Atriplex Confertifolia Research Articles

Nutrient uptake and gas exchange of Great Basin plants provide insight into drought adaptations and coexistence

Abstract Aims Although increases in precipitation variability in arid ecosystems are projected due to climate change, the response of desert shrub communities to precipitation change has not been fully elucidated. Such knowledge is important since drought-adapted plants exhibit varied mechanisms of survival that may contribute to species coexistence. Methods We tested the responses of eight drought-adapted plants, a mix of graminoids, shrubs and forbs to three summer precipitation scenarios (1.3, 2.6 and 3.9 cm per month) in a common garden experiment in the Great Basin (Owens Valley, California). Changes in mineral nutrient uptake (carbon, nitrogen, phosphorus, potassium, calcium, magnesium, manganese, copper, boron, zinc, iron and sodium) and gas exchange parameters (photosynthetic rate and stomatal conductance) were investigated in the studied species. Important Findings Two graminoids (Sporobolus airoides and Leymus triticoides) and one salt tolerant shrub species (Atriplex confertifolia) responded to increased water availability with increases in photosynthetic rate and/or stomatal conductance. There was a significant correlation between water availability and uptake of nutrients for five out of eight species. Artemisia tridentata, with higher rates of photosynthesis, contained greater amounts of potassium, copper and boron, while Juncus arcticus, with higher rates of photosynthesis, contained greater amounts of magnesium and iron, and less sodium. Juncus arcticus and three salt-adapted species (A. confertifolia, Distichlis spicata and S. airoides) exhibited correlations with stomatal conductance and concentrations of nutrients. Results indicate that differential physiological response mechanisms to increased moisture and associated nutrient uptake strategies in drought-adapted species may mediate coexistence under increased summer precipitation.

Chiseling Away at the Dogma of Dietary Specialization in Dipodomys Microps

Dipodomys microps, the chisel-toothed kangaroo rat, is heralded as one of few mammalian herbivores capable of dietary specialization. Throughout its range, the diet of D. microps is thought to consist primarily of Atriplex confertifolia (saltbush), a C4 plant, and sparing amounts of C3 plants. Using stable isotopes of carbon and nitrogen as natural diet tracers, we asked whether D. microps is an obligate specialist on saltbush. We analyzed hair samples of D. microps for isotopes from historic and recent museum specimens (N = 66). A subset of samples (N = 17) from 2017 that were associated with field notes on plant abundances were further evaluated to test how local saltbush abundance affects its inclusion in the diet of D. microps. Overall, we found that the chisel-toothed kangaroo rat facultatively specializes on saltbush and that the degree of specialization has varied over time and space. Moreover, saltbush abundance dictates its inclusion in the diet. Furthermore, roughly a quarter of the diet is comprised of insects, and over the past century, insects have become more prevalent and saltbush less prevalent in the diet. We suggest that environmental factors such as climate change and rangeland expansion have caused D. microps to include more C3 plants and insects.

Response of Four Shrub Species to Different Water Source Components in an Arid Environment

Shrubland species in the Great Basin (USA) depend on soil water recharged from precipitation and/or groundwater for survival and growth. Climate warming and possible basin water diversion could alter the amount and timing of water availability to these plants. The objective of this study was to quantify the extent to which each of four co-occurring shrub species, big sage [Artemisia tridentata], rabbitbrush [Ericameria nauseosus], greasewood [Sarcobatus vermiculatus] and shadscale [Atriplex confertifolia)) acquired water from different sources (precipitation, soil vadose zone and/or groundwater) during a growing season. Soil salinity increased linearly with depth over the upper 1.5 m of soil, with salinity ranging from 0.84 to 31.70 dSm-1 in saturation extracts (R2=0.78, p<0.001). Changes in soil water both with depth and time during the growing period indicated that all species accessed soil water from precipitation recharge. Evapotranspiration totals for the growing period exceeded total precipitation by 137 mm, indicating that plants also used water stored deeper within the vadose zone and/or from groundwater (particularly) by the phreatophyte greasewood. Delta18O in the soil solution declined linearly with depth over the upper 100 cm (R2=0.80, p<0.001). Delta18O values in greasewood corresponded closely to Delta18O values measured deeper in the vadose zone and groundwater. Output from a mixing model indicated a decrease in groundwater reliance for greasewood from 30% in July to 2% in September, with a major shift to deeper soil water in the vadose zone (180 cm depth) (38% in July to 97% in September). Our data suggested that the four shrub species at our site were able to coexist because of their different spatial, temporal, and physiological uses of available soil water, reflecting possible water resource partitioning based on differences in response to precipitation, ability to extract water at deeper depths and variable tolerance to elevated levels of soil salinity to access groundwater.

