Fertile island development around perennial shrubs across a Mojave Desert chronosequence

Spatial heterogeneity has been examined only recently as a factor in studies of ecosystem processes. The effect of this factor on desert organisms is exemplified in the ""fertile island"" a concentration of essential soil nutrients under perennial shrubs. In this study we examined fertile island patterns in undisturbed desert areas and on abandoned roadways to see at what rate and magnitude these fertile patches reappeared after disturbance. Small-scale examination of patterns of soil P, organic matter, moisture, and bulk density showed that soils beneath Larrea tridentata (DC.) Cov. shrubs on the roads lacked the tight circular gradient in these variables that was characteristic of soils beneath control shrubs, even 88 years after road abandonment. The nature of the initial soil disturbance altered both spatial patterns of soil N and temporal patterns of fertile island development. Fertile island patterns for total soil N, available P, and organic matter were more circular than patterns for bulk density, texture, or pH. We suggest that patterns of soil heterogeneity may develop first for elements that may be limiting to desert shrub growth (N, P, organic matter), followed by spatial development in other less limiting soil factors (bulk density, texture, pH).     

Relating soil chemistry and plant relationship in wooded draws of the northern Great Plains

Soils of the green ash/chokecherry habitat type in northwestern South Dakota were evaluated for 22 properties to determine whether any could be correlated with density of chokecherry ( Prunus virginiana ) and snowberry ( Symphoricarpos occidentalis ). Surface soils were moderately fertile, with high levels of all elements except phosphorus and nitrogen. Soils were fine textured with moderately high cation exchange capacity and saturation percentages. However, soils were nonsaline-nonalkaline with low amounts of exchangeable sodium. None of the soil properties showed good correlation with chokecherry and snowberry densities. Greatest correlations were found between each of the shrub species and grass.     

Fungi associated with soils collected beneath and between pinyon and juniper canopies in New Mexico

The soil fungal community beneath pinyon ( Pinus edulis Engelm.) and one-seeded juniper ( Juniperus monosperma [Engelm.] Sar.) tree canopies is described and compared with fungi from adjacent interspace soils dominated by blue grama ( Bouteloua gracilis [H. B. K.] Lag.). Significantly higher organic matter contents and fungal propagule levels were found in soils beneath pinyon and juniper trees than in interspace soils. Soils under pinyon and juniper trees contained similar chemical, physical, and biological properties and, consequently, many groups of fungi in common (64% of the species isolated were common to both). In contrast soil fungi in adjacent interspace soils were vastly different from those collected in soils beneath pinyon and juniper canopies (44% and 48% species in common, respectively). Soil fungi that were isolated more often from pinyon-juniper soils than from interspace soils included Absidia sspp., Beauvaria spp., Gliocladium spp., Mucor spp., Penicillium cyclopium , P. fasciculata , P. frequentans , P. restrictum , Thamnidium spp., and Trichoderma spp. Soil fungi that were isolated more often in interspace soils than in pinyon or juniper soils included Aspergillus alutaceus spp., A. fumigatus , some Fusarium spp., Penicillium luteum , and P. talaromyces .     

Competition Between Adult and Seedling Shrubs of Ambrosia dumosa in the Mojave Desert, Nevada

Seeds of the perennial shrub Ambrosia dumosa germinated in abundance following 11 days of rain during August 1983 at a study site in the northern Mojave Desert. Seedling establishment, growth, and reproduction were observed in natural vegetation and in an area that had been previously cleared of vegetation. For 5,527 A. dumosa seedlings, percent survival in April 1986 averaged 3% in the undisturbed vegetation and 58% in the denuded area. Seedlings occupying the cleared area had grown to sizes up to 0. 1 m 3 m by October 1984; some produced flowers and fruit in the spring of 1985. Surviving seedlings in the undisturbed vegetation were all smaller than 0.001 m 3 and did not reproduce. These pronounced differences in growth, survival, and reproduction associated with the presence or absence of adult shrubs demonstrated an intense competition that is incompatible with indications of mild competition from nearest-neighbor analyses. I therefore hypothesize that competition for water occurred, not by competition for water in two dimensions but by rapid use of a common resource, as if several people were drinking with straws from a common cup. This temporal mechanism would strongly favor adults over seedlings.     

