and wind-driven erosion, is not equivalent to another bare patch index based on soil surface measurements, which is relevant for overland water flow-driven erosion. Per cent grass cover, per cent long-lived grass cover, per cent cover of vegetative reproducers, and a relative preference index all yielded sensitive indicators of desertification. The usefulness of some indicators, such as organic litter, depends upon recent environmental history. Other indicators, such as per cent cover due to grazing-resistant species, appear not to be sensitive to desertification and need further development. Range site type may also be important in determining indicator responses, where some indicators were sensitive to a range site while others were not. Overall, we have identified several sensitive indicators of desertification based on vegetational characteristics in varied range sites in the northern Chihuahuan Desert.

}, keywords = {bare patch index, Chihuahuan Desert, ecosystem health, indicators, litter}, doi = {10.1006/jare.1998.0391}, url = {/files/bibliography/607.pdf}, author = {A.G. de Soyza and Walter G. Whitford and Jeffrey E. Herrick and Justin Van Zee and Kris Havstad} } @conference {51302, title = {Sensitive indicators of desertification: examples of tests in the Chihuahuan Desert}, booktitle = {International Symposium and Workshop on Combating Desertification: Connecting Science with Community Action}, year = {1997}, month = {May 12-16, 1997}, pages = {1}, address = {Tucson, AZ}, abstract = {Desertification is recognized as a suite of symptoms of degradation of rangelands. We selected a suite of indicators related to important ecosystem processes and tested these indicators at baseline sites with long, well-documented, histories of land management. Data were collected at grazing intensity gradients centered at livestock watering points, livestock exclosures, sites with documented change in vegetative cover and composition, and sites dominated by exotic species. Field measurements that provided the data for computing indicator metrics were collected at a site by a two-person team in less than 6 hours. Indicator metrics that proved to be sensitive to desertification provide calibration values of assessment and monitoring at sites where management and ecosystem status history are unknown. Indicators that were consistent and reliable include size and weighted skewness of unvegetated patches, spatial distribution and orientation of unvegetated patches, surface cover characteristics, total vegetative cover, shrub cover/grass cover, cover of native species/cover of exotic species, forage value index, and cover short-lived species/cover long-lived species.}, author = {A.G. de Soyza and Justin Van Zee and Walter G. Whitford} } @article {51303, title = {Sensitivity testing of indicators of ecosystem health}, journal = {Ecosystem Health}, volume = {3}, number = {1}, year = {1997}, month = {March 1997}, pages = {44-53}, abstract = {

The sensitivities of three indicators of ecosystem health were evaluated at several sites in the Jornada Basin of the Chihuahuan Desert. The size of bare patches, proportion of total grass cover contributed by long-lived perennial grasses and soil stability are interdependent indicators of ecosystem functions related to the retention and use of water and nutrients. Sensitivity tests were chosen using data collectedalong disturbance gradients and then tested using independent, ungrazed exclosures and adjacent grazed pastures. The mean size of bare soil patches was sensitive to anthropogenic disturbance. When bare soil patch data were transformed usiing natural logarithms, the skewness of the frequency distribution weighted by mean bare patch size could be used to indicate early disturbance to the ecosystem. The proportion of total vegetation that was long-lived also was sensitive to anthropogenic disturbance and appears to be a good indicator of ecosystem degradation. The slake test for soil surface stability was extremely sensitive to disturbance and may serve as an early-warning indicator of soil degradation for the coarse-textured soils which were evaluated.

}, doi = {10.1111/j.1526-0992.1997.00705.pp.x}, url = {/files/bibliography/509.pdf}, author = {A.G. de Soyza and Walter G. Whitford and Jeffrey E. Herrick} } @article {51304, title = {Variation in Creosotebush (Larrea tridentata) canopy morphology in relation to habitat, soil fertility and associated annual plant communities}, journal = {American Midland Naturalist}, volume = {137}, year = {1997}, month = {1997}, pages = {13-26}, abstract = {

Differences in creosotebush (Larrea tridentata) crown morphology may reflect chances in relative demand for water vs. nutrient resources, coinciding with shrub growth and development. Creosotebushes with inverted, cone-shaped crowns were more abundant in water-limited environments; whereas, hemispherical-shaped creosotebushes were more abundant in less water-limited environments. Cone-shaped creosotebushes accumulated substantially less litter under their canopies than did creosotebushes with hemispherical-shaped crowns. Soil nutrient concentrations under conical shrubs were similar to those in intershrub spaces and both of these were less than soil nutrient concentrations under hemispherical shrubs. In ecosystems where overland flow of water exerted a greater influence on movement of organic litter than did wind, shrub shape had little effect on long-term litter accumulation. No persistent differences in the biomass or diversity of ephemeral taxa exploiting undershrub areas were found, probably because the positive effects of greater nutrient resources under hemispherical shrubs were offset by the limitations imposed by the larger, more dense canopies of hemispherical shrubs. Overall, creosotebush morphology affected litter accumulation patterns and soil nutrient patterns and must be considered when assessing the heterogeneity of desert ecosystems in the southwestern U.S.