Spatially explicit patterns in a dryland's soil respiration and relationships with climate, whole plant photosynthesis and soil fertility

Arid and semiarid ecosystems play a significant role in regulating global carbon cycling, yet our understanding of the controls over the dominant pathways of dryland CO2 exchange remains poor. Substantial amounts of dryland soil are not covered by vascular plants and this patchiness in cover has important implications for spatial patterns and controls of carbon cycling. Spatial variation in soil respiration has been attributed to variation in soil moisture, temperature, nutrients and rhizodeposition, while seasonal patterns have been attributed to changes in moisture, temperature and photosynthetic inputs belowground. To characterize how controls over respiration vary spatially and temporally in a dryland ecosystem and to concurrently explore multiple potential controls, we estimated whole plant net photosynthesis (Anet) and soil respiration at four distances from the plant base, as well as corresponding fine root biomass and soil carbon and nitrogen pools, four times during a growing season. To determine if the controls vary between different plant functional types for Colorado Plateau species, measurements were made on the C4 shrub, Atriplex confertifolia, and C3 grass, Achnatherum hymenoides. Soil respiration declined throughout the growing season and diminished with distance from the plant base, though variations in both were much smaller than expected. The strongest relationship was between soil respiration and soil moisture. Soil respiration was correlated with whole plant Anet, although the relationship varied between species and distance from plant base. In the especially dry year of this study we did not observe any consistent correlations between soil respiration and soil carbon or nitrogen pools. Our findings suggest that abiotic factors, especially soil moisture, strongly regulate the response of soil respiration to biotic factors and soil carbon and nitrogen pools in dryland communities and, at least in dry years, may override expected spatial and seasonal patterns.

How specialized is a diet specialist? Niche flexibility and local persistence through time of the Chisel‐toothed kangaroo rat

Summary Rapid environmental changes are putting many species at risk, particularly niche specialists. In response, species can shift their ranges or remain in place by taking advantage of new resources. The potential for specialists to undergo in situ niche shifts is not well understood yet can buffer species from the effects of long‐term environmental change over centuries and millennia. In the Great Basin of western North America, the Chisel‐toothed kangaroo rat, Dipodomys microps, is a folivore thought to be an obligate specialist on the desert shrub Atriplex confertifolia. Because of its association with A. confertifolia, D. microps is presumed to have tracked the shrub as it moved south during the last glacial maximum. However, recent phylogeographic evidence indicates that D. microps did not shift or contract its range into a southern refugium. Here we evaluate the role that niche flexibility may have played in allowing this presumed dietary specialist to cope with a changing environment and resource base. We do so using carbon and nitrogen isotopes measured in D. microps bone collagen from modern and fossil specimens spanning the last 8000 years at Two Ledges Chamber (TLC) in northwestern Nevada. δ13C values indicate that, contrary to expectation, the population of D. microps at TLC consumes a variety of plants other than A. confertifolia, an isotopically distinct C4 shrub, and has done so for millennia. Mixing models suggest that the proportion of C4 in the diet was highest (∼35%) in the middle Holocene, and has declined towards the recent, especially over the last 30 years. δ15N values are consistently elevated through time, suggesting that D. microps at TLC are potentially also consuming a high proportion of insects. Our results indicate that this population of dietary specialists has greater niche flexibility than previously assumed. This implies that, at the species level, even presumed niche specialists may be capable of undergoing niche shifts over centennial to millennial time‐scales in response to changing environmental conditions, and highlights the unique role that historical and paleontological data can play in establishing resource‐use baselines of the past. A lay summary is available for this article.