Ecological differences of C 3 and C 4 plant species from central Utah in habitats and mineral composition

Six study sites were established in each of three community life form types (grass, forb, and shrub) containing as dominants or subdominants either C 3 and/or C 4 plants. Soil and vegetation samples were analyzed for total nitrogen, phosphorus, magnesium, calcium, potassium, sodium, zinc, iron, copper, and manganese. Discriminant analysis and analysis of variance statistics were used to evaluate differences in mineral content of soils and plant tissues. C 4 plants in all study sites assimilated higher concentrations of potassium, iron, and calcium than did C 3 plants. Forbs in all sites contained the highest concentrations of minerals, followed by shrubs and grasses.     

Ecological impacts of seed harvester ants on soil attributes in a Larrea -dominated shrubland

The influence of seed harvester ant ( Pogonomyrmex rugosus ) colonies on soil properties and soil surface and moisture characteristics was investigated through comparison of adjacent, nonnest (reference, 4 m beyond ant colony) areas in Las Vegas, Nevada. Effects of ant colonies on both terrace and slope sites were investigated. Soil moisture content and soil bulk density in a creosote bush ( Larrea tridentata )--dominated shrubland were significantly lower, while soil temperature, soil organic matter, and percent pore space were significantly higher in soils with ant nests relative to adjacent reference soils. Soil pH and texture did not differ significantly between nest and reference soils. Among soil surface characteristics, percent bare soil and rock (gravel, cobble, and boulder) cover were not significantly different between nest and reference soils. In evaluating soil moisture characteristics, soils with ant nests had a significantly higher water infiltrability and greater depth of water penetration, but a significantly lower area of water spread (surface-water runoff) at both terrace and slope sites. Between the 2 geomorphic surfaces, water infiltrability and depth of water penetration were significantly greater at the terrace than at the slope. Water-borne soil movement (fluvial erosion) was significantly greater at the slope than terrace but did not differ significantly between nest and reference soils. The presence of active P. rugosus colonies in the L. tridentata -dominated shrubland altered certain soil properties and appeared to have a protective influence on the soil by fostering more infiltration and less runoff of surface water in southern Nevada.     

Facilitation of Yucca brevifolia recruitment by Mojave Desert shrubs

Patterns of recruitment for Yucca brevifolia (Joshua tree) were investigated on 3 elevational transects, 1000-2000 m, in the Spring and Sheep Mountain ranges of southern Nevada. Yucca brevifolia is distributed throughout a broad range of plant communities dominated by Larrea tridentata and Ambrosia dumosa at low elevations, Coleogyne ramosissima at middle elevations, and an Artemisia-Pinus-Juniperus community at upper elevations. The density of Y. brevifolia gradually increased from the lowest elevations, peaked at 1600 m, and remained at intermediate levels at high elevations until reaching an abrupt upper elevational limit at 2000 m. Open substrate dominated the study areas; however, a large majority of Y. brevifolia seedlings were found growing under the canopy of other woody shrubs. This pattern of recruitment did not vary by site or elevation. Thirty-five species of perennial shrubs were identified in the study areas, 16 of which were found in association with at least 1 Y. brevifolia seedling. However, 4 shrubs were found in a nurse plant relationship with Y. brevifolia above the frequency predicted by either their canopy area or numerical dominance. Seedlings exhibited significant variation in aspect, relative to the center of the nurse shrub. In Lee and Lucky Strike canyons, recruitment occurred predominantly on the east and west sides of nurse shrubs, indicating the importance of specific microhabitats. Local presence of specific perennial shrubs resulted in higher levels of recruitment, causing a distinct pattern of community development, resumably through amelioration of abiotic stresses.     

