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1.
J Adv Model Earth Syst ; 13(7): e2020MS002421, 2021 Jul.
Artículo en Inglés | MEDLINE | ID: mdl-34434490

RESUMEN

The Western United States is dominated by natural lands that play a critical role for carbon balance, water quality, and timber reserves. This region is also particularly vulnerable to forest mortality from drought, insect attack, and wildfires, thus requiring constant monitoring to assess ecosystem health. Carbon monitoring techniques are challenged by the complex mountainous terrain, thus there is an opportunity for data assimilation systems that combine land surface models and satellite-derived observations to provide improved carbon monitoring. Here, we use the Data Assimilation Research Testbed to adjust the Community Land Model (CLM5.0) with remotely sensed observations of leaf area and above-ground biomass. The adjusted simulation significantly reduced the above-ground biomass and leaf area, leading to a reduction in both photosynthesis and respiration fluxes. The reduction in the carbon fluxes mostly offset, thus both the adjusted and free simulation projected a weak carbon sink to the land. This result differed from a separate observation-constrained model (FLUXCOM) that projected strong carbon uptake to the land. Simulation diagnostics suggested water limitation had an important influence upon the magnitude and spatial pattern of carbon uptake through photosynthesis. We recommend that additional observations important for water cycling (e.g., snow water equivalent, land surface temperature) be included to improve the veracity of the spatial pattern in carbon uptake. Furthermore, the assimilation system should be enhanced to maximize the number of the simulated state variables that are adjusted, especially those related to the recommended observed quantities including water cycling and soil carbon.

3.
New Phytol ; 229(5): 2586-2600, 2021 03.
Artículo en Inglés | MEDLINE | ID: mdl-33118171

RESUMEN

Evergreen conifer forests are the most prevalent land cover type in North America. Seasonal changes in the color of evergreen forest canopies have been documented with near-surface remote sensing, but the physiological mechanisms underlying these changes, and the implications for photosynthetic uptake, have not been fully elucidated. Here, we integrate on-the-ground phenological observations, leaf-level physiological measurements, near surface hyperspectral remote sensing and digital camera imagery, tower-based CO2 flux measurements, and a predictive model to simulate seasonal canopy color dynamics. We show that seasonal changes in canopy color occur independently of new leaf production, but track changes in chlorophyll fluorescence, the photochemical reflectance index, and leaf pigmentation. We demonstrate that at winter-dormant sites, seasonal changes in canopy color can be used to predict the onset of canopy-level photosynthesis in spring, and its cessation in autumn. Finally, we parameterize a simple temperature-based model to predict the seasonal cycle of canopy greenness, and we show that the model successfully simulates interannual variation in the timing of changes in canopy color. These results provide mechanistic insight into the factors driving seasonal changes in evergreen canopy color and provide opportunities to monitor and model seasonal variation in photosynthetic activity using color-based vegetation indices.


Asunto(s)
Tracheophyta , Clima , Bosques , América del Norte , Fotosíntesis , Hojas de la Planta , Estaciones del Año
4.
Sci Data ; 7(1): 225, 2020 07 09.
Artículo en Inglés | MEDLINE | ID: mdl-32647314

RESUMEN

The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.

5.
Proc Natl Acad Sci U S A ; 116(28): 14071-14076, 2019 07 09.
Artículo en Inglés | MEDLINE | ID: mdl-31235581

RESUMEN

The fluxes of energy, water, and carbon from terrestrial ecosystems influence the atmosphere. Land-atmosphere feedbacks can intensify extreme climate events like severe droughts and heatwaves because low soil moisture decreases both evaporation and plant transpiration and increases local temperature. Here, we combine data from a network of temperate and boreal eddy covariance towers, satellite data, plant trait datasets, and a mechanistic vegetation model to diagnose the controls of soil moisture feedbacks to drought. We find that climate and plant functional traits, particularly those related to maximum leaf gas exchange rate and water transport through the plant hydraulic continuum, jointly affect drought intensification. Our results reveal that plant physiological traits directly affect drought intensification and indicate that inclusion of plant hydraulic transport mechanisms in models may be critical for accurately simulating land-atmosphere feedbacks and climate extremes under climate change.