Community Structure and Demography in a Saline Pan-Dune Mosaic in the Western Mojave Desert

A unique saline ecological system formed by an extensive mosaic of small clay pans and low stable dunes exists within Edwards Air Force Base in the western Mojave Desert of California. This ecosystem lies between the large Rosamond and Rogers dry lakes on the old Pleistocene lakebed of Lake Thompson. Plant communities on the low and relatively stable dunes were broadly classed as saltbush scrub, with a total canopy cover of 30–36%. Atriplex confertifolia was the typical dominant, with Ericameria nauseosa as an important associate. Taller dunes of younger age and less saline soils had similar plant cover, but a distinct plant community with Atriplex canescens and Krascheninnikovia lanata as dominants and Yucca brevifolia as a common associate. Flat areas adjacent to the dunes were dominated by a virtual monoculture of Atriplex confertifolia. Aboveground shrub and bunchgrass biomass including dead material varied from 503 to 1204 kg ha−1, low in comparison to similar plant communities in the Great Basin. The absence of small saplings and seedlings of many shrub species suggests successful establishment is highly episodic. Seedlings were abundant only in Isocoma acradenia.

Plant Phenology and Water Relations in a Saline Pan-Dune Mosaic in the Western Mojave Desert

An extensive mosaic of small clay pans and low stable dunes exists within Edwards Air Force Base in the western Mojave Desert of California. This pan-dune ecosystem, positioned between the large Rosamond and Rogers dry lakes on the old Pleistocene lakebed of Lake Thompson, provides an opportunity to study the seasonal patterns in growth and flowering phenology and water relations in a saltbush scrub community dominated by a mix of C3 and C4 species. C3 shrubs initiated vegetative growth in February, with current shoot and leaf production largely ceasing in most species by mid-April. The deeply rooted Tetradymia stenolepis and Ericameria nauseosa continued vegetative growth through May. C4 species including Atriplex and the grass Distichilis spicata typically began vegetative growth in mid- to late March, 2–6 weeks later than the first group and continued this growth in most cases through June and July. Reproductive growth commonly did not begin until June and continued into July and/or August. Shrub species showed a general pattern of slowly declining water potentials (increased stress) through the late spring and summer months, reflecting more limited soil water availability. Smaller changes in predawn water potential over this period were seen in the deeply rooted species. Soil moisture availability in the widespread Atriplex confertifolia showed a pattern of variation between pan, dune and open flat microhabtats.

Patterns of Water Use by Great Basin Plant Species Under Summer Watering

We analyzed temporal and spatial patterns of water use by a functionally-diverse group of Great Basin plant species and determined their water use rates at the whole-plant and individual-leaf scales under variable summer watering. Species studied were the desert grasses Distichlis spicata and Sporobolus airoides, the desert shrubs Artemisia tridentata, Ericameria nauseosa, and Atriplex confertifolia; the wetland/riparian plants Juncus arcticus, Leymus triticoides, and Salix exigua; and the annual exotic Salsola tragus. Plant species were individually grown in 5.8 m2 plots in a common garden in eastern California. Three irrigation treatments in the form of monthly pulses were applied during the summer: low (1.3 cm), medium (2.6 cm), and high (3.9 cm), in addition to a nonirrigated control. Whole-plant water uptake characteristics were determined by soil water depletion at different soil depths, while leaf transpiration was determined by gas exchange. Whole-plant water extraction and leaf transpiration varied similarly among species. Desert shrubs had low water extraction (35 to 395 g m−2 day−1) and were not affected by irrigation. The desert grasses and riparian/wetland species had higher water extraction, increasing with irrigation levels. L. triticoides and J. arcticus had the highest water extraction overall (>2,000 g m−2 day−1). Desert shrubs relied 10 times more on deeper water sources than herbaceous species. The average T/ET was 31%, but varied by species. Summer available water in environments such as the Great Basin favors desert grasses and riparian/wetland species, but not desert shrubs. The observed species differences provide alternatives for water and vegetation management.