Microhabitat-specific controls on soil respiration and denitrification in the Mojave Desert: the role of harvester ant nests and vegetation

Seed harvesting ants ( Pogonomyrmex rugosus ) concentrate organic matter and nutrients near their nests and create biogeochemical hotspots in desert soil. We examined factors regulating denitrification and soil respiration in a Mojave Desert ecosystem to determine the role harvester ant colonies play in nitrogen loss and carbon mineralization. Organic matter and nutrient storage were significantly greater in colonies than under the dominant vegetation (i.e., Pleuraphis rigida , a bunch grass) and in bare soil, with standing stocks of inorganic nitrogen in colonies nearly 4-fold greater than in the other microhabitats. Soil respiration, measured with laboratory incubations, was below detection limits under ambient soil conditions. Respiration rate in soil from bare patches and under grass was limited by water and labile organic carbon, with secondary nitrate limitation evident only once carbon limitation was alleviated. Soil respiration in ant nest soil was limited by water and labile organic carbon only. Denitrification, measured by the acetylene block technique, was elevated in bare soil and under grass with the addition of nitrate. Ant nest soil also responded to a nitrate addition; however, denitrification rate was greatest with addition of glucose. Ordinarily desert soil has low rates of respiration and denitrification due to dry ambient conditions. However, ant colonies likely function as important sites for nitrogen loss and carbon mineralization following rain storms, especially if storms coincide with seed set and the temporally pulsed input of organic matter into colonies.     

Vegetation, soils, and hydrogeomorphology of riparian patch types of a dryland river

From a landscape perspective, riparian corridors can be viewed as mosaics of vegetation patches. We delineated 10 patch types within the floodplain of the San Pedro River (Arizona) on the basis of physiognomy, dominant overstory species, and tree size class; and we assessed differences in hydrogeomorphology, vegetation structure, plant species richness, and soil chemistry and texture. Patches of tamarisk ( Tamarix ), an introduced species, fell within the continuum of variation shown by other patch types in the landscape mosaic. Among the tree-dominated types, cottonwood-willow ( Populus-Salix ) and tamarisk patches were inundated more frequently than mesquite ( Prosopis ) patches, while cottonwood-willow patches had shallower groundwater than tamarisk or mesquite patches. Due to the wetter conditions, cottonwood-willow patches had a high relative abundance of wetland and exotic species in the understory. Tamarisk patches and wet shrublands ( Baccharis salicifolia—Salix exigua ) had high woody stem densities while cottonwood-willow patches had dense canopy cover. In association with differences in canopy cover, cottonwood-willow patches had low herbaceous species richness but high woody species richness, while tamarisk patches had high herbaceous and low woody species richness. Soil electrical conductivity, silt content, organic matter content, and available phosphorus increased from young to old stands of both tamarisk and cottonwood-willow, often resulting in greater differences between patches of different size/age class than between patches with different dominant species. Surface soil salinity (electrical conductivity) was low in all patches, including those dominated by tamarisk. Nitrate was abundant in soils of tamarisk patches (perhaps reflecting their high clay content) and wet shrubland patches. Dry shrublands ( Hymenoclea-Ericameria ) and wet shrublands were similar to young forest patches in having coarse soils with little organic matter.     

Observations on the distribution of microorganisms in desert soil

Population estimates of fungi, bacteria, and actinomycetes in desert soil were determined with respect to soil depth and distance from shrubs. In general the highest numbers of microbes were found at the shrub base; the lowest numbers were found in the interspaces. While the total number of organisms usually declined in deeper soil, the relative importance of the actinomycetes increased. These population trends are attributed to substrate availability and utilization and interspecific interactions. As the soils became drier and warmer the total number of microorganisms decreased. Mold populations remained at about the same level during the study. While the numbers of both bacteria and actinomycetes declined, the relative importance of the actinomycetes increased.        

Amphipod diversity at three Tunisian lagoon complexes in relation to environmental conditions

The amphipod communities of different wetland types belonging to three coastal lagoons complexes in northern Tunisia, Ichkeul (37°06′ to 37°14′ N, 09°35′ to 09°56′ E), Ghar El Melh (37°06′ to 37°10′ N, 10°08′ to 10°14′ E) and Korba (36°34′ to 36°38′ N, 10°52′ to 10°54′ E), were studied with respect to species composition and abundance and their relationship with abiotic environmental characteristics, namely air and soil temperature and humidity, soil grain size and the soil content of organic matter and heavy metals. Both highest abundance and highest diversity of amphipods were observed in the lagoon complex of Ichkeul, which is characterized by higher contents of organic matter but also by higher heavy metal concentrations in the soil than the lagoon complexes of Ghar El Melh and Korba. Although amphipod abundance does not seem to linearly depend on environmental parameters, unimodal Canonical Correspondence Analysis suggests that amphipod abundance is related to grain size of the soil, to the soil content of organic matter and to several heavy metals.     

