Response of sagebrush steppe species to elevated CO 2 and soil temperature

Elevated atmospheric CO 2 may cause long-term changes in the productivity and species composition of the sagebrush steppe. Few studies, however, have evaluated the effects of increased CO 2 on growth and physiology of species important to this ecosystem. Since the response of plants to elevated CO 2 may be limited by environmental factors, soil temperature was also examined to determine if low soil temperatures limit CO 2 response. To determine how CO 2 and soil temperature affect the growth of species native to the sagebrush steppe, bottlebrush squirreltail [ Elymus elymoides (Raf.) Swezey], Thurber needlegrass ( Stipa thurberiana Piper), and Wyoming big sagebrush ( Artemisia tridentata ssp. wyomingensis Beetle) were grown in ambient (374 mL L -1 ) or high (567 mL L -1 ) CO 2 and low (13° C) or high (18° C) soil temperature for approximately 4 months. Although soil temperature affected the growth of squirreltail and needlegrass, temperature did not modify their response to elevated CO 2 . Total biomass of sagebrush was consistent across soil temperature and CO 2 treatments, reflecting its slow-growing strategy. All 3 species had higher leaf water-use efficiency at elevated CO 2 due to higher net photosynthesis and lower transpiration rates. We conclude that elevated CO 2 and soil warming may increase the growth of grasses more than shrubs. Field studies in the sagebrush steppe are necessary to determine if differences in biomass, resulting from changes in CO 2 and soil temperature, are exhibited in the field.     

Potential contribution of respiration by Anabrus simplex (Mormon crickets) to net CO 2 exchange in three Great Basin ecosystems

Disturbance events can significantly influence net CO 2 exchange (NCE) in ecosystems. High densities of Anabrus simplex (Mormon crickets) periodically afflict large areas of the western USA; their sheer numbers could make them a significant source of CO 2 . We modeled cricket respiration at the ecosystem level using air and body temperatures and insect gas exchange measurements. Cricket CO 2 efflux values were compared to ecosystem CO 2 flux from eddy covariance measurements in 3 Great Basin ecosystems: a juniper woodland, a sagebrush shrubland, and a crested wheatgrass pasture. Mean respiration from Mormon crickets was 0.96 g CO 2 ? m –2 d –1 . Since Mormon crickets are present when NCE is otherwise near 0, they can potentially alter NCE between 20% (juniper woodland) and 60% (crested wheatgrass pasture). Transient pests such as Mormon crickets can be an important component of NCE.     

High rates of photosynthesis in the desert shrub Chrysothamnus nauseosus ssp. albicaulis

  Basic aspects of photosynthesis were investigated in white rubber rabbitbrush ( Chrysothamnus nauseosus (Pallas) Britt. ssp. albicaulis ), a common C 3 deciduous shrub native to arid regions of the western U.S. Under favorable field conditions, net photosynthesis (P n ) ranged from 36 to 73 mgCO 2 · dm – 2 · hr – 1 , which is relatively high for a woody species. The leaves from the actively growing flowering shoots exhibited higher P n than those on the vegetative shoots. P n also varied according to the age of the leaves and the location of the plants. P n did not light saturate even at quantum flux densities (QFD) equivalent to full sunlight. The light compensation point was relatively high (ca 100 μ mol · m – 2 · S – 1 ), perhaps due to the presence of a tomentose vestiture on the leaf surface. At high QFD's, the stomatal conductance was high (ca 520 mmol · m – 2 · s – 1 ) for a woody species. RUBP - carboxylase content of the leaves ranged from 20 to 22 mg per gram F.W., which is similar to that found in most C 3 crop species. These results suggest that rabbitbrush is able to maintain high rates of P n , at least under nonstressed conditions.           

Trichoptera and other macroinvertebrates in springs of the Great Basin: species composition, richness, and distribution

Detailed surveys of aquatic invertebrates in 28 springs along the western border of the Great Basin were made using pan traps, emergence traps, black lights, and benthic sampling devices from 1994 to 1998. Although all macroinvertebrate groups were collected, caddisflies (Insecta: Trichoptera) were sampled most intensively. Fifty-eight species of caddisflies were collected from the 28 springs, with up to 18 species from a single spring. Although several springs had very similar physicochemical characteristics, none had identical trichopteran species composition. Using Wards minimum variance clustering technique, and with all macroinvertebrates identified to the lowest possible level, we found 3 taxa assemblages. These assemblages occurred in springs with the following physical conditions: (1) warm water (15.9° C), low elevation (1794 m); (2) cold water (8.5° C), mid-elevation (2334 m); and (3) cold water (6.6° C), high elevation (1794 m). Although the warm water group had a distinct assemblage of invertebrates (amphipods, gastropods, Trichoptera), distinct assemblages did not separate the 2 cold water groups. Discriminant analysis indicated that temperature, conductivity, alkalinity, and elevation were most responsible for discriminating group 1 from groups 2 and 3; temperature and elevation distinguished the latter 2 groups. Spring permanence, lack of disturbance, and cold water temperatures were the factors most responsible for explaining higher species richness of Trichoptera.     

