Mojave Desert Ecosystem Research Articles

Spatio-Temporal Patterns of Joshua Tree Stand Structure and Regeneration Following Mojave Desert Wildfires

Eastern Joshua tree (Yucca jaegeriana) plays a central role in the ecology of the Mojave Desert ecosystem. However, the emergence of invasive grass fire-cycles in the last several decades brings into question Joshua trees’ tolerance and resilience to changing fire regimes. This study’s objective was to examine the effects of wildfires on the structure and regeneration potential of Joshua trees forests. We examined the density, size class structure, and regeneration response of Joshua tree populations on a network of one kilometer transects along the boundaries of four independent wildfires and the interior of the largest fire that occurred 15 years ago (2005) in the northeast Mojave Desert. Burned edge and burned interior transects had 23- and 4.1-fold lower Joshua tree stand densities than unburned transects. The more robust recovery of stand density along burned interior transects compared to burned edge transects appears to be primarily driven by more prolific vegetative sprouting. Our data show that Joshua trees can sprout vegetatively following fire, but it is not a strong or consistent post-fire resprouter. Limiting the spread of invasive annual grasses and novel fire regimes will be critical to maintaining healthy Joshua tree populations into the future, particularly on the edge of its ecological range.

FACTORS INFLUENCING OCCUPANCY AND DETECTION RATES OF MOUNTAIN LIONS IN THE MOJAVE DESERT OF CALIFORNIA

We collected and analyzed a 6-year remote game camera data set from the Mojave National Preserve for determining factors influencing occupancy and detection rates for mountain lions (Puma concolor) in the Mojave Desert. We detected mountain lions in 37 of 186 sampling periods at 36 sites monitored by remote game cameras. Detection rates decreased rapidly with increasing distance to shrub cover, and there was no relationship between season and detection rates. Occupancy was found to be constant across sampling periods. Our results suggest mountain lions are present in the Mojave National Preserve year-round. Our analyses can serve to further the understanding of mountain lion spatial ecology and community-level interactions in the Mojave Desert ecosystem. Lastly, our analyses represent the first occupancy and detection estimates for mountain lions in the Mojave Desert and can serve as a baseline for tracking populations over time.

Greater soil carbon and nitrogen in a Mojave Desert ecosystem after 10 years exposure to elevated CO2

We investigated patterns and possible mechanisms in the greater soil organic carbon (SOC) and total nitrogen (N) after 10 years exposure to elevated atmospheric CO2 concentration ([CO2]) at the Nevada Desert FACE Facility, USA, reported by Evans et al. (2014). Differences in SOC under elevated and ambient [CO2] depended on cover types, and most evident in soils under deciduous shrubs. Greater SOC was observed in soils close to the surface as well as 0.4 to 0.8 m suggesting elevated [CO2] stimulated production of aboveground litter, root litter, and rhizodeposition. Greater total N under elevated [CO2] was most evident under deciduous shrubs as well as plant interspace that had extensive coverage of biological soil crusts. Greater total N in the top soil profile under elevated [CO2] was accompanied with reduced δ15N by 0.4‰, suggesting elevated [CO2] stimulated N2-fixation and/or reduced N loss. We conclude that, in arid ecosystems under elevated [CO2], shrubs play a major role in determining C sink capacity in desert soils, and N limitation for vascular plants is unlikely to occur due to sustained net N supply.

Plant and microbial biomarkers suggest mechanisms of soil organic carbon accumulation in a Mojave Desert ecosystem under elevated CO2