Growth, water productivity, and biomass allocation of Great Basin plants as affected by summer watering

ABSTRACTThis study measured water‐use efficiency of wildland vegetation at the whole‐plant level during two contrasting years to better understand ecosystem responses to precipitation fluctuations in the Great Basin, USA. Biome‐representative species included grasses (Distichlis spicata, Leymus triticoides, and Sporobolus airoides), desert shrubs (Artemisia tridentata, Atriplex confertifolia, and Ericameria nauseosa), wetland/riparian plants (Glycyrrhiza lepidota, Juncus arcticus, and Salix exigua), and an exotic annual (Salsola tragus). Plants were grown in 5·8 m2 plots in a common garden in eastern California. Four watering treatments were applied monthly during two summers: control (no water other than natural precipitation), low (1·3 cm), medium (2·6 cm), and high (3·9 cm). Water‐use efficiency, here termed water to production (WTPa), was the ratio of water transpired to aboveground biomass produced. Biomass production was 50% lower and WTPa was five times higher during 2009 than 2010. WTPa decreased with watering during 2009 but increased with watering during 2010. Year differences determined vegetation productivity and response to summer watering and were related to the lower winter/spring precipitation during 2009 than 2010. Desert shrubs were more drought tolerant than grasses and wetland plants. Yet, an increase in summer precipitation would primarily benefit herbaceous species and not desert shrubs. Desert shrubs achieved greater standing crop but lower root‐to‐shoot ratio (RSR) than herbaceous species. Nonetheless, the grass S. airoides had the greatest standing crop overall, mainly because of its greater root production (RSR 5·5). Species differences in growth, WTPa, and biomass allocation should be considered in land management and conservation practices. Copyright © 2012 John Wiley & Sons, Ltd.

Natural Polyploidization within Tetraploid and Hexaploid Populations of the Desert ShrubAtriplex confertifolia

ABSTRACT. Shadscale (Atriplex confertifolia) is a wind-pollinated dioecious shrub of western North America with an unusual development of apparently autoploid races, showing all even ploidy levels from 2x to 12x (base x = 9). Of these, tetraploid races are the most frequently encountered, with octoploids the next most common, and hexaploids being much less common. In this study, the occurrence of neopolyploid individuals within tetraploid and hexaploid populations of shadscale was examined at natural sites in order to investigate the pathways by which octoploids may be formed and to find a possible explanation for the predominance of octoploids over hexaploids. The frequencies of apparent neopolyploid individuals among adults and the frequencies of their backcrosses were tabulated, as was the occurrence of neopolyploidy among seeds of female plants from some of the same populations. The neopolyploids encountered in these surveys were almost exclusively those expected from unions involving single unreduced...

Landform, soil, and plant relationships to nitrate accumulation, Central Nevada

Nitrate (NO 3 − 1 ) accumulates in Haplocambids and Torrifluvents in inset fan and fan skirt positions in central Nevada. The soils store as much as 17,600 kg of NO 3 − 1 N ha − 1 within the upper 208 cm. This paper provides an explanation. These Holocene soils receive NO 3 − 1 N from mineralization of organic matter and other NO 3 − 1 N sources including snowmelt. The NO 3 − 1 is delivered to soils in the first part of snowmelt in run-off from the higher surfaces. The last part of the melt and the run-off, when sufficient, serve to move the NO 3 − 1 out of the root zone. Winter fat (Krascheninnikovia lanata), the most valuable winter grazing plant in the Great Basin, is the common plant on NO 3 − 1 N rich soils. The soils are loamy or sandy and lack horizons restricting water penetration or biological denitrification zones. Hence, some NO 3 − 1 is free to leach deeply past plant roots. Playas, wet floodplains, deeply gullied inset fans and well-developed soils accumulate little NO 3 − 1 except where the latter soils are capped by desert pavements and rarely, if ever become saturated with water. Soils with argillic or petrocalcic horizons or duripans on summits of alluvial fan remnants loose NO 3 − 1 through denitrification, or incorporate it in plants, commonly accumulating less than 50 kg of NO 3 − 1 N ha − 1 . These soils however do accumulate salt as shown by their shadscale saltbush Atriplex confertifolia, bud sagebrush Picrothamnus desertorum, and four-wing saltbush Atriplex conescens shrub cover.

