Mojave Desert Shrubs Research Articles

Water use of two Mojave Desert shrubs under elevated CO2

SummaryPlant responses to elevated atmospheric CO2 have been characterized generally by stomatal closure and enhanced growth rates. These responses are being increasingly incorporated into global climate models that quantify interactions between the biosphere and atmosphere, altering climate predictions from simpler physically based models. However, current information on CO2 responses has been gathered primarily from studies of crop and temperate forest species. In order to apply responses of vegetation to global predictions, CO2 responses in other commonly occurring biomes must be studied. A Free Air CO2 Enrichment (FACE) study is currently underway to examine plant responses to high CO2 in a natural, undisturbed Mojave Desert ecosystem in Nevada, USA. Here we present findings from this study, and its companion glasshouse experiment, demonstrating that field‐grown Ephedra nevadensis and glasshouse‐grown Larrea tridentata responded to high CO2 with reductions in the ratio of transpirational surface area to sapwood area (LSR) of 33% and 60%, respectively. Thus, leaf‐specific hydraulic conductivity increased and stomatal conductance remained constant or was increased under elevated CO2. Field‐grown Larrea did not show a reduced LSR under high CO2, and stomatal conductance was reduced in the high CO2 treatment, although the effect was apparent only under conditions of unusually high soil moisture. Both findings suggest that the common paradigm of 20–50% reductions in stomatal conductance under high CO2 may not be applicable to arid ecosystems under most conditions.

Soil moisture extraction by evergreen and drought-deciduous shrubs in the Mojave Desert during wet and dry years

Annual and seasonal evapo-transpiration (ET) were compared among Mojave Desert shrubs with different leaf phenologies over a 3-year period during which annual precipitation varied from well below average to more than twice average. During the wet year, soil wetting fronts reached maximum depths of 0·75 m to >1·95 m, depending on soil texture at the study sites. The evergreen shrubs Larrea tridentata and Ephedra nevadensis , and the drought-deciduous shrubAmbrosia dumosa , were able to extract soil water in a uniform manner to depths >1 m. For stands of the deciduous shrubLycium pallidum , a soil texture change at c. 0·75 m impeded percolation of water below that depth. There were no significant differences (p<0·05) in annual ET between the evergreen shrubs Larrea and Ephedra relative to the drought-deciduous shrubs Ambrosia and Lycium during the 3 years of the study. Early in the growing season, extraction of soil water from beneath plant canopies was slightly greater than from shrub interspaces forAmbrosia , Ephedra , and Lycium , but not for Larrea . For all species, annual soil water extraction from beneath plant canopies was not significantly different than that from shrub interspaces. The lower limit of soil water extraction (Le) for the study sites varied from 4 to 10 volumetric per cent, depending on soil texture, and did not differ significantly among species. For all species, Lewas reached within 6 to 12 months following twice average precipitation during the period of November 1994 to March 1995. We conclude that ET in the Mojave Desert is dependent largely on winter precipitation and the amount of soil water available during the growing season rather than on species composition.

Photosynthetic down‐regulation in Larrea tridentata exposed to elevated atmospheric CO2: interaction with drought under glasshouse and field (FACE) exposure

The photosynthetic response of Larrea tridentata Cav., an evergreen Mojave Desert shrub, to elevated atmospheric CO2 and drought was examined to assist in the understanding of how plants from water‐limited ecosystems will respond to rising CO2. We hypothesized that photosynthetic down‐regulation would disappear during periods of water limitation, and would, therefore, likely be a seasonally transient event. To test this we measured photosynthetic, water relations and fluorescence responses during periods of increased and decreased water availability in two different treatment implementations: (1) from seedlings exposed to 360, 550, and 700 μmol mol–1 CO2 in a glasshouse; and (2) from intact adults exposed to 360 and 550 μmol mol–1 CO2 at the Nevada Desert FACE (Free Air CO2 Enrichment) Facility. FACE and glasshouse well‐watered Larrea significantly down‐regulated photosynthesis at elevated CO2, reducing maximum photosynthetic rate (Amax), carboxylation efficiency (CE), and Rubisco catalytic sites, whereas droughted Larrea showed a differing response depending on treatment technique. Amax and CE were lower in droughted Larrea compared with well‐watered plants, and CO2 had no effect on these reduced photosynthetic parameters. However, Rubisco catalytic sites decreased in droughted Larrea at elevated CO2. Operating Ci increased at elevated CO2 in droughted plants, resulting in greater photosynthetic rates at elevated CO2 as compared with ambient CO2. In well‐watered plants, the changes in operating Ci, CE and Amax resulted in similar photosynthetic rates across CO2 treatments. Our results suggest that drought can diminish photosynthetic down‐regulation to elevated CO2 in Larrea, resulting in seasonally transient patterns of enhanced carbon gain. These results suggest that water status may ultimately control the photosynthetic response of desert systems to rising CO2.