Influence of soil frost on infiltration of shrub coppice dune and dune interspace soils in southeastern Nevada

The influence of soil frost on the infiltration rate of shrub coppice dune and dune interspace soils was evaluated near Crystal Springs, Nevada, using simulated rainfall. The infiltration rate of the coppice dune soil was greater than the dune interspace soil under frozen or unfrozen conditions. Because of different vegetation cover and surface soil characteristics, coppice dune and dune interspace soils responded differently to freezing, thus imposing a spatial and temporal response to infiltration rate. Infiltration rate of soils with porous concrete frost increased as the soils thawed during simulated rainfall, but soils with nonporous concrete frost allowed very little infiltration to occur. Both coppice dune and dune interspace soils that were classified in January as having granular frost had a higher infiltration rate than the same unfrozen soils in March.     

Salvage logging and replanting reduce understory cover and richness compared to unsalvaged unplanted sites at Mount St. Helens, Washington

The 1980 eruption of Mount St. Helens killed trees in a broad 600-km2 swath north of the crater. Over most of the blast zone, dead trees were salvage logged and Abies procera was planted, except in areas within the Mount St. Helens National Volcanic Monument. We compared salvage-replanted sites and unsalvaged sites in 1 area of the blast zone where the sites were adjacent by using twenty-five 200-m 2 plots for each treatment. Salvaged-replanted plots had significantly lower herb and shrub cover, richness, diversity, litter depth, downed woody debris, nitrate, and phosphate. Salvaged-replanted sites also had significantly more stumps, bare area, and moss cover than unsalvaged plots. Soil organic matter and nonnative species cover did not differ. Nonnative species were not important components of any plots. Nitrate, total nitrogen, organic matter, and litter were correlated with the major patterns of species distribution in a canonical correspondence analysis of the salvaged-replanted plots. In the unsalvaged plots, slope, downed woody debris, and elevation were correlated with the major patterns of species distribution.     

Some edaphic relations of southeastern Idaho wildlands

Soil samples from the Al horizon and dominant subsoil horizon at 190 sites were analyzed for Ca, Mg, K, P, and organic matter contents in conjunction with a soil resource inventory of the Caribou National Forest. Vegetative composition and production data were compared to the edaphic factors to derive relationships useful to the land manager. Organic matter was effective in distinguishing between soil orders and was positively correlated to vegetative production. Vegetative cycling of the nutrients P and K was most pronounced in soils of the order Mollisols, to a lesser degree in the Alfisols, and not at all in the Entisols. The K/Mg ratio of the soil showed a negative correlation to grass production and a positive correlation with shrub production. An interpretative table is provided to aid determinations of high versus low values for the edaphic factors studied.     

Roadside foraging by kangaroo rats in a grazed short-grass prairie landscape

We used live-trapping and foraging experiments to describe use of roadside vegetation by kangaroo rats ( Dipodomys ordii ) in short-grass prairie in Colorado, and to determine whether this species' perception of predation risk differed between roadside habitats, where predators were expected to be most active, and locations 120 m into adjacent grazed pastures. Giving-up densities (GUDs) were measured in paired seed trays placed beneath saltbush ( Atriplex canescens ) shrubs and in the open, 3 m from shrubs along transects in roadside and pasture locations. Trials were conducted on nights with a partial (1st-quarter), full, and new moon to assess how intensity of risk affected microhabitat use. Kangaroo rats were much more abundant in areas with saltbush cover than in grazed prairie, and were 4–6 times more numerous along roadsides in saltbush areas than in other locations. On dark nights and in pasture locations, foraging rates of kangaroo rats were similar in shrub trays and open trays. The fewest seeds were removed (i.e., GUDs were highest), however, in open trays along roads on moonlit nights, suggesting that kangaroo rats recognized potential risks associated with roadsides on bright nights. The high numbers of kangaroo rats along roads suggest that benefits associated with these habitats (ease of digging, dust bathing, higher soil seed banks) exceed the costs associated with higher risk of mortality from predators and vehicles. Our results demonstrate how foraging decisions differ depending on the spatial and temporal contexts in which behaviors are measured, and underscore the potential value of less common habitats such as road margins for increasing landscape-scale diversity and wildlife habitat in grazed grasslands.     

