Kimberlee L. Sparks

Spatial and temporal controls of soil respiration rate in a high-elevation, subalpine forest

Abstract We examined soil respiration to determine what measurable environmental variables can be used to predict variation in soil respiration rates, spatially and temporally, at a high-elevation, mixed conifer, subalpine forest site at the Niwot Ridge Ameriflux Site in Colorado. For three summers, soil respiration rates were measured using soil collars and a portable gas-exchange system. Transects of the collars were established to ensure spatial characterization of the litter-repleted areas beneath tree crowns and the litter-depleted open spaces between tree crowns. Soil temperature and soil moisture were both identified as important drivers of soil respiration rate, but were found to confound each other and to function as primary controls at different scales. Soil temperature represents a primary control seasonally, and soil moisture represents a primary control interannually. Spatially, organic layer thickness, ammonium concentration, water content, and the microbial and soil soluble carbon pools were found to predict variation from point to point. Soil microbial biomass strongly correlated to soil respiration rate, whereas root biomass was identified as a weak predictor of respiration rate and only when controlling for other variables. Spatial variation in soil respiration rate is highly determined by the depth of the soil organic horizon, which in this ecosystem varies predictably according to distance from trees. The conclusions that can be drawn from the study provide the foundation for the development of future models of soil respiration driven by fundamental variables of the climate and soil microenvironment.

Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest: implications for tropospheric chemistry

Tropical forest soils are known to emit large amounts of reactive nitrogen oxide compounds, often referred to collectively as NOy (NOy = NO + NO2 + HNO3 + organic nitrates). Plants are known to assimilate and emit NOy and it is therefore likely that plant canopies affect the atmospheric concentration of reactive nitrogen compounds by assimilating or emitting some fraction of the soil-emitted NOy. It is crucial to understand the magnitude of the canopy effects and the primary environmental and physiological controls over NOy exchange in order to accurately quantify regional NOy inventories and parameterize models of tropospheric photochemistry. In this study we focused on nitrogen dioxide (NO2), which is the component of NOy that most directly catalyzes the chemistry of O3 dynamics, one of the most abundant oxidative species in the troposphere, and which has been reported as the NOy species that is most readily exchanged between plants and the atmosphere. Leaf chamber measurements of NO2 flux were measured in 25 tree species growing in a wet tropical forest in the Republic of Panama. NO2 was emitted to the atmosphere at ambient NO2 concentrations below 0.53–1.60xa0ppbv (the NO2 compensation point) depending on species, with the highest rate of emission being 50xa0pmol m–2 s–1 at <0.1xa0ppbv. NO2 was assimilated by leaves at ambient NO2 concentrations above the compensation point, with the maximum observed uptake rate being 1,550xa0pmol m–2 s–1 at 5xa0ppbv. No seasonal variation in leaf NO2 flux was observed in this study and leaf emission and uptake appeared to be primarily controlled by leaf nitrogen and stomatal conductance, respectively. When scaled to the entire canopy, soil NO emission rates to the atmosphere were estimated to be maximally altered ±19% by the overlying canopy.

Tree species effects on ecosystem water-use efficiency in a high-elevation, subalpine forest