Surface Dust Impacts on Gas Exchange in Mojave Desert Shrubs

Windblown dust, an environmental problem in many disturbed arid lands, has the potential to affect the physiological performance of desert shrubs. Physiological parameters of gas exchange for three species (Larrea tridentata, Hymenoclea salsola and Atriplex canescens) were measured at a Mojave Desert site, at which both undisturbed and heavily dusted individual shrubs occurred. Maximum rates of net photosynthesis (A) of dusted organs were reduced to 21% of those of control plants in resinous leaflets of Larrea, to 44% in resinous leaves and photosynthetic stems of Hymenoclea, and to 58% in non-resinous C 4 leaves of Atriplex, which have vesiculated trichomes. Dusted plants of all three species showed reduced maximum leaf conductance (g s ), transpiration (E) and instantaneous water-use efficiency (A/E). Intrinsic water-use efficiency (A/g s ) was also reduced, except in Atriplex, in which it remained unchanged. Temperatures of dusted leaves and photosynthetic stems were 2-3°C higher than those of control plants, due to greater absorptance of infra-red radiation. Dust also significantly increased photosynthetically active radiation (PAR) reflectance. Heavily dusted shrubs had smaller leaf areas and greater leaf-specific masses, suggesting that the short-term effects of reduced photosynthesis and decreased water-use efficiency may cause lowered primary production in desert plants exposed to dust during seasons when photosynthesis is occurring.

Root Communication Mechanisms and Intracommunity Distributions of Two Mojave Desert Shrubs

Experimental studies using root observation chambers to observe the effects of encounters between individual roots on root elongation rates have revealed that the interactions among roots of Ambrosia dumosa and Larrea tridentata are more complex than simple competition for a limiting resource. Larrea roots inhibited elongation of either Larrea or Ambrosia roots in their vicinity, and Ambrosia roots inhibited elongation of contacted roots on other Ambrosia plants only. The purpose of the study reported here was to test the hypothesized involvement of inhibitory substances released by roots in these interroot encounters by attempting to remove such substances by adsorption to activated carbon. The presence of activity carbon caused a significant decrease in the inhibition of elongation of neighboring roots by Larrea roots, but activated carbon had no effect on the intraspecific responses of Ambrosia roots. These results support the hypotheses that the interaction mechanism of Larrea roots involves the release of a readily diffusible, generally inhibitory substance by Larrea roots into the soil, rather than a simple depletion of water or nutrients from around Larrea roots, and that the intraspecific, self—nonselfish—recognizing interaction mechanism of Ambrosia roots is mediated by contact and is fundamentally different from that of Larrea. These findings may enhance our understanding of Mojave desert community structure. The root—mediated allelopathy of Larrea may play a role in producing and maintaining the commonly occurring, regular distributions of Larrea. The complex communication mechanism of Ambrosia roots appears to constitute a detection and avoidance system that may allow this shrub to grow in clumped intraspecific distributions with little or no intraspecific competition for water. The interspecific interference between Larrea and Ambrosia in the field may be mechanistically asymmetrical due to their different root communication mechanisms.

Dynamics of Mojave Desert Shrub Assemblages in the Panamint Mountains, California

We studied shrub communities in the Panamint Mountains of the Mojave Desert to determine whether vegetational changes after disturbance can be characterized as succession according to modern successional theory. We found, on a variety of disturbed and undisturbed sites, that the rate of change was a function of the type and age of disturbance. Recent debris—flow deposits were colonized by shrub assemblages of different species composition than those on the surrounding, older debris—flow deposits and other geomorphically stable surfaces. Colonization of human—disturbed sites was highly variable, but species compositions were different from the predisturbance species composition. In Grayia—Lycium assemblages, Grayia spinosa reasserted its dominance over colonizers relatively quickly. In Coleogyne assemblages, typically found on older geomorphic surfaces, species composition differences persisted considerably longer, depending on the severity of the initial disturbance. Extremely stable assemblages, dominated by Coleogyne ramosissima, occurred on the oldest, least disturbed surfaces. The variability of species composition among disturbed sites was greater than the variability among undisturbed and geomorphically stable sites, in accord with convergent succession. Models of desert succession should consider several factors: (1) colonization is dependent largely on the severity of disturbances and residual biotic components; (2) the time span for recovery may be longer than past periods of climatic and geomorphic stability; and (3) colonizing species may have considerable range in their life—history strategies.