}, keywords = {desert soils, deserts, nitrogen, Plant morphology, plants, shrubs, soil morphology, soil nutrients, Stems, Vegetation canopies}, doi = {10.2307/2426751 }, url = {/files/bibliography/510.pdf}, author = {A.G. de Soyza and Walter G. Whitford and E. Martinez-Meza and Justin Van Zee} } @article {51375, title = {Effects of plant size on photosynthesis and water relations in the desert shrub Prosopis glandulosa (Fabaceae)}, journal = {American Journal of Botany}, volume = {83}, number = {1}, year = {1996}, month = {1996}, pages = {99-105}, abstract = {

The Jornada del Muerto Basin of southern New Mexico has undergone a marked transition of plant communities. Shrubs now dominate areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete with similar-sized grass plants. This is followed by a phase in which established shrubs sequester nutrients and water within their biomass and alter soil resources under their canopy, creating {\textquotedblleft}islands{\textquotedblright} of fertility. We studied gas exchange and water relations of small and large shrubs growing in the Jornada del Muerto as a function of varying soil moistsure during the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrubs following high summer rainfall in July and highest seasonal soil moisture. High rates of carbon assimilation and water use would be an advantage for small shrubs competing with grasses when shallow soil moisture was plentiful. Large shrubs had greater net assimilation and water-use efficiency and lower xylem water potential than small shrubs following a dry period in September when soil moisture was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water as soil moisture is depleted. Although the study provides evidence of differences in physiological responses of different-sized shrubs, there was not support for the hypothesis that small shrubs are more closely coupled to variation in soil moisture availability than large shrubs. Small shrubs may actually be less coupled to soil moisture than large shrubs and thus avoid conditions when continued transpiration could not be matched by equivalent water uptake.

}, keywords = {Atmospheric moisture, desert soils, deserts, grasses, plants, Rain, shrubs, soil depth, soil water, Water use efficiency}, doi = {10.2307/2445960}, url = {/files/bibliography/471.pdf}, author = {A.G. de Soyza and A. C. Franco and R. A. Virginia and J. F. Reynolds and Walter G. Whitford} } @proceedings {51374, title = {Effects of summer drought on the water relations, physiology, and growth of large and small plants of Prosopis glandulosa and Larrea tridentata}, volume = {General Technical Report INT-GTR-338}, year = {1996}, month = {1996}, pages = {220-223}, publisher = {USDA Forest Service, Intermountain Research Station, Gen. Tech. Rep. INT-GTR-338}, address = {Las Cruces, NM}, abstract = {

Large and small plants of creosotebush and mesquite were subjected to drought during summer 1993. Large and small plants responded to drought with lowered xylem water potential and lowered photosynthetic gas exchange. Large plants appear to maintain a reduced but constant photosynthetic rate during drought, and responded to precipitation with increased photosynthesis. How-ever, small plants attempted to maintain a high photosynthetic rate throughout the season, responding less strongly to \ precipitation events, but probably depleting reserves more and causing photosynthesis to decline more when droughted. Our results suggest that \ large, established plants are better able to withstand drought than are small, establishing shrubs.

}, url = {/files/bibliography/472.pdf}, author = {A.G. de Soyza and Walter G. Whitford and R. A. Virginia and J. F. Reynolds}, editor = {J.R. Barrow and McArthur, E. Durant and R. E. Sosebee and R. J. Tausch} } @conference {51423, title = {Effects of summer drought on the water relations, physiology and growth of large and small plants of Prosopis glandulosa and Larrea tridentata}, booktitle = {Wildland Shrub Symposium}, year = {1995}, month = {May 23-25, 1995}, pages = {19}, abstract = {Large and small plants of creosotebush (Lorreatridentata) and mesquite (Prosopisglandulosa) were subjected to summer drought by erecting rain-exclusion shelters with transparent plastic roofs over the experimental plants. The root systems of control and experimental plants of both species were isolated from the surrounding soil mass to a depth of approximately 1.5 m, with a backfilled, plastic-lined trench approximately 0.6 m from the plant crown dripline. The rainout shelters were very effective in drying the soil as measured with a neutron atennuation probe at 0.3 and 0.9 m. Both large and small plants responded to drought with lowered xylem water potential and lowered photosynthetic gas exchange, but the gas-exchange physiology of large plants appeared to be somewhat uncoupled from the short-term drought we induced. Stem growth was much greater in control plants, and in experimental plants growth appeared to be greater in small plants than in large plants. Our results are largely consistent with shrub resource islands theory which suggests that large, established plants with greater accumulated resources are better able to withstand episodes of short-term drought than are small, establishing shrubs.}, keywords = {Larrea tridentata, physiology, prosopis glandulosa, summer drought, water relations}, author = {A.G. de Soyza and Walter G. Whitford and A. C. Franco and R. A. Virginia and J. F. Reynolds} } @conference {51422, title = {Responses of plant assemblages to disturbance gradients in the northern Chihuahuan Desert}, booktitle = {Winter Meeting of the British Ecological Society}, year = {1995}, month = {December 19-21, }, pages = {71}, address = {University of Sheffield, GB}, abstract = {At three locations in the Chihuahuan Desert, domestic livestock-induced disturbance gradients extend outward from watering points. Soil is more compacted but less stable near the watering points. Plant species richness changes little along the gradient, but species diversity and plant biomass does increase dramaatically with distance from the watering point.}, keywords = {Chihuahuan Desert, disturbance gradients, plant assemblages, species diversity}, author = {A.G. de Soyza and Justin Van Zee and Walter G. Whitford and Jeffrey E. Herrick} }