Coordination of branch orientation and photosynthetic physiology in the Joshua tree ( Yucca brevifolia )

Despite the profusion of light in deserts, morphological adaptations to increase light interception are common among desert plants. We studied branch orientation and related physiological parameters in the Mojave Desert Joshua tree, Yucca brevifolia (Agavaceae). Azimuth and inclination were measured on all leaf rosettes of 44 Y. brevifolia trees. Interception of solar radiation was modeled for leaves in hypothetical rosettes facing due south and due north in December, March, and June. Carbon isotope discrimination, nitrogen content, and conductance of water vapor were measured in leaves from north- and south-facing rosettes. Rosette azimuths were nonrandom; rosettes predominantly faced southeast. North-facing rosettes were more steeply inclined than those facing south. The preponderance of south-facing rosettes reduces self-shading and increases interception of solar radiation during the winter-spring growth period. Stomatal conductance was higher for leaves in south-facing than in north-facing rosettes. Nevertheless, discrimination against 13 C was less in leaves of south-facing rosettes, indicating that average intercellular CO 2 concentration was also lower. South-facing whorls had higher leaf nitrogen content. Greater allocation of nitrogen to leaves in south-facing whorls probably results in those leaves having a greater photosynthetic capacity than their north-facing counterparts. Orientation of rosettes to increase interception of sunlight during the period most favorable for photosynthesis, coupled with allocation of nutrients to maintain a higher photosynthetic capacity in those rosettes, should significantly increase whole-plan carbon gain in Y. brevifolia .     

Rhodanese activity in Lotus corniculatus s.l.

Rhodanese (EC 2.8.1.1) is a sulphurtransferase which catalyses in vitro the formation of thiocyanate from cyanide and thiosulphate or some other sulphur donor. One proposed function of this multifunctional enzyme in vivo is detoxification of cyanide.

Lotus corniculatus s.l. is genetically polymorphic for cyanogenesis. Here we present the first investigation of such a polymorphic species group for its rhodanese content.

Enzymatic thiocyanate formation was easily detected in the leaves of Lotus alpinus and L. corniculatus from various natural habitats in Switzerland. All samples taken from these two species were found to contain a higher level of rhodanese activity than samples from the leaves of the six cultivated higher plants examined. Most interestingly, the differences observed within the Lotus species appear to be correlated with the polymorphism of cyanogenesis: cyanogenic plants AcLi showed much higher rhodanese activities than the acyanogenic types Acli and acLi. This correlation suggests that rhodanese provides a mechanism to detoxify cyanide in cyanogenic plants.     

Respiration characteristics differ among cheatgrass ( Bromus tectorum L.) populations

Cheatgrass ( Bromus tectorum L.) is a dominant weed that has increased the frequency of wildfire in the Great Basin since its introduction about 100 yr ago. This study examines characteristics of respiratory metabolism in several different populations. Seeds from 6 populations were germinated and metabolic heat rates (q) and dark respiration rates (R co2 ) of all seedlings were measured calorimetrically at 15° and 25° C or (for 3 populations) at 5° intervals from 5° to 35° C. Growth rates, substrate carbon conversion efficiencies, and Arrhenius temperature coefficients were calculated from the data. Results show that cheatgrass metabolism is most efficient at temperatures near 0° C; at temperatures above 20-25° C, efficiency goes to zero. Cheatgrass populations differ in their temperature dependencies of substrate carbon conversion efficiency and predicted growth rate. Measurements of respiratory heat and CO 2 rates as functions of temperature can be made relatively quickly and used to aid understanding of metabolic adaptation by invasive and native species to the environment.     