The Effect of Axis Splitting on Xylem Pressure Potentials and Water Movement in the Desert Shrub Ambrosia dumosa (Gray) Payne (Asteraceae)

Predawn xylem pressure potential (XPP) measurements were used to investigate the effects of axis splitting on water relations of the Mojave Desert shrub Ambrosia dumosa. Differences among branches within shrubs were significant in all large shrubs with split axes at least once during the summer but not in intermediate or small shrubs with unsplit axes. A subset of plants received 24 liters of water delivered to one side of the plants from buckets every other week during the summer. Two of three large, watered shrubs showed significant differences among branches at least once during the summer, but XPPs were not higher than in unwatered shrubs. No significant differences among branches of intermediate and small, watered shrubs occurred; both intermediate and small shrubs, however, showed increased XPPs and produced small, leafy, terminal shoots in response to being watered. The pathway of water movement was investigated with a dye that ascended sectorially in all plants.

Density and Diversity in Mojave Desert Rodent and Shrub Communities

The mechanisms allowing for coexistence of animal species, and particularly the number of species coexisting in a given habitat, have been explored from many viewpoints. Evolutionary ecologists have investigated the process whereby the species has acquired, the necessary adaptations for exploitation of the habitat (e.g. Bowman 1961; Selander 1966). Biogeographical ecologists have explored the packing of species in an environment and the effects of speciation and immigration upon community composition (e.g. Pianka 1967; Brown 1973). Thirdly, community ecologists have examined species interactions within a relatively stable community to determine how resources are allocated among the species (e.g. MacArthur & MacArthur 1961; Paine 1966; Brown & Lieberman 1973). Most of these studies have concluded that the species composition of a community is the resultant of a complex interaction between all three parameters, i.e. evolutionary, biogeographical and ecological factors. Accordingly, researchers such as Pianka (1967) and Brown (1973) have chosen to simultaneously analyse the effects of all three parameters upon animal species diversity. An alternative method of community analysis, wherein one of the three parameters is observed in closer detail, requires that the remaining two factors be conclusively eliminated as variables. In this study I have chosen to focus upon the ecological mechanisms of granivorous rodent coexistence in the Mojave Desert of southern California. The study transect has been selected to range over a geographically small area of physically similar desert habit such that all rodent species of the study are (at least potentially) evolutionarily and biogeographically able to exploit all study areas. The rodent complpment of each of the study areas is therefore determined by the interactions of the ecologies of the species and, particularly, the effect of certain varying parameters (such as rainfall and seed abundance) upon each species' competitive success. Granivorous rodent species of desert habitats vary remarkably little in general behavior and morphology. The rigors of the desert environment have necessitated physiological adaptations in most species for existence with little or no free water intake. High diurnal temperatures have required that nearly all North American desert rodent species forage at night and spend the day in burrows. Additionally, anti-predatory manoeuvers in areas of sparse vegetation have necessitated adaptations in most species for rapid locomotion in open spaces. The net result of these and other adaptations for desert survival in rodents is convergence toward very similar ecologies. It is the intent of this study to analyze the abundance and diversity of rodents in several Mojave Desert communities in the attempt to uncover predictable patterns related to

A Comparison of Seasonal Primary Production of Mojave Desert Shrubs During Wet and Dry Years