Asunto(s)
Atmósfera/química , Cambio Climático , Ecosistema , Transpiración de Plantas/fisiología , Agricultura , Carbono/química , Carbono/metabolismo , Sequías , Humanos , Desarrollo de la Planta , Hojas de la Planta/química , Hojas de la Planta/crecimiento & desarrollo , Plantas/genética , Suelo/química , Agua/química , Agua/metabolismo
6.
Proc Natl Acad Sci U S A ; 116(24): 11640-11645, 2019 06 11.
Artículo en Inglés | MEDLINE | ID: mdl-31138693

RESUMEN

Northern hemisphere evergreen forests assimilate a significant fraction of global atmospheric CO2 but monitoring large-scale changes in gross primary production (GPP) in these systems is challenging. Recent advances in remote sensing allow the detection of solar-induced chlorophyll fluorescence (SIF) emission from vegetation, which has been empirically linked to GPP at large spatial scales. This is particularly important in evergreen forests, where traditional remote-sensing techniques and terrestrial biosphere models fail to reproduce the seasonality of GPP. Here, we examined the mechanistic relationship between SIF retrieved from a canopy spectrometer system and GPP at a winter-dormant conifer forest, which has little seasonal variation in canopy structure, needle chlorophyll content, and absorbed light. Both SIF and GPP track each other in a consistent, dynamic fashion in response to environmental conditions. SIF and GPP are well correlated (R 2 = 0.62-0.92) with an invariant slope over hourly to weekly timescales. Large seasonal variations in SIF yield capture changes in photoprotective pigments and photosystem II operating efficiency associated with winter acclimation, highlighting its unique ability to precisely track the seasonality of photosynthesis. Our results underscore the potential of new satellite-based SIF products (TROPOMI, OCO-2) as proxies for the timing and magnitude of GPP in evergreen forests at an unprecedented spatiotemporal resolution.


Asunto(s)
Fotosíntesis/fisiología , Ciclo del Carbono/fisiología , Clorofila/fisiología , Clima , Ecosistema , Monitoreo del Ambiente/métodos , Fluorescencia , Bosques , Complejo de Proteína del Fotosistema II/fisiología , Estaciones del Año , Luz Solar
7.
Nature ; 561(7724): 538-541, 2018 09.
Artículo en Inglés | MEDLINE | ID: mdl-30232452

RESUMEN

Plants influence the atmosphere through fluxes of carbon, water and energy1, and can intensify drought through land-atmosphere feedback effects2-4. The diversity of plant functional traits in forests, especially physiological traits related to water (hydraulic) transport, may have a critical role in land-atmosphere feedback, particularly during drought. Here we combine 352 site-years of eddy covariance measurements from 40 forest sites, remote-sensing observations of plant water content and plant functional-trait data to test whether the diversity in plant traits affects the response of the ecosystem to drought. We find evidence that higher hydraulic diversity buffers variation in ecosystem flux during dry periods across temperate and boreal forests. Hydraulic traits were the predominant significant predictors of cross-site patterns in drought response. By contrast, standard leaf and wood traits, such as specific leaf area and wood density, had little explanatory power. Our results demonstrate that diversity in the hydraulic traits of trees mediates ecosystem resilience to drought and is likely to have an important role in future ecosystem-atmosphere feedback effects in a changing climate.