Leaf area ratios for selected rangeland plant species

Leaf area estimates are required by hydrologic erosion, and growth/yield simulation models and are important to the understanding of transpiration interception, CO 2 fixation, and the energy balance for native plant communities. Leaf biomass (g) to leaf area (mm 2 ) linear regression relationships were evaluated for 15 perennial grasses, 12 shrubs, and 1 tree. The slope coefficient (β 0 ) of the linear regression equation is a ratio of leaf area to leaf biomass and is defined as the leaf area ratio [LAR = one-sided leaf area (mm 2 )/oven-dry leaf weight (g)]. LAR represents β 0 in each regression equation, where Y = β 0 (X). Linear regression relationships for leaf area were computed ( r 2 = .84-.98) for all 28 native range species after full leaf extension. Within-plant estimates of leaf area for mesquite ( Prosopis glandulosae Torr. var. glandulosae [Torr.] Cockll.) or lime prickly ash ( Zanthoxylum fagara [L.] Sarg.) were not significantly different ( P ≤ .05). LARs for three of the shrubs and the tree were established at four different phenological stages. There were no significant differences ( P ≤ .05) in LARs for lime prickly ash, mesquite, and Texas persimmon ( Diospyros texana Scheele) after full leaf extension during the growing season. The LAR relationship for Texas persimmon changed significantly after full leaf extension. LAR relationships for Texas colubrina ( Colubrina texensis [T. & G.] Gray) changed in response to water stress.     

Historical variations in δ 13 C leaf of herbarium specimens in the southwestern U.S.

The uncontrolled, global increase in atmospheric CO 2 concentration (ca 80 ppmv) and decline in δ 13 C air (ca 1.5%) since industrialization provide experimental boundary conditions by which to assess physiological response of vegetation. To examine consequences of these global atmospheric changes in the southwestern U.S., 350 specimens of Atriplex confertifolia, A. canescens, Ephedra viridis, Pinus edulis, P. flexilis, Juniperus scopulorum, and Quercus turbinella of precisely known age spanning the last 150 years were acquired from 9 herbaria. Cellulose analysis of δ 13 C plant and estimation of isotopic discrimination (Δ) permitted calculation of water-use efficiency (A/g). The δ 13 C plant chronologies of C 4 Atriplex spp. show some promise as a reliable proxy for δ 13 C air because their mean trends approximate the known δ 13 C air chronology. However, the high variability would necessitate multiple samples at any time period to accurately represent the mean. The generally increasing A/g trends of the 5 C 3 species are particularly pronounced for P. edulis and, after the 1950s, for J. scopulorum, but less evident for P. flexilis, E. viridis, and Q. turbinella, evidencing possible differences in species response to rising CO 2 concentrations. The trends are statistically noisy, however, possibly due to complex microclimates, extreme seasonality, and great interannual variability typical of the southwestern U.S. Herbarium specimens, at least in the Southwest, may be less useful for precise detection of direct CO 2 effects on plant physiology than tree rings, where the variability can be constrained to a single individual over time.     

The male of Culicoides reevesi Wirth, with a redescription of the female and new seasonal activity, distribution, and biting records (Diptera: Ceratopogonidae)

The previously unknown male of the biting midge, Culicoides reevesi Wirth, is described and illustrated; the female is also redescribed and this species is reassigned to the leoni group. Previously known from California, Arizona, and New Mexico, C. reevesi is recorded for the 1st time from Utah ( new record ). Females of this aggressive, hematophagous species were collected while biting humans during evening crepuscular periods in California. Females exhibited a strong attraction to CO 2 traps, and seasonal surveillance demonstrated that host-seeking occurred from late May until mid-October in both California and Utah. Small numbers of males were also collected in CO 2 traps; however, both sexes showed little attraction to ultraviolet and incandescent light traps.     

Secondary production estimates of benthic insects in three cold desert streams

We studied aquatic insect production in three cold desert streams in southeastern Washington. The size-Frequency (SF) and P/B methods were used to assess production, which is expressed by taxon, functional group, and trophic level. Dipterans (midges and black flies) were the most productive taxa, accounting for 40-70% of the total insect production. Production by collectors and detritivores was the greatest of all functional groups and trophic levels, respectively, in all study streams. Insects with rapid development times and multiple cohorts are very important in cold desert streams; they were major contributors to the total insect production. Total insect production rates in our study streams (14-23 g DW·m -2 ·yr -1 ) were greater than those found in Deep Creek, Idaho (1.2 g DW·m -2 ·yr -1 ), the only other cold desert stream for which production data are available. Our values also were generally greater than published data for most cold/mesic (3-27 g DW·m -2 ·yr -1 ) and humid/mesic (3-25 g DW·m -2 ·yr -1 ) streams, but lower than in Sonoran Desert Streams (>120 g DW·m -2 ·yr -1 ) or New Zealand streams (～40 g DW·m -2 ·yr -1 ). Our data support the contention of others that production, rather than density or biomass, is the most accurate and meaningful way to assess the role of these organisms in lotic ecosystems.     