Ecosystem water-use efficiency (eWUE; the ratio of net ecosystem productivity to evapotranspiration rate) is a complex landscape-scale parameter controlled by both physical and biological processes occurring in soil and plants. Leaf WUE (lWUE; the ratio of leaf CO2 assimilation rate to transpiration rate) is controlled at short time scales principally by leaf stomatal dynamics and this control varies among plant species. Little is known about how leaf-scale variation in lWUE influences landscape-scale variation in eWUE. We analyzed approximately seven thousand 30-min averaged eddy covariance observations distributed across 9xa0years in order to assess eWUE in two neighboring forest communities. Mean eWUE was 19% lower for the community in which Engelmann spruce and subalpine fir were dominant, compared to the community in which lodgepole pine was dominant. Of that 19% difference, 8% was attributed to residual bias in the analysis that favored periods with slightly drier winds for the spruce-fir community. In an effort to explain the remaining 11% difference, we assessed patterns in lWUE using C isotope ratios. When we focused on bulk tissue from older needles we detected significant differences in lWUE among tree species and between upper and lower canopy needles. However, when these differences were scaled to reflect vertical and horizontal leaf area distributions within the two communities, they provided no power to explain differences in eWUE that we observed in the eddy covariance data. When we focused only on bulk needle tissue of current-year needles for 3 of the 9xa0years, we also observed differences in lWUE among species and in needles from upper and lower parts of the canopy. When these differences in lWUE were scaled to reflect leaf area distributions within the two communities, we were able to explain 6.3% of the differences in eWUE in 1xa0year (2006), but there was no power to explain differences in the other 2xa0years (2003 and 2007). When we examined sugars extracted from needles at 3 different times during the growing season of 2007, we could explain 3.8–6.0% of the differences in eWUE between the two communities, but the difference in eWUE obtained from the eddy covariance record, and averaged over the growing season for this single year, was 32%. Thus, overall, after accounting for species effects on lWUE, we could explain little of the difference in eWUE between the two forest communities observed in the eddy covariance record. It is likely that water and C fluxes from soil, understory plants, and non-needle tissues, account for most of the differences observed in the eddy covariance data. For those cases where we could explain some of the difference in eWUE on the basis of species effects, we partitioned the scaled patterns in lWUE into two components: a component that is independent of canopy leaf area distribution, and therefore only dependent on species-specific differences in needle physiology; and a component that is independent of species differences in needle physiology, and only dependent on species-specific influences on canopy leaf area distribution. Only the component that is dependent on species influences on canopy leaf area distribution, and independent of inherent species differences in needle physiology, had potential to explain differences in eWUE between the two communities. Thus, when tree species effects are important, canopy structure, rather than species-specific needle physiology, has more potential to explain patterns in eWUE.

The effect of long-term exposure to elevated CO2 on nitrogen gas emissions from Mojave Desert soils

[1]xa0In arid regions, emissions of nitrogen (N) gases are important to long-term soil fertility and regional atmospheric chemistry, making alterations in N gas emissions an important aspect of ecosystem response to climate change. Studies at the Nevada Desert FACE Facility suggest that rising atmospheric CO2 concentrations impact ecosystems N dynamics in the Mojave Desert; our objective was to identify whether those responses translate into changes in trace N gas emissions. We measured soil fluxes of reactive N gases (NO, NOy, NH3) and N2O in plots receiving long-term fumigation with ambient and elevated (550 ppm) CO2. Reactive N gas emissions were significantly lower under elevated CO2 during high soil moisture conditions in the spring and fall. The strongest responses occurred in the islands of fertility created by the dominant shrub Larrea tridentata, where fluxes were 3–5 ng N m−2 s−1 lower in elevated CO2 plots. Changes in total N gas emissions were driven by reduced NO and NH3 emissions, with smaller changes in NOy efflux and little to no production of N2O. Lower N gas emissions under elevated CO2 reflect changes in plant and microbial demand for N, suggesting increased uptake or immobilization coupled with decreased rates of N mineralization and nitrification. This response of N gas efflux to elevated CO2 in the growing season suggests that in deserts, elevated CO2 promotes ecosystem retention of N during periods of peak biological demand. Concomitantly, exposure to elevated CO2 alters inputs of new reactive gases into the atmosphere, potentially impacting local atmospheric processes.

Population-Level Differentiation in Growth Rates and Leaf Traits in Seedlings of the Neotropical Live Oak Quercus oleoides Grown under Natural and Manipulated Precipitation Regimes

Widely distributed species are normally subjected to spatial heterogeneity in environmental conditions. In sessile organisms like plants, adaptive evolution and phenotypic plasticity of key functional traits are the main mechanisms through which species can respond to environmental heterogeneity and climate change. While extended research has been carried out in temperate species in this regard, there is still limited knowledge as to how species from seasonally-dry tropical climates respond to spatial and temporal variation in environmental conditions. In fact, studies of intraspecific genetically-based differences in functional traits are still largely unknown and studies in these ecosystems have largely focused on in situ comparisons where environmental and genetic effects cannot be differentiated. In this study, we tested for ecotypic differentiation and phenotypic plasticity in leaf economics spectrum (LES) traits, water use efficiency and growth rates under natural and manipulated precipitation regimes in a common garden experiment where seedlings of eight populations of the neotropical live oak Quercus oleoides were established. We also examined the extent to which intraspecific trait variation was associated with plant performance under different water availability. Similar to interspecific patterns among seasonally-dry tropical tree species, live oak populations with long and severe dry seasons had higher leaf nitrogen content and growth rates than mesic populations, which is consistent with a “fast” resource-acquisition strategy aimed to maximize carbon uptake during the wet season. Specific leaf area (SLA) was the best predictor of plant performance, but contrary to expectations, it was negatively associated with relative and absolute growth rates. This observation was partially explained by the negative association between SLA and area-based photosynthetic rates, which is contrary to LES expectations but similar to other recent intraspecific studies on evergreen oaks. Overall, our study shows strong intraspecific differences in functional traits in a tropical oak, Quercus oleoides, and suggests that precipitation regime has played an important role in driving adaptive divergence in this widespread species.