Asunto(s)
Aclimatación/fisiología , Biodiversidad , Sequías , Bosques , Árboles/anatomía & histología , Árboles/fisiología , Agua/metabolismo , Atmósfera/química , Cambio Climático , Retroalimentación , Hojas de la Planta/anatomía & histología , Hojas de la Planta/metabolismo , Madera/anatomía & histología , Madera/metabolismo
8.
Oecologia ; 187(4): 1025-1039, 2018 08.
Artículo en Inglés | MEDLINE | ID: mdl-29955987

RESUMEN

Stable isotope ratios of H and O are widely used to identify the source of water, e.g., in aquifers, river runoff, soils, plant xylem, and plant-based beverages. In situations where the sampled water is partially evaporated, its isotope values will have evolved along an evaporation line (EL) in δ2H/δ18O space, and back-correction along the EL to its intersection with a meteoric water line (MWL) has been used to estimate the source water's isotope ratios. Here, we review the theory underlying isotopic estimation of source water for evaporated samples (iSWE). We note potential for bias from a commonly used regression-based approach for EL slope estimation and suggest that a model-based approach may be preferable if assumptions of the regression approach are not valid. We then introduce a mathematical framework that eliminates the need to explicitly estimate the EL-MWL intersection, simplifying iSWE analysis and facilitating more rigorous uncertainty estimation. We apply this approach to data from the US EPA's 2007 National Lakes Assessment. We find that data for most lakes are consistent with a water source similar to annual runoff, estimated from monthly precipitation and evaporation within the lake basin. Strong evidence for both summer- and winter-biased sources exists, however, with winter bias pervasive in most snow-prone regions. The new analytical framework should improve the rigor of iSWE in ecohydrology and related sciences, and our initial results from US lakes suggest that previous interpretations of lakes as unbiased isotope integrators may only be valid in certain climate regimes.


Asunto(s)
Agua Subterránea , Ríos , Lagos , Isótopos de Oxígeno , Nieve
9.
Proc Natl Acad Sci U S A ; 115(12): 2912-2917, 2018 03 20.
Artículo en Inglés | MEDLINE | ID: mdl-29507190

RESUMEN

Cities are concentrated areas of CO2 emissions and have become the foci of policies for mitigation actions. However, atmospheric measurement networks suitable for evaluating urban emissions over time are scarce. Here we present a unique long-term (decadal) record of CO2 mole fractions from five sites across Utah's metropolitan Salt Lake Valley. We examine "excess" CO2 above background conditions resulting from local emissions and meteorological conditions. We ascribe CO2 trends to changes in emissions, since we did not find long-term trends in atmospheric mixing proxies. Three contrasting CO2 trends emerged across urban types: negative trends at a residential-industrial site, positive trends at a site surrounded by rapid suburban growth, and relatively constant CO2 over time at multiple sites in the established, residential, and commercial urban core. Analysis of population within the atmospheric footprints of the different sites reveals approximately equal increases in population influencing the observed CO2, implying a nonlinear relationship with CO2 emissions: Population growth in rural areas that experienced suburban development was associated with increasing emissions while population growth in the developed urban core was associated with stable emissions. Four state-of-the-art global-scale emission inventories also have a nonlinear relationship with population density across the city; however, in contrast to our observations, they all have nearly constant emissions over time. Our results indicate that decadal scale changes in urban CO2 emissions are detectable through monitoring networks and constitute a valuable approach to evaluate emission inventories and studies of urban carbon cycles.

