We quantified the temporal trend and climatic sensitivity of vegetation phenology in dryland ecosystems in the US Great Basin during 1982–2011. Our results indicated that vegetation greenness in the Great Basin increased significantly during the study period, and this positive trend occurred in autumn but not in spring and summer. Spatially, increases in vegetation greenness were more apparent in the northwestern, southeastern, and eastern Great Basin but less apparent in the central and southwestern Great Basin. In addition, the start of growing season (SOS) was not advanced while the end of growing season (EOS) was delayed significantly at a rate of 3.0 days per decade during the study period. The significant delay in EOS and lack of earlier leaf onset caused growing season length (GSL) to increase at a rate of 3.0 days per decade. Interestingly, we found that the interannual variation of mean vegetation greenness calculated for the period of March to November (spring, summer, and autumn – SSA) was not significantly correlated with mean surface air temperature in SSA but was strongly correlated with total precipitation. On a seasonal basis, the variation of mean vegetation greenness in spring, summer, and autumn was mainly attributable to changes in pre-season precipitation in winter and spring. Nevertheless, climate warming appeared to play a strong role in extending GSL that, in turn, resulted in the upward trend in mean vegetation greenness. Overall, our results suggest that changes in wintertime and springtime precipitation played a stronger role than temperature in affecting the interannual variability of vegetation greenness, while climate warming was mainly responsible for the upward trend in vegetation greenness we observed in Great Basin dryland ecosystems during the 30-year period from 1982 to 2011.

The distribution of chemical species in desert soils is a complex function of atmospheric inputs, rock weathering, chemical equilibria, soil leaching, and plant cycling. In this study of the surface 1 m of desert soils at the Nevada Test Site, we examined the distribution of soluble soil elements beneath plant canopies and in intershrub zones. We extracted water-soluble chemical species using saturation extracts that were equilibrated overnight and more dilute extracts that were incubated for 1 mo. All soil horizons were supersaturated with calcite and undersaturated with gypsum. Calcite supersaturation was a function of soil organic C (inhibitor) and water content (inhibitor diluent). It is critical that the latter two properties be explicitly recognized in modeling CaCO3 mobility through soils. Plant-essential cations (K, Ca, and Mg) were highest in concentration in surface soils beneath plant canopies. Sodium, Cl, NO3, and SO4 were generally highest in concentration around 1-m depth. The dissimilar patterns of essential nutrient accumulations between the cations (Ca, K, and Mg) and the anions (NO3 and SO4) were probably due to the greater mobility of anions than cations. Also, these surface soils were "N saturated" due to their low N retention capacity, which accounts, at least in part, for the anomalous NO3 accumulations at depths > 1 m in Mojave Desert soils. Furthermore, once NO3 or SO4 have leached to deeper soil depths, they may be relatively unavailable for plant uptake. The soil elemental patterns are consistent with elemental plant requirements and elemental mobility.

Salt flux through soils can significantly influence local and global processes. For example, desert soils can atypically concentrate NO3- Lit depth in soil profiles. CaCO3 precipitation/dissolution can play significant roles as either sinks or sources of global carbon. The objectives of this work were to develop a salt-flux model for long-term (> 1000 years) simulations of desert soils and examine the consequences of climate, soils, system inputs, and land-use change on salt movement in arid soils.The field study was conducted at the Nevada Test Site in the northern Mojave Desert. New additions to the CALGYP model allowing: for site-specific parameterization included stochastic rainfall model, salt inputs, soil water-holding capacities, and soil CO2 profiles. New ions added to the model included Na+, K+, Mg2+, Cl-, and NO3-.About 81% of Ca2+ input remained within the surface 1.0m of soil as CaCO3, which argues in favor of soil CaCO3 serving as a recalcitrant sink for global carbon. In contrast, approximate to 99.96% of Na+, K+, Mg2+, Cl-, NO3-, and SO42- ions leached to soil depths > 1.0m and 94.3% leached to soil depths >2.0m. This is true despite only 1.64% of the rainfall leached beyond 1.0 m and 0.020% of the rainfall leached beyond 2.0 m. The leachability of NO3- and Cl- to soil depths > 2.0 m agrees with NO3- and Cl- accumulations at depth in Mojave Desert soils (1.3-2.7 m). Simulation of extreme events and years with a stochastic rainfall model and accurate soil water-holding capacities are critical for modeling water and salt flux through soils. (C) 2007 Elsevier Ltd. All rights reserved.

Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO2]. Net ecosystem CO2 exchange (NEE) was measured in the eighth year of CO2 enrichment at the Nevada Desert Free-Air CO2 Enrichment (FACE) Facility between the months of December 2003-December 2004. On most dates mean daily NEE (24 h) (μ mol CO2 m(-2) s(-1)) of ecosystems exposed to elevated atmospheric CO2 were similar to those maintained at current ambient CO2 levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO2] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO2]. Mean daily NEE varied seasonally across both CO2 treatments, increasing from about 0.1 μ mol CO2 m(-2) s(-1) in December to a maximum of 0.5-0.6 μ mol CO2 m(-2) s(-1) in early spring. Maximum NEE in ecosystems exposed to elevated CO2 occurred 1 month earlier than it did in ecosystems exposed to ambient CO2, with declines in both treatments to lowest seasonal levels by early October (0.09± 0.03 μ mol CO2 m(-2) s(-1)), but then increasing to near peak levels in late October (0.36± 0.08 μ mol CO2 m(-2) s(-1)), November (0.28± 0.03 μ mol CO2 m(-2) s(-1)), and December (0.54± 0.06 μ mol CO2 m(-2) s(-1)). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO2 uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free-living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO2 treatments were net CO2 sinks in 2004, but exposure to elevated CO2 reduced CO2 sink strength by 30% (positive net ecosystem productivity=127± 17 g C m(-2) yr(-1) ambient CO2 and 90± 11 g C m(-2) yr(-1) elevated CO2, P=0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO2- along with the extensive coverage of arid and semi-arid ecosystems globally - points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.

Two commercially available ion exchange resin (IER) devices - Unibest resin capsules (Unibest, Inc., Bozeman, MT) and Plant Root Simulator (PRS) probe-ion exchange membranes (Western Ag Innovations, Inc., Saskatoon, Canada) - for measuring soil nutrient availability were compared to traditional soil NH4+ and NO3- measurements during incubation of an Oklahoma tallgrass prairie soil at two temperatures (16degrees and 25degreesC) and two moisture contents (15 and 25% by weight). Nitrate dominated the soil mineral N pool in soils and in both IER devices. Soil extractable and resin capsule mineral N showed significant responses to both temperature and moisture whereas PRS probe mineral N showed responses to moisture only. Both devices were more sensitive to moisture than soil mineral N was. Neither device related well to N mineralization or the patterns of extractable mineral N over time. Possible reasons for the differences include the integration of soil mineral N over time in the IERs as opposed to snapshots in time for soil mineral N, spatial variation within the incubated soils, and the importance of solution contact with IERs.

The atmosphere is an important pathway by which mercury is transported and distributed to pristine ecosystems. The significance of anthropogenic versus natural mercury contributions to the atmosphere is controversial, and the importance of re-emission of deposited mercury from ecosystems is not known. Here we present a continuous year-long data set of gaseous elemental mercury exchange between intact soil–plant monoliths of tallgrass prairie and the atmosphere. Mercury fluxes were measured using large open-flow gas exchange chambers (7.3×5.5×4.5 m3, L×W×D). Approximately 60 μg m−2 of elemental gaseous mercury was lost from four replicate grassland ecosystems (9 m2 surface area each) to the atmosphere over the course of 1 yr. Deposition was an important flux in the winter and emissions were dominant in spring, summer, and fall. Solar radiation and air temperature were most strongly correlated with mercury emissions. Gaseous elemental mercury losses to the atmosphere exceeded other measured fluxes of mercury in and out of the grassland ecosystems. These results indicate that mercury emissions from uncontaminated terrestrial ecosystems to the atmosphere may be a significant source of atmospheric mercury. We hypothesize that most of the mercury being emitted is previously deposited mercury and that re-emissions of mercury from terrestrial ecosystems is an important process whereby mercury is continually cycled between the air and terrestrial ecosystems.