Implications of symbiotic nitrogen fixation by desert plants

There is little information available regarding symbiotic nitrogen fixation by desert plants, particularly with respect to the group of nodulated non-legumes. Early investigations of nodules on non-legumes were contemporary with early studies of the Leguminoseae; however, comparatively little is known even now regarding the extent to which non-legumes fix nitrogen. A review of literature regarding changes in concepts of nitrogen fixation by legumes and nodulated non-legumes is presented. Evidence from recent studies on nodulated non-legumes, including those in desert areas, indicates some 31 species of 21 genera in 12 new or previously unrecognized plant families show nodulation with implications of nitrogen fixation. Some biological aspects of symbiotic nitrogen fixation are discussed, and reports in the literature of nitrogen fixation in natural plant populations are reviewed. Nitrogen fixation reactions in the desert environment are considered in terms of the areas of investigation needed to help define (1) the magnitude of nitrogen fixation, (2) the extent of nodulation in non-legume plants, and (3) factors affecting this nodulation in the desert ecosystem.     

Effect of salinity and planting density on physiological responses of Allenrolfea occidentalis

Physiological responses of Allenrolfea occidentalis to salinity and seedling density were investigated. Effects of salinity (0, 200, 400, 600, 800, and 1000 mM NaCl) and 3 planting densities (2000, 4000, 6000 plants m -2 ) on the growth, survival, and ecophysiology of A. occidentalis , a stem succulent inland halophyte, were studied under controlled greenhouse conditions. Plants were grown in a sand culture using subirrigation. Dry mass of roots was highest at 600 mM NaCl at low density (2000 plants m -2 ), but declined as salinity increased. Tissue water content was highest at the 200 mM NaCl treatment and decreased with increased salinity. Water potential of the plants became more negative with increasing salinity due to the accumulation of NaCl in the leaves. Inorganic ions, especially Na + and Cl - , contributed substantially to dry mass. Na + and Cl - concentration in shoots and roots increased when NaCl level was increased while K + , Ca ++ , Mg ++ , SO 4 -- , and NO 3 - contents decreased. Net photosynthesis increased at low salinity (200 mM), but photosynthesis at other salinities was not significantly different from the control. While A. occidentalis is very salt tolerant and photosynthesis functioned reasonably well at high salinities, extremely high salinity did decrease dry mass of roots and shoots.     

Correlation of neighborhood relationships, carbon assimilation, and water status of sagebrush seedlings establishing after fire

Interactions of Artemisia tridentata ssp. vaseyana (mountain big sagebrush) and neighboring herbs may affect community development following fire in sagebrush steppe. Dry mass, photosynthesis, and water relations were measured for seedlings of A. tridentata ssp. vaseyana occurring at different distances from neighboring herbs in the initial growing seasons following fire, when herbs dominate plant community cover. Seedling mass significantly increased as distance to neighboring herbs increased, although a low r 2 indicated that mass was also affected by other sources of variation. Carbon assimilation ( A net ) was also greater for A. tridentata ssp. vaseyana seedlings in microsites farther from herbs, except during those sampling dates when A net and water availability were at low levels. Contrary to our expectations, water status of seedlings was not correlated with their distances to neighboring herbs, and supplemental watering did not affect the slope of the relationship between seedling mass and distance to neighboring herbs. These findings suggest that negative relationships between A. tridentata ssp. vaseyana seedlings and herbs establishing after fire are not likely due to competition over water and may instead result from interactions over factors not measured here (e.g., nutrients).     

Functional characteristics of wilderness streams twenty years following wildfire

We compared functional attributes of streams draining catchments burned by wildfire 20 years previously to those of streams in unburned catchments. Long-term analyses of channel profiles indicated most channel change occurred within the first 10 years after fire with little subsequent change the following 10 years. Much of the standing dead timber had fallen, and its effect on stream morphology was directly related to stream size, with important ramifications for future years as decay progresses. The volume of wood in the active channel was 5X higher in a 3rdorder burn stream than in other burn or reference streams, but > 80% of this wood was still bridging the stream. Retention of leaves was strongly associated with channel morphology and location of debris dams. Sediment respiration was significantly greater (1.7X) in streams of burned catchments, resulting from greater amounts of loosely attached organic matter in the sediments of these streams. In concordance with respiration results, coefficients of exchange (k ex ) were almost 5X higher in burn streams than in reference streams, although estimates of transient storage were similar between stream types. We expect the input of large woody debris to increase in the next 10 years in fire-impacted streams as bridging trees collapse into the stream, thereby enhancing channel complexity and habitat heterogeneity, instream metabolism and retention, and consequently stream function. The results emphasize the importance of landscape history, such as large-scale wildfires, on present patterns and processes in stream ecosystems.     

