Therese N. Charlet

Elevated CO2 increases productivity and invasive species success in an arid ecosystem.

Arid ecosystems, which occupy about 20% of the earths terrestrial surface area, have been predicted to be one of the most responsive ecosystem types to elevated atmospheric CO2 and associated global climate change. Here we show, using free-air CO2 enrichment (FACE) technology in an intact Mojave Desert ecosystem, that new shoot production of a dominant perennial shrub is doubled by a 50% increase in atmospheric CO2 concentration in a high rainfall year. However, elevated CO 2 does not enhance production in a drought year. We also found that above-ground production and seed rain of an invasive annual grass increases more at elevated CO2 than in several species of native annuals. Consequently, elevated CO2 might enhance the long-term success and dominance of exotic annual grasses in the region. This shift in species composition in favour of exotic annual grasses, driven by global change, has the potential to accelerate the fire cycle, reduce biodiversity and alter ecosystem function in the deserts of western North America.

Increases in desert shrub productivity under elevated carbon dioxide vary with water availability

Productivity of aridland plants is predicted to increase substantially with rising atmospheric carbon dioxide (CO2) concentrations due to enhancement in plant water-use efficiency (WUE). However, to date, there are few detailed analyses of how intact desert vegetation responds to elevated CO2. From 1998 to 2001, we examined aboveground production, photosynthesis, and water relations within three species exposed to ambient (around 38 Pa) or elevated (55 Pa) CO2 concentrations at the Nevada Desert Free-Air CO2 Enrichment (FACE) Facility in southern Nevada, USA. The functional types sampled—evergreen (Larrea tridentata), drought-deciduous (Ambrosia dumosa), and winter-deciduous shrubs (Krameria erecta)—represent potentially different responses to elevated CO2 in this ecosystem. We found elevated CO2 significantly increased aboveground production in all three species during an anomalously wet year (1998), with relative production ratios (elevated:ambient CO2) ranging from 1.59 (Krameria) to 2.31 (Larrea). In three below-average rainfall years (1999–2001), growth was much reduced in all species, with only Ambrosia in 2001 having significantly higher production under elevated CO2. Integrated photosynthesis (mol CO2 m−2 y−1) in the three species was 1.26–2.03-fold higher under elevated CO2 in the wet year (1998) and 1.32–1.43-fold higher after the third year of reduced rainfall (2001). Instantaneous WUE was also higher in shrubs grown under elevated CO2. The timing of peak canopy development did not change under elevated CO2; for example, there was no observed extension of leaf longevity into the dry season in the deciduous species. Similarly, seasonal patterns in CO2 assimilation did not change, except for Larrea. Therefore, phenological and physiological patterns that characterize Mojave Desert perennials—early-season lags in canopy development behind peak photosynthetic capacity, coupled with reductions in late-season photosynthetic capacity prior to reductions in leaf area—were not significantly affected by elevated CO2. Together, these findings suggest that elevated CO2 can enhance the productivity of Mojave Desert shrubs, but this effect is most pronounced during years with abundant rainfall when soil resources are most available.

ELEVATED ATMOSPHERIC CO2 DOES NOT CONSERVE SOIL WATER IN THE MOJAVE DESERT

Numerous studies, including those of desert plants, have shown reduced stomatal conductance under elevated atmospheric CO2. As a consequence, soil water has been postulated to increase. Soil water was measured for .4 yr at the Nevada Desert Free Air CO2 Enrichment (FACE) Facility to determine if elevated atmospheric CO2 conserves soil water for a desert scrub community in the Mojave Desert. We measured soil water in the top 0.2 and 0.5 m of soil with time domain reflectometry and to 1.85 m with a neutron probe for the three treatments at Desert FACE: elevated CO2 (550 mmol/mol), blower control (ambient CO2), and non-ring treatments. The treatment main effect was not significant in any analyses of variance. Although the treatment 3 date interaction was significant for soil water in the top 0.5 m of soil, the expected greater soil water for elevated CO 2 vs. ambient CO2 only occurred on one sampling date. In contrast, soil water for that same depth was significantly lower under elevated CO 2 on six dates. Thus, we infer that increased water use from increased primary productivity (and therefore leaf area) under elevated CO 2 offset the decreased water use from reduced stomatal conductance, and hence soil water was not conserved under elevated CO2 in the Mojave Desert, unlike other ecosystems.