Modeling analyses suggest that an increase in growth rate of atmospheric CO2 concentrations during an anomalously warm year may be caused by a decrease in net ecosystem production (NEP) in response to increased heterotrophic respiration (Rh). To test this hypothesis, 12 intact soil monoliths were excavated from a tallgrass prairie site near Purcell, Oklahoma, USA and divided among four large dynamic flux chambers (Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs)). During the first year, all four EcoCELLs were subjected to Oklahoma air temperatures. During the second year, air temperature in two EcoCELLs was increased by 4°C throughout the year to simulate anomalously warm conditions. This paper reports on the effect of warming on soil CO2 efflux, representing the sum of autotrophic respiration (Ra) and Rh.

Increases in net primary production (NPP) may not necessarily result in increased C sequestration since an increase in uptake can be negated by concurrent increases in ecosystem C losses via respiratory processes. Continuous measurements of net ecosystem C exchange between the atmosphere and two experimental cheatgrass (Bromus tectorum L.) ecosystems in large dynamic flux chambers (EcoCELLs) showed net ecosystem C losses to the atmosphere in excess of 300 g C m−2 over two growing cycles. Even a doubling of net ecosystem production (NEP) after N fertilization in the second growing season did not compensate for soil C losses incurred during the fallow period. Fertilization not only increased C uptake in biomass but also enhanced C losses through soil respiration from 287 to 469 g C m−2, mainly through an increase in rhizosphere respiration. Fertilization decreased dissolved inorganic C losses through leaching of from 45 to 10 g C m−2.

•  Stimulation of root growth under elevated CO2 has been hypothesized to enhance soil water uptake under water-limiting conditions. The objectives of this study were to quantify the effects of rising CO2 on root development and soil water uptake in Larrea tridentata and to quantify root proliferation into small water patches. •  Seedling communities of L. tridentata were grown in rhizotrons under controlled environmental conditions at three CO2 concentrations (280, 360, and 600 µl l-1 ). Patches of water were applied to small areas of the root systems in the rhizotrons and to L. tridentata shrubs in the field. •  Rising CO2 significantly stimulated root length production, but only in the lower half of the soil profile. Stimulation of root production led to faster depletion of soil water. Neither mature shrubs nor seedlings responded to water-enriched soil patches via root proliferation. •  The results of our study indicate that rising CO2 may accelerate seedling root growth in L. tridentata, could lead to proportionally greater investment of roots in deeper soil layers and may enhance water acquisition.

Increased fire frequency in the Great Basin of North America's intermountain West has led to large-scale conversion of native sagebrush (Artemisia tridentata Nutt.) communities to postfire successional communities dominated by native and non-native annual species during the last century. The consequences of this conversion for basic ecosystem functions, however, are poorly understood. We measured net ecosystem CO2 exchange (NEE) and evapotranspiration (ET) during the first two dry years after wildfire using a 4-m diameter (16.4 m(3)) translucent static chamber (dome), and found that both NEE and ET were higher in a postfire successional ecosystem (-0.9-2.6 mu mol CO2 m(-2)s(-1) and 0.0-1.0 mmol H-2 O m(-2)s(-2), respectively) than in an adjacent intact sagebrush ecosystem (-1.2-2.3 mu mol CO2 m(-2)s(-1) and -0.1-0.8 mmol H2O m(-2)s(-2), respectively) during relatively moist periods. Higher NEE in the postfire ecosystem appears to be due to lower rates of above-ground plant respiration while higher ET appears to be caused by higher surface soil temperatures and increased soil water recharge after rains. These patterns disappeared or were reversed, however, when the conditions were drier. Daily net ecosystem productivity (NEP; g C m(-2)d(-1)), derived from multiple linear regressions of measured fluxes with continuously measured climate variables, was very small (close to zero) throughout most of the year. The wintertime was an exception in the intact sagebrush ecosystem with C losses exceeding C gains leading to negative NEP while C balance of the postfire ecosystem remained near zero. Taken together, our results indicate that wildfire-induced conversion of native sagebrush steppe to ecosystems dominated by herbaceous annual species may have little effect on C balance during relatively dry years (except in winter months) but may stimulate water loss immediately following fires.