We investigated how properties of soil organic matter (SOM) were altered after 10 years exposure to elevated atmospheric CO2 concentration ([CO2]) in a Mojave Desert ecosystem, using plant and microbial biomarkers. We focused on roles of Larrea tridentata, the dominant evergreen shrub which form islands of fertility, and biological soil crusts which have extensive cover in plant interspace. Soils to 5 cm in depth were collected under L. tridentata and plant interspace, and biological soil crusts to 0.5 cm in depth under three cover types, Pleuraphis rigida, a C4 grass, shrubs and plant interspace. Soil organic carbon contents were not significantly different between elevated and ambient [CO2]. However, significantly higher abundance of n-alkanes, a major constituent of foliage wax material, occurred in the elevated compared to ambient [CO2] plots in the soils collected under L. tridentata, but no such difference was found in the soils at plant interspace or the biological soil crusts. There was no significant difference in abundance of microbial phospholipid fatty acids between the CO2 treatments in the soils of either cover types. However, neutral lipid fatty acid abundance was significantly higher under elevated than ambient [CO2] in the soils under L. tridentata, whereas no such significant difference was observed at plant interspace. These results emphasize important roles of the dominant shrubs in SOM formation under elevated [CO2] in arid ecosystems. Elevated [CO2] stimulated growth of L. tridentata in wet years, and aboveground litter deposition via senescence contributed to SOM formation in islands of fertility. In addition, elevated [CO2] stimulated soil microbial turnover rates in rhizosphere of L. tridentata, which left more soil microbial necromass, a major SOM source. We concluded that responses of dominant shrubs to elevated [CO2] can stimulate SOM formation in arid ecosystems, but biological soil crusts may have limited capacity.

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.

Do increased summer precipitation and N deposition alter fine root dynamics in a Mojave Desert ecosystem?

Climate change is expected to impact the amount and distribution of precipitation in the arid southwestern United States. In addition, nitrogen (N) deposition is increasing in these regions due to increased urbanization. Responses of belowground plant activity to increases in soil water content and N have shown inconsistent patterns between biomes. In arid lands, plant productivity is limited by water and N availability so it is expected that changes in these factors will affect fine root dynamics. The objectives of this study were to quantify the effects of increased summer precipitation and N deposition on fine root dynamics in a Mojave Desert ecosystem during a 2-year field experiment using minirhizotron measurements. Root length density, production, and mortality were measured in field plots in the Mojave Desert receiving three 25mm summer rain events and/or 40kg Nha(-1) yr(-1) . Increased summer precipitation and N additions did not have an overall significant effect on any of the measured root parameters. However, differences in winter precipitation resulting from interannual variability in rainfall appeared to affect root parameters with root production and turnover increasing following a wet winter most likely due to stimulation of annual grasses. In addition, roots were distributed more deeply in the soil following the wet winter. Root length density was initially higher under canopies compared to canopy interspaces, but converged toward the end of the study. In addition, roots tended to be distributed more deeply into the soil in canopy interspace areas. Results from this study indicated that increased summer precipitation and N deposition in response to climate change and urbanization are not likely to affect fine root dynamics in these Mojave Desert ecosystems, despite studies showing aboveground plant physiological responses to these environmental perturbations. However, changes in the amount and possibly distribution of winter precipitation may affect fine root dynamics.

Effects of elevated atmospheric CO 2 on rhizosphere soil microbial communities in a Mojave Desert ecosystem

The effects of elevated atmospheric carbon dioxide [CO 2] on microbial communities in arid rhizosphere soils beneath Larrea tridentata were examined. Roots of Larrea were harvested from plots fumigated with elevated or ambient levels of [CO 2] using Free-Air CO 2 Enrichment (FACE) technology. Twelve bacterial and fungal rRNA gene libraries were constructed, sequenced and categorized into operational taxonomical units (OTUs). There was a significant decrease in OTUs within the Firmicutes (bacteria) in elevated [CO 2], and increase in Basiomycota (fungi) in rhizosphere soils of plots exposed to ambient [CO 2]. Phylogenetic analyses indicated that OTUs belonged to a wide range of bacterial and fungal taxa. To further study changes in bacterial communities, Quantitative Polymerase Chain Reaction (QPCR) was used to quantify populations of bacteria in rhizosphere soil. The concentration of total bacteria 16S rDNA was similar in conditions of enriched and ambient [CO 2]. However, QPCR of Gram-positive microorganisms showed a 43% decrease in the population in elevated [CO 2]. The decrease in representation of Gram positives and the similar values for total bacterial DNA suggest that the representation of other bacterial taxa was promoted by elevated [CO 2]. These results indicate that elevated [CO 2] changes structure and representation of microorganisms associated with roots of desert plants.