Alterations of nitrogen dynamics under elevated carbon dioxide in an intact Mojave Desert ecosystem: Evidence from nitrogen-15 natural abundance

We examined soil and vegetation N isotopic composition (δ15N) and soil inorganic N availability in an intact Mojave desert ecosystem to evaluate potential effects of elevated atmospheric CO2 on N cycling. Vegetation from the dominant perennial shrub Larrea tridentata under elevated CO2 was enriched in 15N. Over a 7-month sampling period, Larrea δ15N values increased from 5.7±0.1‰ to 9.0±1.1‰ with elevated CO2; under ambient conditions, δ15N values of shrubs increased from 4.9±0.3‰ to 6.6±0.7‰. No difference was found in soil δ15N under elevated and ambient CO2. Soil δ15N values under the drought deciduous shrubs Lycium spp. were greatest (7.2±0.3‰), and soil under the C4 perennial bunchgrass Pleuraphis rigida had the lowest values (4.5±0.2‰). Several mechanisms could explain the enrichment in 15N of vegetation with elevated CO2. Results suggest that microbial activity has increased with elevated CO2, enriching pools of plant-available N and decreasing N availability. This hypothesis is supported by a significant reduction of plant-available N under elevated CO2. This indicates that exposure to elevated CO2 has resulted in significant perturbations to the soil N cycle, and that plant δ15N may be a useful tool for interpreting changes in the N cycle in numerous ecosystems.

No cumulative effect of 10 years of elevated [CO2] on perennial plant biomass components in the Mojave Desert

Elevated atmospheric CO2 concentrations ([CO2 ]) generally increase primary production of terrestrial ecosystems. Production responses to elevated [CO2 ] may be particularly large in deserts, but information on their long-term response is unknown. We evaluated the cumulative effects of elevated [CO2 ] on primary production at the Nevada Desert FACE (free-air carbon dioxide enrichment) Facility. Aboveground and belowground perennial plant biomass was harvested in an intact Mojave Desert ecosystem at the end of a 10-year elevated [CO2 ] experiment. We measured community standing biomass, biomass allocation, canopy cover, leaf area index (LAI), carbon and nitrogen content, and isotopic composition of plant tissues for five to eight dominant species. We provide the first long-term results of elevated [CO2 ] on biomass components of a desert ecosystem and offer information on understudied Mojave Desert species. In contrast to initial expectations, 10 years of elevated [CO2 ] had no significant effect on standing biomass, biomass allocation, canopy cover, and C : N ratios of above- and belowground components. However, elevated [CO2 ] increased short-term responses, including leaf water-use efficiency (WUE) as measured by carbon isotope discrimination and increased plot-level LAI. Standing biomass, biomass allocation, canopy cover, and C : N ratios of above- and belowground pools significantly differed among dominant species, but responses to elevated [CO2 ] did not vary among species, photosynthetic pathway (C3 vs. C4 ), or growth form (drought-deciduous shrub vs. evergreen shrub vs. grass). Thus, even though previous and current results occasionally show increased leaf-level photosynthetic rates, WUE, LAI, and plant growth under elevated [CO2 ] during the 10-year experiment, most responses were in wet years and did not lead to sustained increases in community biomass. We presume that the lack of sustained biomass responses to elevated [CO2 ] is explained by inter-annual differences in water availability. Therefore, the high frequency of low precipitation years may constrain cumulative biomass responses to elevated [CO2 ] in desert environments.