We designed, constructed, calibrated and field-tested a lightweight (30kg), 4.2m diameter, 16.4m3 polyethylene-covered dome static chamber ecosystem gas exchange cuvette that can quantify ecosystem CO2 and water vapour fluxes as low as 0.1μmol CO2m−2s−1 and 0.1mmol H2Om−2s−1 with little impact on environmental conditions. Fluxes measured in May 2001 in an intact Great Basin sagebrush ecosystem at midday were significantly higher than in an adjacent post-wildfire successional ecosystem, with observed ranges from –0.71 to 1.49μmol CO2m−2s−1 for CO2 and from –0.09 to 0.53mmol H2Om−2s−1 for water vapour.

Use of plant phenological variables in models predicting evapotranspiration (ET) has largely relied on relatively simple (e.g., linear) relationships which may not be sufficiently accurate to predict small—yet ecologically significant—changes in plant phenology that are expected to occur in response to global climate change. A dearth of experimental data reflects the difficulties in quantifying these relationships against the background of large environmental variability that occurs in the field. Our main objective was to quantify how plant phenology (leaf area index [LAI] and root length density [RLD]) affect ET and its components during an entire vegetation cycle in large-scale model grassland (Bromus tectorum) ecosystems using the Ecologically Controlled Enclosed Lysimeter Laboratory (EcoCELL)—a unique open flow and mass balance laboratory. We also aimed to compare the three methods employed by the EcoCELL laboratory to measure ecosystem ET (whole-ecosystem gas exchange, weighing lysimetry, and weighing lysimetry combined with time domain reflectometry [TDR]) in order to independently confirm the performance of the unique gas exchange technology. Cumulative ET during the 190 days of the experiment measured with the three different methods compared very well with each other (mean errors <1%). We found that ET reached maximum levels at relatively low LAI (2–3), but as LAI increased beyond this value, small increase in transpiration were more than offset by decreases in soil evaporation, thereby causing declines in ET. A combined rectangular hyperbola (effects on transpiration) and linear (effects on soil evaporation) function between LAI and ET accounted for almost 90% of all variability in measured daily ET. RLD showed relationships to ET similar to those observed for LAI due to high covariance between RLD and LAI, but root length densities did not explain any additional variability in daily ET beyond that explained by LAI under the well-watered conditions of the experiment. Taken together, our results show that: (i) the EcoCELL mesocosm laboratory can precisely and accurately quantify hydrologic processes of large soil–plant monoliths under controlled environmental conditions; (ii) plant canopy phenological changes affect ecosystem ET, and the contribution of transpiration, in non-linear ways; (iii) these non-linear responses must be accounted for when assessing the consequences of changes in plant phenology—e.g., due to global environmental change—on ecosystem hydrology.