Effect of excluding shredders on leaf litter decomposition in two streams

The effect of excluding shredders on leaf processing rates was studied in a Rocky Mountain stream in Utah and a cold desert stream in Washington. Experimentally excluding shredders significantly decreased the processing rate in both streams. Processing rates (k) were higher in the desert stream, and it is postulated that this is related to increased microbial activity due to the higher water temperatures.     

A heterotrophic desert stream? The role of sediment stability

In autumn 1998 stream metabolism was measured in the Hassayampa River, Arizona, a Sonoran Desert stream, using single-station diel oxygen curves and an oxygen mass balance model. Oxygen consumption rates of parafluvial and channel sediments were determined with respiration chambers. Bedload of channel sediments (sand) prevented significant primary production by benthic autotrophs, despite favorable nutrient, light, and temperature conditions. Ecosystem respiration was relatively low (1.50 g O 2 m -2 d -1 ) and presumably fueled by production in the riparian zone and riverine marshes. Respiration rates in the parafluvial zone and in channel sediments ranged from 0.6 to 1.4 g O 2 m -3 sediment h -1 . Sediment organic matter (ash-free dry mass) was 4.0 ± 1.8 kg m -3 sediment and did not significantly differ between the channel and the parafluvial zone. Results indicate that heterotrophic processes may dominate the metabolism of desert stream segments over extended periods of time if unstable sandy bed sediments prevail.     

Field acetylene reduction rates of Lupinus argenteus along an elevational gradient

The effects of elevation and environmental parameters associated with elevation on the dinitrogen fixation rates, N 2 [C 2 H 2 ], of a North American lupine species were evaluated. Lupinus argenteus Pursh plants growing at five elevations on Francis Peak, Utah, were sampled weekly throughout the growing season. The highest elevation site had the greatest frequency of nodulated plants. Influence of the environmental and physiological variables on fixation activity varied with elevation and the mean time of sampling. Soil moisture, nodule dry weight, and air temperature were important. Less than one-half of the observed variation in fixation rates, N 2 [C 2 H 2 ], was accounted for by variation in the associated independent variables. Nevertheless this study documented that nodulated plants of L. argenteus were capable of actively fixing nitrogen at a range of elevations on Francis Peak.     

On Liljeborgia fissicornis (M. Sars, 1858) and three related new species from Scandinavia,with a hypothesis on the origin of the group fissicornis

The large and common Scandinavian amphipod Liljeborgia fissicornis (M. Sars) is split into four species: the deep-water L. caliginis sp. nov. and L. charybdis sp. nov., and the shallow-water L. fissicornis and L. ossiani sp. nov. The poorly known species L. polosi Barnard and Karaman, recorded from the deep Canadian Basin of the Arctic Ocean, is considered as belonging to the same group. All those northern species are completely devoid of eyes, while similar species from the Antarctic and sub-Antarctic continental shelf do have eyes. It is hypothesized that the group fissicornis, which is specific to cold waters, derives from ancestors living on the continental shelf of the Southern Ocean. These ancestors would have adapted to deep-sea environments, losing their eyes completely. Then they would have migrated northwards through the cold abyss, and reached the cold but shallow waters of the Arctic/sub-Arctic continental shelf, without redeveloping visual organs.     

Grass rhizosheaths: associated bacterial communities and potential for nitrogen fixation

Rhizosheaths are structures composed of mucilage secreted from plants and adherent soil particles that form a cylinder around the root. Using scanning electron microscopy (SEM), we observed bacteria associated with rhizosheaths of the grasses Achnatherum hymenoides, Calamovilfa longifolia, Hesperostipa comata, and Pascopyrum smithii from a sand dune area in Harding County in northwestern South Dakota. The greatest numbers of bacteria, observed with SEM (529 mm –2 ), and the greatest number of culturable bacteria (9.9 × 10 7 CFU ? g –1 or 5178 CFU ? mm –2 ) were on rhizosheaths of C. longifolia. Rhizosheaths of all the grasses examined contained a higher density of bacteria than the surrounding soil. Nitrogen fixation, as assayed by reduction of acetylene to ethylene, was present in some rhizosheaths. Bacterial nif H gene sequences amplified from bacteria associated with rhizosheaths were most similar to those from Alcaligenes latus and Mesorhizobium loti .