Cytotoxicity of Atriplex confertifolia.

The search for cancer treatment continues to be a global effort. As part of this global effort, many natural products have been tested against cancer cell lines, mostly from tropically located plants. This study reports that extracts of Atriplex confertifolia (Torr. and Frem.) S. Watson (Chenopodiaceae), a native North American plant (also known as shadscale or saltbush), has significant bioactivity against human breast cancer cell lines MCF-7, MDA-MB 435, MDA-MB 231, and HeLa cells (cervical cancer cells). The bioactivity of A. confertifolia extracts on these cells lines was compared to an FDA-approved cancer drug (Onxol®) and an industry-standard leukocyte control cell line. Active portions of the extracts were found primarily in the polar fractions of the plant. A dose-response curve of the extracts displayed significant cell death similar to Onxol®. The plant extracts did not significantly inhibit the viability of the leukocyte cell line. In a timed study, over 90% of cell lines MDA-MB 435 and HeLa died after 24 hours. Cell death appears to result from apoptosis.

Effects of fire and environmental variables on plant structure and composition in grazed salt desert shrublands of the Great Basin (USA)

Fire in shrub-dominated portions of the Great Basin, largely fueled by non-native annuals such as Bromus tectorum, has become an important structuring force altering vegetation composition and soil characteristics. The extent to which fire affects native species in drier portions of the Great Basin, termed salt desert, is poorly documented. We conducted a survey of grazed salt desert habitat in northwestern Nevada 5 years after wildfires burned 650,000 ha, with the goal of investigating community response to fire and factors correlating with post-fire recovery. We found that recruitment of a dominant shrub, Artemisia spinescens, is severely restricted following fire: it occurred in only 2 of the 24 burned sites. The co-dominant shrub, Atriplex confertifolia, occurred in most burned sites although on average its percent cover was one-third lower than adjacent unburned sites. Biotic soil crust cover was four times lower, and non-native species cover 5 times higher, in burned sites compared to unburned. Ordination analyses confirmed differences among plant communities in burned versus unburned sites, with environmental variables soil conductivity, plant litter, soil potassium (K +) and pH explaining 38% of the variance in community composition. However, we found no environmental predictors of recovery for native species in burned sites. Future recruitment is likely to be further limited, as fire frequency in the salt desert is expected to increase with invasion by non-native annual grasses and with global climate change.

Comparison of respiratory and growth characteristics of two co-occurring shrubs from a cold desert, Coleogyne ramosissima (blackbrush) and Atriplex confertifolia (shadscale)

Coleogyne ramosissima Torr. (blackbrush) and Atriplex confertifolia [Torr. & Frem.] Wats. (shadscale) are cold desert shrubs from different families. Despite very different life histories they often grow in close geographic proximity in the Great Basin and the Colorado Plateau between 800 and 2000m elevation. The purpose of this study is to compare the ecophysiology of slow growing and reproducing blackbrush with the ecophysiology of faster growing and reproducing shadscale. Metabolic heat and carbon dioxide production rates were measured on leaf tissue from wild plants and on lab-grown seedlings at temperatures from 10 to 35°C at 5°C intervals. Heat of combustion, ash content, and carbon and nitrogen contents were also measured. Substrate carbon conversion efficiencies and anabolic (or growth) rates were calculated from the respiration data. The growth rate of blackbrush was found to be approximately half that of shadscale because of lower respiration rate, but blackbrush begins growing earlier in the spring and can grow at higher temperatures when water is available. Blackbrush was observed to reproduce heavily when winter and spring precipitation is abundant.