Rising atmospheric CO2 has been predicted to reduce litter decomposition as a result of CO2‐induced reductions in litter quality. However, available data have not supported this hypothesis in mesic ecosystems, and no data are available for desert or semi‐arid ecosystems, which account for more than 35% of the Earth's land area. The objective of our study was to explore controls on litter decomposition in the Mojave Desert using elevated CO2 and interannual climate variability as driving environmental factors. In particular, we sought to evaluate the extent to which decomposition is modulated by litter chemistry (C:N) and litter species and tissue composition. Naturally senesced litter was collected from each of nine 25 m diameter experimental plots, with six plots exposed to ambient [CO2] or 367 μL CO2 L−1 and three plots continuously fumigated with elevated [CO2] (550 μL CO2 L−1) using FACE technology beginning in April 1997. All litter collected in 1998 (a wet, or El Niño year; 306 mm precipitation) was pooled as was litter collected in 1999 (a dry year; 94 mm). Samples were allowed to decompose for 4 and 12 months starting in May 2001 in mesh litterbags in the locations from which litter was collected. Decomposition of litter produced under elevated CO2 and ambient CO2 did not differ. Litter produced in the wetter year showed more rapid initial decomposition (over the first 4 months) than that produced in the drier year (27±2% yr−1 or 7.8±0.7 g m−2 yr−1 for 1998 litter; 18±3% yr−1 or 2.2±0.4 g m−2 yr−1 for 1999 litter). C:N ratios of litter produced under elevated CO2 (wet year: 37±0.5; dry year: 42±2.5) were higher than those of litter produced under ambient CO2 (wet year: 34±1.1; dry year: 35±1.4). Litter production in the wet year (amb. CO2: 25.1±1.1 g m−2 yr−1; elev. CO2: 35.0±1.1 g m−2 yr−1) was more than twice as high as that in the dry year (amb. CO2: 11.6±1.7 g m−2, elev. CO2: 13.3±3.4 g m−2), and contained a greater proportion of Lycium pallidum and a lower proportion of Larrea tridentata than litter produced in the dry year. Decomposition, viewed across all treatments, decreased with increasing C:N ratios, decreased with increasing proportions of Larrea tridentata and increased with increasing proportions of Lycium pallidum and Lycium andersonii. Because litter C:N did not vary by litter production year, and CO2 did not alter decomposition or litter species/tissue composition, it is likely that the impact of year‐to‐year variation in precipitation on the proportion of key plant species in the litter may be the most important way in which litter decomposition will be modulated in the Mojave Desert under future rising atmospheric CO2.

The effects of elevated carbon dioxide (CO2 ) on plant litter are critical determinants of ecosystem feedback to changing atmospheric CO2 concentrations. We measured concentrations of nitrogen (N) and carbon (C) and calculated C : N ratios of green leaves of two desert perennial shrubs, and the same quality parameters plus lignin and cellulose content of leaf litter from four shrub species exposed to elevated CO2 (FACE technology; Hendrey & Kimball, 1994) for 3 years in an intact Mojave Desert ecosystem. Shrubs tested were Larrea tridentata , Lycium pallidum , Lycium andersonii and Ambrosia dumosa . We calculated resorption efficiency from green tissue and leaf litter N data and measured lignin and cellulose content in litter in the last year study. Green leaves of L. tridentata grown under elevated CO2 had significantly lower N concentrations and higher C : N ratios than shrubs grown in ambient conditions in 1999 (P < 0.05). Lycium pallidum green leaves grown under elevated CO2 had significantly lower N concentrations and higher C : N ratios than shrubs grown under ambient conditions in 2000 (P < 0.05). There was no CO2 effect on C content of either species. We found no effect of CO2 on N or C content, C : N ratios, or lignin or cellulose concentrations in leaf litter of L. tridentata , L. pallidum , L. andersonii , or A. dumosa . There was no significant effect of CO2 on estimates of shrub resorption efficiency. There was a seasonal effect on green tissue and litter tissue quality for L. tridentata , with lower tissue N content in summer than in spring or winter months. These data suggest that any productivity increases with elevated CO2 in desert ecosystems may not be limited by lower leaf litter quality and that resorption efficiency calculations are best performed on an individual leaf basis.

The objective of this study was to determine effects of elevated CO2 and soil nutrient availability on growth and plant tissue quality in four grass species, Agrostis stolonifera, Anthoxanthum odoratum, Festuca rubra and Poa pratensis, native in Western European calcareous grassland. Plants were grown for 65 days in the greenhouse in pots with untreated soil from calcareous grassland under ambient (350 ppm) and elevated (700 ppm) CO2 either with or without fertilisation. In general, elevated CO2 increased plant height, total biomass, starch and sugar concentrations, and decreased water and nitrogen concentrations. However, the response to CO2-enrichment depended strongly on the grass species investigated. Fertilisation enhanced most effects of elevated CO2 Biomass production in fertilised plants increased more under elevated CO2 than in unfertilised plants whereas leaf nitrogen concentration of fertilised plants decreased more at elevated CO2 than it did in unfertilised plants. Furthermore, a species-specific response to elevated CO2, depending on soil nutrient availability was detected in starch and sugar concentrations (three-way interaction). The data from this study indicate that grass species vary in their response to elevated CO2 in biomass production and tissue quality. Furthermore, increasing nutrient availability can substantially alter effects of elevated CO2. Since the investigated grass species are important larval food-plants of insect herbivores on calcareous grassland, the observed species-specific reactions to CO2-enrichment and high nutrient availability in tissue quantity and quality are discussed with respect to their effects on insect performance and thus abundance and biodiversity of these insects.