Soil water availability and microsite mediate fungal and bacterial phospholipid fatty acid biomarker abundances in Mojave Desert soils exposed to elevated atmospheric CO2

[1] Changes in the rates of nitrogen (N) cycling, microbial carbon (C) substrate use, and extracellular enzyme activities in a Mojave Desert ecosystem exposed to elevated atmospheric CO2 suggest shifts in the size and/or functional characteristics of microbial assemblages in two dominant soil microsites: plant interspaces and under the dominant shrub Larrea tridentata. We used ester-linked phospholipid fatty acid (PLFA) biomarkers as a proxy for microbial biomass to quantify spatial and temporal differences in soil microbial communities from February 2003 to May 2005. Further, we used the 13C signature of the fossil CO2 source for elevated CO2 plots to trace recent plant C inputs into soil organic matter (SOM) and broad microbial groups using δ13C (‰). Differences between individual δ13CPLFA and δ13CSOM for fungal biomarkers indicated active metabolism of newer C in elevated CO2 soils. Total PLFA-C was greater in shrub microsites compared to plant interspaces, and CO2 treatment differences within microsites increased under higher soil water availability. Total, fungal, and bacterial PLFA-C increased with decreasing soil volumetric water content (VWC) in both microsites, suggesting general adaptations to xeric desert conditions. Increases in fungal-to-bacterial PLFA-C ratio with decreasing VWC reflected functional group-specific responses to changing soil water availability. While temporal and spatial extremes in resource availability in desert ecosystems contribute to the difficulty in identifying common trends or mechanisms driving microbial responses in less extreme environments, we found that soil water availability and soil microsite interacted with elevated CO2 to shift fungal and bacterial biomarker abundances in Mojave Desert soils.

Transitory effects of elevated atmospheric CO2 on fine root dynamics in an arid ecosystem do not increase long‐term soil carbon input from fine root litter

Experimental increases in atmospheric CO₂ often increase root production over time, potentially increasing soil carbon (C) sequestration. Effects of elevated atmospheric CO₂ on fine root dynamics in a Mojave desert ecosystem were examined for the last 4.5 yr of a long-term (10-yr) free air CO₂ enrichment (FACE) study at the Nevada desert FACE facility (NDFF). Sets of minirhizotron tubes were installed at the beginning of the NDFF experiment to characterize rooting dynamics of the dominant shrub Larrea tridentata, the codominant shrub Ambrosia dumosa and the plant community as a whole. Although significant treatment effects occurred sporadically for some fine root measurements, differences were transitory and often in opposite directions during other time-periods. Nonetheless, earlier root growth under elevated CO₂ helped sustain increased assimilation and shoot growth. Overall CO₂ treatment effects on fine root standing crop, production, loss, turnover, persistence and depth distribution were not significant for all sampling locations. These results were similar to those that occurred near the beginning of the NDFF experiment but unlike those in other ecosystems. Thus, increased C input into soils is unlikely to occur from fine root litter under elevated atmospheric CO₂ in this arid ecosystem.

Cyanobacteria in Soils from a Mojave Desert Ecosystem

The Fort Irwin National Training Center in the Mojave Desert of California contains soils that represent a diversity of disturbance regimes, including some soil sites that have been protected from anthropogenic disturbance for many years. Previous studies of the soils of this military installation discussed biogeography and ecology of the crusts but did not conduct detailed study of the species present. In this study, we selected 6 sites in and around the Fort Irwin Training Center to be the subject of intensive isolation efforts. These sites contained at least some development of microbiotic soil crust. Out of the more than 90 cyanobacterial isolates characterized, only 23 distinctive morphospecies were differentiated. Of these, only 13 were identified to a previously described species, the others being comparable to previously described aquatic species (cf.) or so dissimilar to described taxa that they were given numbers as identification. Leptolyngbya was the most species rich and also the most commonly isolated genus. Twelve filamentous genera were found; no coccoid representatives occurred in our samples. All taxa are described and illustrated. This study is significant as it is the first since the 1960s to examine cyanobacterial taxonomy of Mojave Desert soils in a systematic fashion. Our unidentified taxa are very likely new species but will require molecular sequence analysis before they can be named.