10.
Tree Physiol ; 37(7): 984-995, 2017 07 01.
Artículo en Inglés | MEDLINE | ID: mdl-28549168

RESUMEN

Productivity of conifers in seasonally snow-covered forests is high before and during snowmelt when environmental conditions are optimal for photosynthesis. Climate change is altering the timing of spring in many locations, and changes in the date of transition from winter dormancy can have large impacts on annual productivity. Sap flow methods provide a promising approach to monitor tree activity during the cold season and the winter-spring and fall-winter transitions. Although sap flow techniques have been widely used, cold season results are generally not reported. Here we examine the feasibility of using the Granier thermal dissipation (TD) sap flux density method to monitor transpiration and dormancy of evergreen conifers during the cold season. We conducted a laboratory experiment which demonstrated that the TD method reliably detects xylem water transport (when it occurs) both at near freezing temperature and at low flow rate, and that the sensors can withstand repeated freeze-thaw events. However, the dependence between sensor output and water transport rate in these experiments differed from the established TD relation. In field experiments, sensors installed in two Abies forests lasted through two winters and a summer with low failure. The baseline (no-flow) sensor output varied considerably with temperature during the cold season, and a new baseline algorithm was developed to accommodate this variation. The Abies forests differed in elevation (2070 and 2620 m), and there was a clear difference in timing of initiation and cessation of transpiration between them. We conclude that the TD method can be reliably used to examine water transport during cold periods with associated low flow conditions.


Asunto(s)
Abies/fisiología , Frío , Bosques , Estaciones del Año , Nieve , Cambio Climático , Transpiración de Plantas , Árboles/fisiología , Agua , Xilema/fisiología
11.
Glob Chang Biol ; 23(10): 4204-4221, 2017 10.
Artículo en Inglés | MEDLINE | ID: mdl-28295911

RESUMEN

Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO2 sink. However, such analyses are poorly constrained by measured CO2 exchange in drylands. Here we address this observation gap with eddy covariance data from 25 sites in the water-limited Southwest region of North America with observed ranges in annual precipitation of 100-1000 mm, annual temperatures of 2-25°C, and records of 3-10 years (150 site-years in total). Annual fluxes were integrated using site-specific ecohydrologic years to group precipitation with resulting ecosystem exchanges. We found a wide range of carbon sink/source function, with mean annual net ecosystem production (NEP) varying from -350 to +330 gCm-2 across sites with diverse vegetation types, contrasting with the more constant sink typically measured in mesic ecosystems. In this region, only forest-dominated sites were consistent carbon sinks. Interannual variability of NEP, gross ecosystem production (GEP), and ecosystem respiration (Reco ) was larger than for mesic regions, and half the sites switched between functioning as C sinks/C sources in wet/dry years. The sites demonstrated coherent responses of GEP and NEP to anomalies in annual evapotranspiration (ET), used here as a proxy for annually available water after hydrologic losses. Notably, GEP and Reco were negatively related to temperature, both interannually within site and spatially across sites, in contrast to positive temperature effects commonly reported for mesic ecosystems. Models based on MODIS satellite observations matched the cross-site spatial pattern in mean annual GEP but consistently underestimated mean annual ET by ~50%. Importantly, the MODIS-based models captured only 20-30% of interannual variation magnitude. These results suggest the contribution of this dryland region to variability of regional to global CO2 exchange may be up to 3-5 times larger than current estimates.


Asunto(s)
Ecosistema , Bosques , Dióxido de Carbono , América del Norte , Temperatura
12.
Oecologia ; 181(4): 1221-31, 2016 08.
Artículo en Inglés | MEDLINE | ID: mdl-27102809

RESUMEN

Natural abundance nitrate (NO3 (-)) isotopes represent a powerful tool for assessing denitrification, yet the scale and context dependence of relationships between isotopes and denitrification have received little attention, especially in surface soils. We measured the NO3 (-) isotope compositions in soil extractions and lysimeter water from a semi-arid meadow and lawn during snowmelt, along with the denitrification potential, bulk O2, and a proxy for anaerobic microsites. Denitrification potential varied by three orders of magnitude and the slope of δ(18)O/δ(15)N in soil-extracted NO3 (-) from all samples measured 1.04 ± 0.12 (R (2) = 0.64, p < 0.0001), consistent with fractionation from denitrification. However, δ(15)N of extracted NO3 (-) was often lower than bulk soil δ(15)N (by up to 24 ‰), indicative of fractionation during nitrification that was partially overprinted by denitrification. Mean NO3 (-) isotopes in lysimeter water differed from soil extractions by up to 19 ‰ in δ(18)O and 12 ‰ in δ(15)N, indicating distinct biogeochemical processing in relatively mobile water versus soil microsites. This implies that NO3 (-) isotopes in streams, which are predominantly fed by mobile water, do not fully reflect terrestrial soil N cycling. Relationships between potential denitrification and δ(15)N of extracted NO3 (-) showed a strong threshold effect culminating in a null relationship at high denitrification rates. Our observations of (1) competing fractionation from nitrification and denitrification in redox-heterogeneous surface soils, (2) large NO3 (-) isotopic differences between relatively immobile and mobile water pools, (3) and the spatial dependence of δ(18)O/δ(15)N relationships suggest caution in using NO3 (-) isotopes to infer site or watershed-scale patterns in denitrification.