The objectives of this study were to quantify changes in leaf freezing resistance and carbohydrate concentrations caused by long-term (6 years) exposure to elevated CO2 (ambient: 360 mul l(-1), elevated. 600 mul l(-1)) in five dominant plant species growing in situ in a native temperate grassland. Across all five species tested from three functional groups, the mean temperature at which all leaves were damaged (T-100) significantly (P = 0.016) increased from -9.6 to -8.5 degreesC under elevated CO2, and a similar marginally significant (P = 0.079) reduction was observed for the mean temperature that caused 50% leaf damage (T-50), from -6.7 to -6.0 degreesC. The mean temperature at which initial leaf damage was observed (T-o) was not significantly influenced by elevated CO2. Although concentrations of soluble sugars (+25%, P = 0.042), starch (+53%, P < 0.001), and total non-structural carbohydrates (TNC, +40%, P < 0.001) were significantly higher under elevated CO2, leaf freezing resistance actually decreased under elevated CO2. Concentrations of soluble sugars were positively correlated with freezing resistance when viewed across all five community dominants, but within any individual species, no such relationships were found. We also found no evidence for our original hypothesis that increased concentrations of soluble sugars increase freezing resistance. Thus, future atmospheric CO2 levels may instead increase the risk of late spring freezing damage. Furthermore, the strong differences in freezing resistance observed among the species, along with decreased freezing resistance, may increase the risk of losing species that have inherently weak freezing resistances from the plant community. (C) 2001 Annals of Botany Company.

initial leaf damage was observed (T0) was not significantly influenced by elevated CO2. Although concentrations of soluble sugars (á25%, Pà 0.042), starch (á53%, P 5 0.001), and total non-structural carbohydrates (TNC, á40%, P 5 0.001) were significantly higher under elevated CO2, leaf freezing resistance actually decreased under elevated CO2. Concentrations of soluble sugars were positively correlated with freezing resistance when viewed across all five community dominants, but within any individual species, no such relationships were found. We also found no evidence for our original hypothesis that increased concentrations of soluble sugars increase freezing resistance. Thus, future atmospheric CO2 levels may instead increase the risk of late spring freezing damage. Furthermore, the strong diÄerences in freezing resistance observed among the species, along with decreased freezing resistance, may increase the risk of losing species that have inherently weak freezing resistances from the plant community. # 2001 Annals of Botany Company

Summary 1 Although plants of Equisetum spp. are generally thought to be of little value to ecosystems, a study of a cold‐temperate Alaskan shrub wetland showed that they acquired and cycled phosphorus and other nutrients more efficiently than other plant community members. While Equisetum plants represented only 5% of the above‐ and below‐ground biomass in the community, they contained 16% of the P and 24% of the K. 2 Equisetum plants accounted for 29% of the P and 39% of the K in annual community foliage litterfall. Over a 2‐year period, losses from Equisetum litter contributed 55, 41, and 75% of the P, K and Ca litter inputs into soil nutrient pools. 3 The ability of different species of Equisetum to acquire nutrients is linked to their deep rooting habit. While the majority of their roots and rhizomes, and particularly the fine roots, were located in the C horizon, the majority of roots and rhizomes of other species were located in the overlying O horizon. The biomass of Equisetum plants was also correlated with edaphic characteristics of the C horizon. 4 The absorption of nutrients from the C horizon by Equisetum helped bring P and other minerals to the soil surface, increasing the amount of minerals in the O horizon and thus making them potentially available to other species, including Myrica gale, Salix spp., and Carex spp. Productivity in this community is limited by P and N, and the ability of Equisetum to act as a nutrient pump may help explain why its net primary productivity is high for a cold‐temperate wetland.