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 (2 x AMB seedlings) had reduced growth rates compared with seedlings from ambient CO2–grown parents (AMB seedlings). As compared to 360, 550 and 700 μmol mol−1 CO2‐stimulated relative growth rate (RGR) for most seedlings, the degree of stimulation was greatest for the AMB seedlings and least for the 2 x AMB seedlings. Instantaneous rates of photosynthesis mirrored the pattern of RGR across the parental CO2 and seedling CO2 treatment combinations. At 360 μmol mol−1 CO2, photosynthetic rates of 2 x AMB seedlings were half that of AMB seedlings, but at 700 μmol mol−1 CO2, their photosynthetic rates were not statistically different. Analysis of A‐Ci response curves indicates that 2 x AMB seedlings had reduced Rubisco activity compared with AMB seedlings, most likely as a result of less total nitrogen investment in leaves. AMB seedlings responded to low levels of nutrient input (1:40 Hoagland’s solution) with increased growth rates and leaf nitrogen content compared with zero nutrient addition. The 2 x AMB seedlings required the application of 1:10 Hoagland’s before an increase in these two parameters, compared with zero nutrient addition. These results indicate that elevated CO2 affects Bromus offspring performance through changes in adult‐seed‐seedling nitrogen dynamics, such that reductions in photosynthesis and growth rates occur in successive generations. Species‐specific allocation patterns that increase or decrease nitrogen allocation to seeds may enhance or diminish the ability of subsequent offspring to respond to an elevated CO2 environment

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

Elevated CO2 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 CO2 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 CO2 Enrichment (FACE) in the field in an intact Mojave Desert ecosystem. We were specifically interested in the differential effects CO2 enrichment would have on short-shoots and actively growing long-shoots during canopy development. Net photosynthesis (Anet) was similar in elevated compared with ambient CO2, but stomatal conductance (gs) was reduced by 27% in both shoot types. L. andersonii growing in elevated CO2 had larger leaves on short-shoots, and more leaves per shoot length on long-shoots. Enhanced leaf growth did not counter lower gs, and midday plant water potential was similar between treatments. In both short- and long-shoots, down-regulation of light-saturated photosynthetic electron transport rate (Jmax) occurred under elevated CO2. However, the balance between rubisco efficiency (estimated by the maximum carboxylation rate of rubisco, Vcmax), and electron transport capacity (Vcmax/Jmax) remained constant in short-shoots, but increased in elevated CO2 grown long-shoots. Apparent quantum requirement was similar, while light-saturated photosynthetic rates (Amax) decreased by approximately 30% under elevated CO2 in both shoot-types. These results suggest that elevated CO2 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

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 (\u2715N) 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 \u2715N values increased from 5.7-0.1‰ to 9.0-1.1‰ with elevated CO2; under ambient conditions, \u2715N values of shrubs increased from 4.9-0.3‰ to 6.6-0.7‰. No difference was found in soil \u2715N under elevated and ambient CO2. Soil \u2715N 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 \u2715N may be a useful tool for interpreting changes in the N cycle in numerous ecosystems

Photosynthetic responses of Mojave Desert shrubs to free air CO2 enrichment are greatest during wet years

It has been suggested that desert vegetation will show the strongest response to rising atmospheric carbon dioxide due to strong water limitations in these systems that may be ameliorated by both photosynthetic enhancements and reductions in stomatal conductance. Here, we report the long-term effect of 55 Pa atmospheric CO2 on photosynthesis and stomatal conductance for three Mojave Desert shrubs of differing leaf phenology (Ambrosia dumosa—drought-deciduous, Krameria erecta—winter-deciduous, Larrea tridentata—evergreen). The shrubs were growing in an undisturbed ecosystem fumigated using FACE technology and were measured over a four-year period that included both above and below-average precipitation. Daily integrated photosynthesis (A(day)) was significantly enhanced by elevated CO2 for all three species, although Krameria erecta showed the greatest enhancements (63% vs. 32% for the other species) enhancements were constant throughout the entire measurement period. Only one species, Larrea tridentata, decreased stomatal conductance by 25–50% in response to elevated CO2, and then only at the onset of the summer dry season and following late summer convective precipitation. Similarly, reductions in the maximum carboxylation rate of Rubisco were limited to Larrea during spring. These results suggest that the elevated CO2 response of desert vegetation is a function of complex interactions between species functional types and prevailing environmental conditions. Elevated CO2 did not extend the active growing season into the summer dry season because of overall negligible stomatal conductance responses that did not result in significant water conservation. Overall, we expect the greatest response of desert vegetation during years with above-average precipitation when the active growing season is not limited to ∼2 months and, consequently, the effects of increased photosynthesis can accumulate over a biologically significant time period