Estimates of net primary productivity were made for five major shrub species in the northern Mojave Desert during 2 years of contrasting moisture regimes using harvest and gas exchange techniques. Production increased by 224% (harvest) to 260% (gas exchange) in 1973 over 1972. Production gains varied among species depending on whether the species was favored by the cool, moist spring of 1973. Lycium andersonii and L. pallidum, which have high photosynthetic rates and start growth in the early spring, showed the greatest increases. Larrea tridentata and Krameria parvifolia, species adapted to growing under warmer, drier conditions, exhibited the smallest changes in production. Ambrosia dumosa, which has high photosynthetic rates but responds more favorably to warmer spring temperatures than the Lycium species, had moderate production gains. Reasons for the different estimates obtained by these two methods are discussed. INTRODUCTION The determination of primary productivity in terrestrial ecosystems is a necessary but difficult step in providing information on the basic. energy or carbon input into biological systems. Productivity estimates are important for predictive models of a general nature in ecosystem analysis. In general, two methods have been used for net primary production estimates. The first method is a direct harvest technique in which whole plants or samples are clipped throughout the growing season, dried and weighed, and productivity calculated on dry weight per unit area. The second method involves measuring the total CO2 or carbon exchange per weight of photosynthetic organs, and calculating productivity as carbon or biomass fixed minus respiratory losses. There have been several measurements of net primary production and biomass in deserts made by the harvest and other methods (Burk and Dick-Peddie, 1973; Chew and Chew, 1965; Pearson, 1965; Rodin and Bazilevich, 1967). Measurements of CO2 exchange rates in desert plants have been made in the Negev Desert (Lange et al., 1969) and in the North American deserts (Bamberg et al., 1975; Caldwell, 1972; Patten and Dinger, 1969; Strain, 1969, 1970; Strain and Chase, 1966). Attempts at relating some of these gas exchange rates to production are only now being done. The US/IBP Desert Biome is currently obtaining production estimates from several sites in the western United States. 1 Present address: EG&G Environmental Consultants, 9025 E. Kenyon Avenue, Denver, Colorado 80237. 2 Present address: University of Idaho, Research and Extension Center, Route 1, Kimberly, Idaho 83341.

Response of the desert shrub Krameria parvifolia after ten years of chronic gamma irradiation

A northern Mojave Desert shrub community was irradiated by a 137Cs source for a ten-year period. Leaf and fruit production, cover, and percent live stem of Krameria parvifolia shrubs were found to respond significantly to a radiation gradient with exposure rates ranging from 0·1 to 10 R/day. Fruit and leaf production were greatly reduced at exposures over 6 R/day. Above 7 R/day 16% of the shrubs were dead compared to 1·2% in a non-irradiated area. Reduced cover, density and live stem values indicate a trend toward a lower status of Krameria in the community at cumulative exposures above 25 kR. Observations indicate that an equilibrium in response to irradiation has not yet occurred. Radiosensitivity of K. parvifolia is attributed in part to its phenology.

Seed Treatment of Mojave Desert Shrubs1

AbstractInformation is needed for revegetation of barren scars left from destruction of native shrubs by recreational vehicles and right‐of‐way clearance. Numerous seed treatments were tested for improvement of field establishment of Mojave Desert shrubs and reduction of establishment costs.Scarification, 4‐hour 60 C and 80 C dry heat, stratification, activated carbon, 3‐hour and 15‐hour soaking, 100 ppm and 1,000 ppm ethylene, hot water (5 min), 0.5 and 6% sodium hypochlorite, 3% thiourea, and 6% hydrogen peroxide were used on the following species considered promising for revegetation: 1) Larrea divaricata Cav., 2) Atriplex polyycarpa (Torr.) Wats., 3) Atriplex canescens (Pursh.) Hutt., 4) Hymenoclea salsola T. &amp; C., 5) Ephedra nevadensis Wats., 6) Ambrosia dumosa (Gray) Payne Franseria dumosa), and 7) Lepidospartum squamatum Gray.Activated carbon was the most effective treatment in improving fruit germination on 5 of 6 species tested (spp. 1, 2, 4, 5, 6). Stratification was effective on improving the germination of 3 of 6 species (spp. 2, 3, 6). Thiourea (3%) was effective in improving the germination of both the carpels and seeds of Larrea divayicata, The 15‐hour soaking, 100 ppm ethylene, 4‐hour 60 C, and scarification treatments improved the germination of only one species each. Treatments having no effect on germination were 3‐hour soaking, 6% hydrogen peroxide, and 1,000 ppm ethylene. The 4‐hour 80 C dry heat depressed the germination of 5 of the 7 species and 6% sodium hypochlorite depressed the germination on all species tested. Hot water (5 min) killed the embryos on all species tested.

Comparative Photosynthetic Production of Mojave Desert Shrubs

Transpirational and net photosynthetic rates of several species of desert shrubs were measured as a function of season and environmental variables at the Nevada Test Site in the northern Mojave Desert. Drought—deciduous species, Ambrosia dumosa (Grey) Payne, Lycium andersonii Grey, and Lycium pallidum Miers, had higher maximum rates and greater water loss than the evergreen, Larrea tridentata Munz, and summer green, Krameria parvifolia Benth., species. Moisture status was the most critical factor determining gas exchange rates and affected temperatre optima and acclimation as the season progressed. Because of a dry spring season, the drought—deciduous species became dormant in late May—early June: the other two species exhibited by mid—June a small positive CO2 uptake during the morning period. Desert plant species, with few exceptions, are extremely labile and exhibit large variability and different adaptive strategies.