Effects of changing precipitation regimes on dryland soil respiration and C pool dynamics at rainfall event, seasonal and interannual scales

Predicted precipitation regime changes in arid ecosystems have the potential to alter soil C balance, but the influence of changes in different aspects of precipitation (amount, seasonality, and intensity) and the factors contributing to such effects are poorly understood. We used a process‐based ecosystem model (PALS) that was modified, parameterized, and evaluated for a Mojave Desert ecosystem in the southwestern US to simulate how dryland soil respiration (Rs) and soil C pool size responded to precipitation changes at multiple temporal scales. The effects of changing precipitation on Rswere largely mediated by the contrasting respiratory response of plants (Ra) and soil microbes (Rh) and confounded by predrought duration, season, soil temperature, plant phenology, and substrate availability. Increases in precipitation amount stimulated Rsand increased the contribution of Rato Rs, whereas reductions in summer rainfall and strong increases in rainfall event size reduced total Rsand decreased the contribution of Rato Rs. Increases in annual rainfall and decreases in summer rainfall benefited dryland soil C sequestration, whereas strong increases in rainfall event size resulted in a loss of soil C, with labile soil C pools being more responsive to precipitation regime changes than recalcitrant C pools at a decadal scale. These simulation results implied that dryland soils may act as C sinks with increased precipitation amount or C sources with decreased precipitation amount, but the strength of the sink/source may be mediated by accompanying shifts in rainfall seasonality and event size distribution.

Large annual net ecosystem CO2 uptake of a Mojave Desert ecosystem

AbstractThe net ecosystem CO2 exchange (NEE) between a Mojave Desert ecosystem and the atmosphere was measured over the course of 2 years at the Mojave Global Change Facility (MGCF, Nevada, USA) using the eddy covariance method. The investigated desert ecosystem was a sink for CO2, taking up 102±67 and 110±70 g C m−2 during 2005 and 2006, respectively. A comprehensive uncertainty analysis showed that most of the uncertainty of the inferred sink strength was due to the need to account for the effects of air density fluctuations on CO2 densities measured with an open‐path infrared gas analyser. In order to keep this uncertainty within acceptable bounds, highest standards with regard to maintenance of instrumentation and flux measurement postprocessing have to be met. Most of the variability in half‐hourly NEE was explained by the amount of incident photosynthetically active radiation (PAR). On a seasonal scale, PAR and soil water content were the most important determinants of NEE. Precipitation events resulted in an initial pulse of CO2 to the atmosphere, temporarily reducing NEE or even causing it to switch sign. During summer, when soil moisture was low, a lag of 3–4 days was observed before the correlation between NEE and precipitation switched from positive to negative, as opposed to conditions of high soil water availability in spring, when this transition occurred within the same day the rain took place. Our results indicate that desert ecosystem CO2 exchange may be playing a much larger role in global carbon cycling and in modulating atmospheric CO2 levels than previously assumed – especially since arid and semiarid biomes make up >30% of Earth's land surface.

Laboratory incubations reveal potential responses of soil nitrogen cycling to changes in soil C and N availability in Mojave Desert soils exposed to elevated atmospheric CO2