The Temperature responses of soil respiration in deserts: A seven desert synthesis

Deserts remain one of the most under-represented ecosystems in soil respiration syntheses (Lloyd and Taylor 1994; Raich and Potter 1995; Chen and Tian 2005) due to their low productivity, low soil respiration rates, and limited available data (Raich and Potter 1995). However, deserts are important to include in large-scale models because drylands cover a quarter of the earth’s land surface (Reynolds 2001), are expanding in area (Dregne 1983), and are rapidly changing. For example, in addition to tremendous human population growth (Geist and Lambin 2004), deserts are experiencing wide-spread woody plant expansion, which has been associated with increases in productivity (Hibbard et al. 2003), soil fertility (McCulley et al. 2004), deep root biomass (Connin et al. 1997), and soil respiration rates (McCulley et al. 2004). Further, climate change is predicted to increase precipitation variability and potentially exacerbate aridity in some desert systems (Christensen et al. 2007; Seager et al. 2007). Such alterations of the hydrological cycle could significantly impact desert ecosystems given that water is the primary driver of biological activity in deserts (e.g., Noy-Meir 1973). Thus, the combined effects of changes in climate, land use, vegetation cover, and desertification make it critical to better understand and quantify desert ecological processes

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 \u3e4 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 μmol/mol), blower control (ambient CO2), and non-ring treatments. The treatment main effect was not significant in any analyses of variance. Although the treatment × date interaction was significant for soil water in the top 0.5 m of soil, the expected greater soil water for elevated CO2 vs. ambient CO2 only occurred on one sampling date. In contrast, soil water for that same depth was significantly lower under elevated CO2 on six dates. Thus, we infer that increased water use from increased primary productivity (and therefore leaf area) under elevated CO2 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

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

Arid ecosystems, which occupy about 20% of the earth\u27s 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 ecosystem4, 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

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

The temperature response of soil respiration in deserts is not well quantified. We evaluated the response of respiration to temperatures spanning 67°C from seven deserts across North America and Greenland. Deserts have similar respiration rates in dry soil at 20°C, and as expected, respiration rates are greater under wet conditions, rivaling rates observed for more mesic systems. However, deserts differ in their respiration rates under wet soil at 20°C and in the strength of the effect of current and antecedent soil moisture on the sensitivity and magnitude of respiration. Respiration increases with temperature below 30°C but declines for temperatures exceeding 35°C. Hot deserts have lower temperature sensitivity than cold deserts, and insensitive or negative temperature sensitivities were predicted under certain moisture conditions that differed among deserts. These results have implications for large-scale modelling efforts because we highlight the unique behaviour of desert soil respiration relative to other systems. These behaviors include variable temperature responses and the importance of antecedent moisture conditions for soil respiration

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

Abstract The temperature response of soil respiration in deserts is not well quantified. We evaluated the response of respiration to temperatures spanning 67°C from seven deserts across North America and Greenland. Deserts have similar respiration rates in dry soil at 20°C, and as expected, respiration rates are greater under wet conditions, rivaling rates observed for more mesic systems. However, deserts differ in their respiration rates under wet soil at 20°C and in the strength of the effect of current and antecedent soil moisture on the sensitivity and magnitude of respiration. Respiration increases with temperature below 30°C but declines for temperatures exceeding 35°C. Hot deserts have lower temperature sensitivity than cold deserts, and insensitive or negative temperature sensitivities were predicted under certain moisture conditions that differed among deserts. These results have implications for large-scale modeling efforts because we highlight the unique behavior of desert soil respiration relative to other systems. These behaviors include variable temperature responses and the importance of antecedent moisture conditions for soil respiration. Electronic supplementary material The online version of this articl