Cheatgrass Invasion in Salt Desert Shrublands: Benefits of Postfire Reclamation

Abstract In 1998, fires burned more than 11 330 ha of rangeland on Dugway Proving Ground in Utah's west desert. Postfire revegetation was implemented in 2 affected salt desert shrub communities (greasewood; Sarcobatus vermiculatus Hook. and black sagebrush/shadscale; Artemisia nova A. Nels; Atriplex confertifolia Torr. & Frem.) to deter cheatgrass ( Bromus tectorum L.) encroachment. We monitored cheatgrass densities for 3 years after the fire in burned drill seeded, burned not-seeded, and unburned plots to assess the rate of invasion and determine the impact on cheatgrass of drill seeding perennial species. Cheatgrass invaded quickly in both shrub sites following the fires. In the greasewood site, drill seeded species germinated but did not establish. This was likely due to a combination of soil salinity and extremely dry weather conditions during the second year of the study. Drill seeded species in the black sagebrush site germinated and established well, resulting in the establishment of 16.5 perennial grasses · m -2 and 1 356 shrubs · ha -1 . Cheatgrass densities were consistently lower in drill seeded versus not-seeded plots, although these were not always statistically different when Bonferroni comparisons were considered. The initial decrease in cheatgrass densities in drill seeded plots may have resulted from soil disturbance coupled with extremely low precipitation rather than competitive effects. Nevertheless, as seeded species mature and increase their competitive ability, we predict long-term suppression of cheatgrass in the absence of further disturbance.

Influence of temporal heterogeneity in nitrogen supply on competitive interactions in a desert shrub community

Soil nutrients in arid systems are supplied to plants in brief pulses following precipitation inputs. While these resource dynamics have been well documented, little is known about how this temporal heterogeneity influences competitive interactions. We examined the impacts of the temporal pattern of N supply on competitive intensity and ability in an N-limited desert shrub community. At our field site, the three codominant shrubs, Atriplex confertifolia, A. parryi, and Sarcobatus vermiculatus, differ in seasonal growth patterns, with A. confertifolia and S. vermiculatus achieving higher growth rates earlier in the growing season than A. parryi. We predicted that these timing differences in maximum growth rate may interact with temporal variation in N supply to alter competitive abilities over time. Seedlings of the two Atriplex species were planted either individually in field plots or as target plants surrounded by neighbor seedlings. After one year of establishment, the same amount of (15)N was applied to plots either as early spring pulses, mid spring pulses or continuously through the second growing season. Competitive effects were observed under continuous and pulsed N supply. Averaged across all target-neighbor treatments, competitive intensity was approximately 1.8-fold greater when N was pulsed compared to when N was supplied continuously, but overall, the outcome of competitive interactions was not influenced by N pulse timing. While the timing of resource supply did not differentially influence the competitive abilities of coexisting species in this system, the temporal pattern of resource supply did alter the intensity of competitive interactions among species. While additional studies in other systems are needed to evaluate the generality of these results, this study suggests that competitive intensity may not necessarily be a direct function of productivity or resource availability as traditionally assumed. Instead, the intensity of competitive interactions in resource-poor systems may depend upon the temporal pattern of resource supply.

Impact of the pathogen Pyrenophora semeniperda on Bromus tectorum seedbank dynamics in North American cold deserts

Summary Bromus tectorum is a dominant winter annual weed in cold deserts of western North America. We followed patterns of seed carry‐over and abundance of the pathogen Pyrenophora semeniperda over 5 years at B. tectorum‐dominated shadscale (Atriplex confertifolia) and sagebrush (Artemisia tridentata) sites in southern Idaho. We hypothesised that more seeds could potentially carry over at the drier shadscale site because of minimal autumn precipitation, but that P. semeniperda, a pathogen that primarily kills dormant seeds, would have more impact at the drier site, where a higher density of dormant seeds would likely be present in the early spring seedbank. Successful first‐year seed carry‐over was higher in years with below‐average autumn precipitation. It was lower at the shadscale site than at the sagebrush site (9% vs.16%). The number of seeds killed during incubation by P. semeniperda averaged three times higher at the drier site and the number of field‐killed seeds averaged almost five times higher. This suggests that pathogen‐related mortality caused the greater decrease in seed carry‐over at the drier site. Mortality risk increased dramatically with seed age. This climate–pathogen interaction apparently limits B. tectorum seedbank carry‐over in cold deserts to 3 years or less. Pyrenophora semeniperda shows potential as a biocontrol agent for B. tectorum in these habitats.