Soil carbon dioxide emissions from the Mojave desert: Isotopic evidence for a carbonate source

Arid soils represent a substantial carbonate pool, and may participate in surface-atmosphere CO2 exchange via a diel cycle of carbonate dissolution and exsolution. We used a Keeling plot approach to determine the substrate δ13C of CO2 emitted from carbonate-dominated soils in the Mojave Desert, and found evidence for a non-respiratory source that increased with surface temperature. In dry soils at 25-30u2009°C, the CO2 substrate had δ13C values of -19.4u2009±u20094.2‰, indicative of respiration of organic material (soil organic matteru2009=u2009-23.1u2009±u20090.8‰). CO2 flux increased with temperature; maximum fluxes occurred above 60u2009°C, where δ13CO2-substrate (-7.2‰u2009±u20092.8‰) approached soil carbonate values (0.2u2009±u20090.2 ‰). In wet soils, CO2 emissions were not temperature dependent, and δ13CO2-substrate was lower in vegetated soils with higher flux rates, higher organic C content and potential root respiration. These data provide the first direct evidence of CO2 emissions from alkaline desert soils derived from an abiotic source, and that diurnal emissions patterns are strongly driven by surface temperature.

Natural selection and neutral evolutionary processes contribute to genetic divergence in leaf traits across a precipitation gradient in the tropical oak Quercus oleoides

The impacts of drought are expanding worldwide as a consequence of climate change. However, there is still little knowledge of how species respond to long‐term selection in seasonally dry ecosystems. In this study, we used QST‐FST comparisons to investigate (i) the role of natural selection on population genetic differentiation for a set of functional traits related to drought resistance in the seasonally dry tropical oak Quercus oleoides and (ii) the influence of water availability at the site of population origin and in experimental treatments on patterns of trait divergence. We conducted a thorough phenotypic characterization of 1912 seedlings from ten populations growing in field and greenhouse common gardens under replicated watering treatments. We also genotyped 218 individuals from the same set of populations using eleven nuclear microsatellites. QST distributions for leaf lamina area, specific leaf area, leaf thickness and stomatal pore index were higher than FST distribution. Results were consistent across growth environments. Genetic differentiation among populations for these functional traits was associated with the index of moisture at the origin of the populations. Together, our results suggest that drought is an important selective agent for Q. oleoides and that differences in length and severity of the dry season have driven the evolution of genetic differences in functional traits.

A multi-analytical approach for determining the provenance of the marbles from Ruspina Roman baths (Monastir, Tunisia)

Although most of the ancient monuments found in Ruspina Roman town situated near the town of Monastir in the Sahil region of Tunisia were built with local materials, the Roman baths in this site were mainly decorated with imported marbles from Greece, Asia, Numidia, Italy and Algeria, indicating the important trade relationships between Ruspina and the other Mediterranean Roman towns. Among the seven types of marbles used in the decoration of Ruspina’s Roman baths, four have been characterized in previous studies: white Proconnesian from Asia, Penthelic and green cipolin from Greece, and Antique yellow from Simitthus in Tunisia. To characterize and identify the other three types, series of analytical methods had to be applied: minero-petrographic analysis of thin sections, chemical analysis, and isotopic analysis of oxygen and carbon. Used together, the methods enabled to identify the marbles studied: the white fine-grained marble corresponds to Carrara white marble from Italy, and the spotted dull grey and greco scritto came from Cap de Garde quarries in Algeria. With the exception on one type (antique yellow), the other six types were imported marbles.

Ethanol isotope method (EIM) for uncovering illegal wine

Isotopic methods have proven to be a powerful analytical tool for the determination of origin and authenticity of wine. In addition, measuring the stable isotope ratio provides useful information for the detection of many illegal practices in the production of wine. The determinations of the stable isotope composition of compounds are based on measuring the relative ratios using isotope ratio mass spectrometry. This article describes a new isotopic method for measuring the δD value of non-exchangeable hydrogen stable isotopes in ethanol for investigating adulteration practices in wine making. With this new method, we are able to determine the addition of water and sugar in wine with higher accuracy, repeatability and reliability.