AbstractElevated atmospheric carbon dioxide (CO2) has the potential to alter soil carbon (C) and nitrogen (N) cycling in arid ecosystems through changes in net primary productivity. However, an associated feedback exists because any sustained increases in plant productivity will depend upon the continued availability of soil N. We took soils from under the canopies of major shrubs, grasses, and plant interspaces in a Mojave Desert ecosystem exposed to elevated atmospheric CO2 and incubated them in the laboratory with amendments of labile C and N to determine if elevated CO2 altered the mechanistic controls of soil C and N on microbial N cycling. Net ammonification increased under shrubs exposed to elevated CO2, while net nitrification decreased. Elevated CO2 treatments exhibited greater fluxes of N2O–N under Lycium spp., but not other microsites. The proportion of microbial/extractable organic N increased under shrubs exposed to elevated CO2. Heterotrophic N2‐fixation and C mineralization increased with C addition, while denitrification enzyme activity and N2O–N fluxes increased when C and N were added in combination. Laboratory results demonstrated the potential for elevated CO2 to affect soil N cycling under shrubs and supports the hypothesis that energy limited microbes may increase net inorganic N cycling rates as the amount of soil‐available C increases under elevated CO2. The effect of CO2 enrichment on N‐cycling processes is mediated by its effect on the plants, particularly shrubs. The potential for elevated atmospheric CO2 to lead to accumulation of NH4+ under shrubs and the subsequent volatilization of NH3 may result in greater losses of N from this system, leading to changes in the form and amount of plant‐available inorganic N. This introduces the potential for a negative feedback mechanism that could act to constrain the degree to which plants can increase productivity in the face of elevated atmospheric CO2.

MICROHABITAT-SPECIFIC CONTROLS ON SOIL RESPIRATION AND DENITRIFICATION IN THE MOJAVE DESERT: THE ROLE OF HARVESTER ANT NESTS AND VEGETATION

Seed harvesting ants (Pogonomyrmex rugosus) concentrate organic matter and nutrients near their nests and create biogeochemical hotspots in desert soil. We examined factors regulating denitrification and soil respiration in a Mojave Desert ecosystem to determine the role harvester ant colonies play in nitrogen loss and carbon mineralization. Organic matter and nutrient storage were significantly greater in colonies than under the dominant vegetation (i.e., Pleuraphis rigida, a bunch grass) and in bare soil, with standing stocks of inorganic nitrogen in colonies nearly 4-fold greater than in the other microhabitats. Soil respiration, measured with laboratory incubations, was below detection limits under ambient soil conditions. Respiration rate in soil from bare patches and under grass was limited by water and labile organic carbon, with secondary nitrate limitation evident only once carbon limitation was alleviated. Soil respiration in ant nest soil was limited by water and labile organic carbon only. Denitrification, measured by the acetylene block technique, was elevated in bare soil and under grass with the addition of nitrate. Ant nest soil also responded to a nitrate addition; however, denitrification rate was greatest with addition of glucose. Ordinarily desert soil has low rates of respiration and denitrification due to dry ambient conditions. However, ant colonies likely function as important sites for nitrogen loss and carbon mineralization following rain storms, especially if storms coincide with seed set and the temporally pulsed input of organic matter into colonies.

Precipitation history and ecosystem response to multidecadal precipitation variability in the Mojave Desert region, 1893–2001

Precipitation varied substantially in the Mojave Desert through the 20th century in a manner broadly similar to the other warm North American deserts. Episodes of drought and prolonged dry conditions (1893–1904, ca. 1942–1975, and 1999-present) alternated with relatively wet periods (1905–ca. 1941 and ca. 1976–1998), probably because of global-scale climate fluctuations. These are the El Niño-Southern Oscillation that affects interannual climate and the Pacific Decadal Oscillation that evidently causes decadal-scale variability such as prolonged dry and wet episodes. Studies done in the late 20th century demonstrate that precipitation fluctuations affected populations of perennial vegetation, annuals, and small herbivores. Landscape rephotography reveals that several species, particularly creosote bush, increased in size and density during the ca. 1976–1998 wet period. A brief, intense drought from 1989 to 1991 and the ongoing drought caused widespread mortality of certain species; for example, chenopods and perennial grasses suffered up to 100% mortality. Drought pruning, the shedding of above-ground biomass to reduce carbon allocation, increased substantially during drought. Overall, drought had the greatest influence on the Mojave Desert ecosystem.