Seasonal timing of N pulses influences N capture in a saltbush scrub community

Limiting resources are generally available in brief temporal pulses in arid systems. We compared the abilities of dominant shrubs in a saltbush scrub community to capture N from pulses and evaluated whether N capture and partitioning within this community is influenced by the seasonal timing of pulses. Based on previous research in agronomic systems we predicted that the ability of a species to capture N following a pulse would depend on when the pulse occurred in relation to plant growth rate and N demand. Supporting this hypothesis, Atriplex confertifolia and Sarcobatus vermiculatus, which had greater growth rates early in the growing season compared to Atriplex parryi, captured more N from early spring pulses than A. parryi. Atriplex parryi, which had higher growth rates later in the growing season, captured more N from mid- and late spring pulses than the other species. These temporal differences in N capture among species, however, also depended on the magnitude of the N pulse. These results suggest that temporal variation in N availability may differentially impact competitive abilities of coexisting species and potentially facilitate species coexistence in arid systems.

Influence of resource pulses and perennial neighbors on the establishment of an invasive annual grass in the Mojave Desert

Invasion by exotic annual grasses is one of the most significant threats to arid ecosystems in the western USA. Current theories of invasibility predict plant communities become more susceptible to invasion whenever there is an increase in the amount of unused resources. The objective of this field study was to examine how resource pulses and temporal variation in resource demand by the native shrub vegetation influences establishment of the invasive annual grass Schismus arabicus. Water and nitrogen were applied as pulses in early spring, mid-spring, or continuously throughout the growing season to plots containing either Atriplex confertifolia or Atriplex parryi shrubs. The effect of resource pulses on Schismus density and biomass was highly dependent on the seasonal timing of the resource pulses and the identity of the neighbor shrub. When resource pulses coincided with high rates of resource capture and growth of the native vegetation, density and biomass of Schismus was reduced. Schismus establishment was greater under continuous resource supply compared to pulsed resource supply, likely because more soil resources were available at a shallow depth when resources were supplied at a continuous low rate. These results suggest that the establishment of invasive annual grasses in arid systems can be influenced by the magnitude and spatial distribution of resource pulses in addition to the seasonal timing of resource pulses.

Development of vegetation in the Central Mojave Desert of California during the late Quaternary

Vegetation analysis of 47 Neotoma (packrat) middens from the core of the central Mojave Desert of California reveal changes in desert plant community composition over the last ca. 24,000 years BP, one of the lowest and most arid locations in North America. Habitat currently dominated by Mojave Desert Larrea tridentata (creosote bush) was occupied prior to ca. 11,500 years BP by Pinus monophylla (piñon), Juniperus osteosperma (Utah juniper), Purshia mexicana (bitterbush), Cercocarpus ledifolius (mountain mahogany), and Prunus fasciculata (desert almond) woodland above 1000 m. P. monophylla– Juniperus woodland was widespread over the southern and central Mojave Desert. However, less than 150 km north of the central Mojave, J. osteosperma steppe dominated the landscape. Paleoecologic records spanning latitude 35°N ca. 37°N document a biogeographic boundary between milder, moister environments to the south and the colder, drier environments to the north. At elevations <1000 m desert scrub species of Lycium cooperi (wolfberry), Salvia mojavensis (Mojave sage) and the mesophytic Prosopis juliflora (mesquite) occurred near the shore of pluvial Lake Mojave from ca. 8810 to 8430 years BP. Larrea is not recorded on the xeric carbonate substrates (605 m) until ca. 6990 years BP, then took over 2000 years to dominate the regional landscape. Modern vegetation associates such as Thamnosma montana (desert rue) and Atriplex confertifolia (shadscale) were established by ca. 4500 years BP. Late Holocene vegetation changes document Neoglacial cooling and wetter conditions than today between ca. 3490 and 2640 years BP, paralleling the regional pluvial lake highstands and marsh deposit records. Decreases of mesophytic Mojave Desert associates in the Granite Mountains suggest climates similar to today's aridity between ca. 1970 and 1440 years BP. Mesophytic species increase in middens dating between ca. 1390 and 700 years BP at higher elevation sites; during the same time Prosopis occurs at lower elevation sites.