Effects of elevated CO2 (FACE) on the functional ecology of the drought-deciduous Mojave Desert shrub, Lycium andersonii

Abstract Elevated CO 2 may improve the productivity of cool-season active (‘drought-deciduous’) shrubs in the deserts of southwestern North America by reducing early-season phenological constraints imposed by low leaf area when photosynthetic capacity is high and later-season physiological limitations from declining photosynthesis and midday water potentials. Altered productivity under elevated CO 2 would depend on the specific responses of short-shoots that only provide early-season leaf area display, and long-shoots which determine annual growth increment in these plants. We measured plant water relations, photosynthetic gas exchange, and growth in short- and long-shoots of the drought-deciduous shrub, Lycium andersonii , under Free Air CO 2 Enrichment (FACE) in the field in an intact Mojave Desert ecosystem. We were specifically interested in the differential effects CO 2 enrichment would have on short-shoots and actively growing long-shoots during canopy development. Net photosynthesis ( A net ) was similar in elevated compared with ambient CO 2 , but stomatal conductance ( g s ) was reduced by 27% in both shoot types. L. andersonii growing in elevated CO 2 had larger leaves on short-shoots, and more leaves per shoot length on long-shoots. Enhanced leaf growth did not counter lower g s , and midday plant water potential was similar between treatments. In both short- and long-shoots, down-regulation of light-saturated photosynthetic electron transport rate ( J max ) occurred under elevated CO 2 . However, the balance between rubisco efficiency (estimated by the maximum carboxylation rate of rubisco, V cmax ), and electron transport capacity ( V cmax / J max ) remained constant in short-shoots, but increased in elevated CO 2 grown long-shoots. Apparent quantum requirement was similar, while light-saturated photosynthetic rates ( A max ) decreased by approximately 30% under elevated CO 2 in both shoot-types. These results suggest that elevated CO 2 lowered investment to photosynthetic electron transport capacity and whole-plant water use, even when leaf growth was stimulated. Such canopy dynamics are likely to enhance the ability of this drought-deciduous species to better cope with the highly variable inter- and intra-annual climate regimes characteristic of North American deserts.

Long-term response of a Mojave Desert winter annual plant community to a whole-ecosystem atmospheric CO2 manipulation (FACE)

Desert annuals are a critically important component of desert communities and may be particularly responsive to increasing atmospheric (CO2 ) because of their high potential growth rates and flexible phenology. During the 10-year life of the Nevada Desert FACE (free-air CO2 enrichment) Facility, we evaluated the productivity, reproductive allocation, and community structure of annuals in response to long-term elevated (CO2 ) exposure. The dominant forb and grass species exhibited accelerated phenology, increased size, and higher reproduction at elevated (CO2 ) in a wet El Niño year near the beginning of the experiment. However, a multiyear dry cycle resulted in no increases in productivity or reproductive allocation for the remainder of the experiment. At the community level, early indications of increased dominance of the invasive Bromus rubens at elevated (CO2 ) gave way to an absence of Bromus in the community during a drought cycle, with a resurgence late in the experiment in response to higher rainfall and a corresponding high density of Bromus in a final soil seed bank analysis, particularly at elevated (CO2 ). This long-term experiment resulted in two primary conclusions: (i) elevated (CO2 ) does not increase productivity of annuals in most years; and (ii) relative stimulation of invasive grasses will likely depend on future precipitation, with a wetter climate favoring invasive grasses but currently predicted greater aridity favoring native dicots.

The Effects of parental CO2 and offspring nutrient environment on initial growth and photosynthesis in an annual grass

Seeds of Bromus madritensis ssp. rubens (red brome, an exotic annual grass in the Mojave Desert), from parents grown at three CO2 levels (360, 550, and 700 μmol mol−1), were grown in factorial CO2 (360, 550, and 700 μmol mol−1) and nutrient (zero addition, 1:40‐strength, and 1:10‐strength Hoagland’s solution) environments to evaluate parental CO2 effects on offspring performance characteristics across a range of developmental environments. We evaluated growth rate, leaf nitrogen content, and photosynthetic gas exchange over a 3‐wk period. Seedlings from elevated‐CO2 parental seed sources ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

The temperature responses of soil respiration in deserts: a seven desert synthesis

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