LizLand: A geomorphic approach to lizard habitat modeling in the Mojave Desert

The macro-habitat preferences of three conspicuous and widely distributed species of lizards ( Aspidoscelis tigris, Callisaurus draconoides, and Uta stansburiana) were examined across four geomorphic landforms (sandy wash, rocky wash, alluvial plain, and alluvial deposit) in the southern Mojave Desert, California. All three species were non-randomly distributed across the four geomorphic landforms. The goal of this study was to develop less ecologically generalized habitat models (LizLand) than the vegetation-based wildlife–habitat relationship models in the California Gap Analysis Program (CA-GAP). Conceptually, LizLand is a geomorphological approach to habitat modeling in arid environments. Specifically, LizLand is a series of spatially explicit habitat models that define and predict habitat for A. tigris, C. draconoides, and U. stansburiana in Joshua Tree National Park and the Marine Air Ground Task Force Training Command, Marine Corp Air Ground Combat Center. LizLand models resulted in higher resolution habitat models with minimal reduction in model accuracy. These models more accurately captured the complexity of the Mojave Desert ecosystem and offered greater ecological resolution in identifying habitat in contrast to the CA-GAP models.

CO2ENRICHMENT REDUCES THE ENERGETIC COST OF BIOMASS CONSTRUCTION IN AN INVASIVE DESERT GRASS

To examine how global change could influence species invasions, we compared the responses of energetic processes and growth of invasive and native grass species to atmospheric CO2 enrichment in an intact Mojave Desert ecosystem. Combined with its modest influence on photosynthetic activity, elevated atmospheric CO2 was associated with a significant reduction in the energetic cost of aboveground biomass construction in invasive Bromus madritensis spp. rubens (red brome) without a concurrent cost reduction in native Vulpia octoflora (six-weeks fescue). Consequently, the invasive grass species grew faster, grew bigger, and produced more seeds with atmospheric CO2 enrichment than the native grass species. As a physiological mechanism of invasive species success driven by CO2 enrichment, such alterations in biomass construction costs combined with increased photosynthetic activity could trigger a shift in the species composition of this ecosystem, and potentially that of other invaded ecosystems, toward increa...

In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability

AbstractRising 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.

Functional ecology of shrub seedlings after a natural recruitment event at the Nevada Desert FACE Facility

AbstractSeedling recruitment is an important determinant of community structure in desert ecosystems. Positive photosynthetic growth and water balance responses to increasing atmospheric carbon dioxide (CO2) concentrations ([CO2]) are predicted to be substantial in desert plants, suggesting that recruitment could be stimulated. However, to date no studies have addressed the response of perennial plant recruitment in natural populations of desert shrubs exposed to elevated [CO2]. In April 1997, we employed Free‐Air Carbon Dioxide Enrichment (FACE) in order to increase atmospheric [CO2] in an undisturbed Mojave Desert ecosystem from ambient (∼∼ 370 µmol mol−1) to elevated CO2 (∼∼ 550 µmol mol−1). From 1997 to 2001 we seasonally examined survival, growth, gas exchange and water potential responses of Larrea tridentata and Ambrosia dumosa seedlings that germinated in Fall, 1997. Recruitment densities were not influenced by [CO2] in either species, although a two‐fold higher adult Ambrosia density under elevated [CO2] resulted in two‐fold higher seedling density (0.87 vs 0.40 seedlings m−2). Mortality was greatest for both species during the first summer (1998), despite above‐average rainfall during the previous Winter–Spring. A significant [CO2] × time interaction revealed that early survival was greater under elevated CO2, whereas a significant species time interaction revealed that overall survival was greater for Ambrosia (28%) than for Larrea (15%), regardless of [CO2]. Microsite (understorey or interspace) alone had no significant influence on survival. Significant species, microsite and species × microsite effects on growth (seedling height, stem diameter and canopy size) were found, but elevated CO2 had minimal impact on these parameters. Photosynthetic rates (Asat) for both species were higher at elevated [CO2] during certain seasons, but not consistently so. These results suggest that increased atmospheric [CO2] may enhance carbon (C) assimilation and survival of aridland perennial shrubs during favourable growing conditions, but that it may not counteract the effects of prolonged drought on mortality.

Effects of elevated carbon dioxide on green leaf tissue and leaf litter quality in an intact Mojave Desert ecosystem

AbstractThe 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.