Geostatistical analysis of resource islands under Artemisia tridentata in the shrub-steppe

Desert plants can influence the pattern of resources in soil resulting in small-scale enriched zones. Although conceptually simple, the shape, size, and orientation of these "resource islands" are difficult to study in detail using conventional sampling regimes. To demonstrate and alternative approach, we sampled soil under and around individual Artemisia tridentata (sagebrush), a dominant shrub of cool desert environments, and analyzed the samples with univariate statistics and geostatistics. Univariate statistics revealed that soil variables like total inorganic-N, soluble-C, and microbial biomass-C were distributed with highest mean values within about 25 cm of the plant axis and significantly lower mean values at distances beyond 60 cm. However, such simple analyses restricted our view of resource islands to identically sized, symmetrical accumulations of soil resources under each plant. Geostatistics provided additional information about spatial characteristics of soil variables. Variography revealed that samples separated by a distance of less than about 70 cm were correlated spatially. Over 75% of the sample variance was attributable to spatial variability. We modeled these spatial relationships and used kriging to predict values for unsampled locations. Resulting maps indicated that magnitude, size, and spatial distribution of soil resource islands vary between individual plants and for different soil properties. Maps, together with cross-variography, further indicate that resource islands under A. tridentata are not always distinguishable from the surrounding soil by sharp transition boundaries and may be asymmetrically distributed around the plant axis.     

Rangeland alpha diversities: Harvey Valley, Lassen National Forest, California

Nutrients in soil covered by Carex exserta sod and in adjacent unvegetated gravel areas were compared at Siberian Outpost, Sequoia National Park, California. The comparisons were part of a study to learn if Carex exserta meadow can be reestablished and if herbaceous cover on gravel areas can be increased. Grazing capacity and aesthetic appeal of denuded areas would be improved by better vegetative cover. The sod had higher concentrations of calcium, copper, iron, magnesium, manganese, nitrogen, potassium, and zinc than did the gravel areas. And it had a higher soil pH and percent organic matter. Sod and gravel did not differ in concentrations of phosphorus and sulfur. The differences were as might be expected between climax and badly deteriorated (or early seral) situations, and the results suggest that fertilization may be a useful treatment.         

Nutrients in Carex exserta sod and gravel in Sequoia National Park, California

Nutrients in soil covered by Carex exserta sod and in adjacent unvegetated gravel areas were compared at Siberian Outpost, Sequoia National Park, California. The comparisons were part of a study to learn if Carex exserta meadow can be reestablished and if herbaceous cover on gravel areas can be increased. Grazing capacity and aesthetic appeal of denuded areas would be improved by better vegetative cover. The sod had higher concentrations of calcium, copper, iron, magnesium, manganese, nitrogen, potassium, and zinc than did the gravel areas. And it had a higher soil pH and percent organic matter. Sod and gravel did not differ in concentrations of phosphorus and sulfur. The differences were as might be expected between climax and badly deteriorated (or early seral) situations, and the results suggest that fertilization may be a useful treatment.         

Comparison of Sage and Sharp-tailed Grouse leks in south central Wyoming

Columbian Sharp-tailed Grouse ( Tympanuchus Phasianellus columnianus ) and Sage Grouse ( Centro-cercus urophasianus ) leks were compared in an area of sympatry in south central Wyoming. Sharp-tailed Grouse leks had more (P < .05 shrub cover, taller shrubs, more forb, grass, and shrub species, and less visibility than did Sage Grouse leks. Reduction in shrub cover or the diversity of herbaceous species could potentially have greater influence on the use of lek sites by Columbian Sharp-tailed Grouse than by Sage Grouse in areas of sympatry in Wyoming.