Asunto(s)
Desnitrificación , Suelo/química , Monitoreo del Ambiente , Nitratos , Isótopos de Nitrógeno , Isótopos de Oxígeno
13.
Environ Sci Technol ; 50(3): 1137-46, 2016 Feb 02.
Artículo en Inglés | MEDLINE | ID: mdl-26744921

RESUMEN

Snowmelt dominates the hydrograph of many temperate montane streams, yet little work has characterized how streamwater sources and nitrogen (N) dynamics vary across wildland to urban land use gradients in these watersheds. Across a third-order catchment in Salt Lake City, Utah, we asked where and when groundwater vs shallow surface water inputs controlled stream discharge and N dynamics. Stream water isotopes (δ(2)H and δ(18)O) reflected a consistent snowmelt water source during baseflow. Near-chemostatic relationships between conservative ions and discharge implied that groundwater dominated discharge year-round across the montane and urban sites, challenging the conceptual emphasis on direct stormwater inputs to urban streams. Stream and groundwater NO3(-) concentrations remained consistently low during snowmelt and baseflow in most montane and urban stream reaches, indicating effective subsurface N retention or denitrification and minimal impact of fertilizer or deposition N sources. Rather, NO3(-) concentrations increased 50-fold following urban groundwater inputs, showing that subsurface flow paths potentially impact nutrient loading more than surficial land use. Isotopic composition of H2O and NO3(-) suggested that snowmelt-derived urban groundwater intercepted NO3(-) from leaking sewers. Sewer maintenance could potentially mitigate hotspots of stream N inputs at mountain/valley transitions, which have been largely overlooked in semiarid urban ecosystems.


Asunto(s)
Agua Subterránea/química , Nitrógeno/análisis , Ríos/química , Ciudades , Desnitrificación , Deuterio/análisis , Ecosistema , Monitoreo del Ambiente/métodos , Agua Subterránea/análisis , Nitratos/análisis , Isótopos de Nitrógeno/análisis , Nieve , Utah
14.
Plant Cell Environ ; 39(7): 1513-23, 2016 07.
Artículo en Inglés | MEDLINE | ID: mdl-26824577

RESUMEN

Bark beetle outbreaks are widespread in western North American forests, reducing primary productivity and transpiration, leading to forest mortality across large areas and altering ecosystem carbon cycling. Here the carbon isotope composition (δ(13) C) of soil respiration (δJ ) was monitored in the decade after disturbance for forests affected naturally by mountain pine beetle infestation and artificially by stem girdling. The seasonal mean δJ changed along both chronosequences. We found (a) enrichment of δJ relative to controls (<1 ‰) in near-surface soils in the first 2 years after disturbance; (b) depletion (1‰ or no change) during years 3-7; and (c) a second period of enrichment (1-2‰) in years 8-10. Results were consistent with isotopic patterns associated with the gradual death and decomposition of rhizosphere organisms, fine roots, conifer needles and woody roots and debris over the course of a decade after mortality. Finally, δJ was progressively more (13) C-depleted deeper in the soil than near the surface, while the bulk soil followed the well-established pattern of (13) C-enrichment at depth. Overall, differences in δJ between mortality classes (<1‰) and soil depths (<3‰) were smaller than variability within a class or depth over a season (up to 6‰).


Asunto(s)
Ciclo del Carbono , Escarabajos , Bosques , Micorrizas/metabolismo , Suelo/química , Animales , Isótopos de Carbono/análisis , Colorado
15.
Plant Cell Environ ; 33(11): 1804-19, 2010 Nov.
Artículo en Inglés | MEDLINE | ID: mdl-20545887

RESUMEN

Carbon isotope ratios (δ¹³C) of heterotrophic and rhizospheric sources of soil respiration under deciduous trees were evaluated over two growing seasons. Fluxes and δ¹³C of soil respiratory CO2 on trenched and untrenched plots were calculated from closed chambers, profiles of soil CO2 mole fraction and δ¹³C and continuous open chambers. δ¹³C of respired CO2 and bulk carbon were measured from excised leaves and roots and sieved soil cores. Large diel variations (>5‰) in δ¹³C of soil respiration were observed when diel flux variability was large relative to average daily fluxes, independent of trenching. Soil gas transport modelling supported the conclusion that diel surface flux δ¹³C variation was driven by non-steady state gas transport effects. Active roots were associated with high summertime soil respiration rates and around 1‰ enrichment in the daily average δ¹³C of the soil surface CO2 flux. Seasonal δ¹³C variability of about 4‰ (most enriched in summer) was observed on all plots and attributed to the heterotrophic CO2 source.


Asunto(s)
Dióxido de Carbono/análisis , Modelos Biológicos , Suelo/química , Árboles/metabolismo , Isótopos de Carbono/análisis , Hojas de la Planta/metabolismo , Raíces de Plantas/metabolismo , Rizosfera , Estaciones del Año , Temperatura
16.
Rapid Commun Mass Spectrom ; 24(7): 894-900, 2010 Apr 15.
Artículo en Inglés | MEDLINE | ID: mdl-20201033

RESUMEN

We present a novel technique in which the carbon isotope ratio (delta(13)C) of soil CO(2) is measured from small gas samples (<5 mL) injected into a stream of CO(2)-free air flowing into a tunable diode laser absorption spectrometer (TDL). This new method extends the dynamic range of the TDL to measure CO(2) mole fractions ranging from ambient to pure CO(2), reduces the volume of sample required to a few mL, and does not require field deployment of the instrument. The measurement precision of samples stored for up to 60 days was 0.23 per thousand. The new TDL method was applied with a simple gas well sampling technique to obtain and measure gas samples from shallow soil depth increments for CO(2) mole fraction and delta(13)C analysis, and subsequent determination of the delta(13)C of soil-respired CO(2). The method was tested using an artificial soil system containing a controlled CO(2) source and compared with an independent method using the TDL and an open soil chamber. The profile and chamber estimates of delta(13)C of an artificially produced CO(2) flux were consistent and converged to the delta(13)C of the CO(2) source at steady state, indicating the accuracy of both methods under controlled conditions. The new TDL method, in which a small pulse of sample is measured on a carrier gas stream, is analogous for the TDL technique to the development of continuous-flow configurations for isotope ratio mass spectrometry. While the applications presented here are focused on soil CO(2), this new TDL method could be applied in a number of situations requiring measurement of delta(13)C of CO(2) in small gas samples with ambient to high CO(2) mole fractions.


Asunto(s)
Dióxido de Carbono/química , Isótopos de Carbono/análisis , Análisis de Inyección de Flujo/métodos , Láseres de Semiconductores , Espectrometría de Masas/instrumentación , Espectrometría de Masas/métodos , Suelo/análisis , Absorción , Isótopos de Carbono/química
17.
New Phytol ; 178(1): 24-40, 2008.
Artículo en Inglés | MEDLINE | ID: mdl-18179603

RESUMEN

Stable carbon isotopes are used extensively to examine physiological, ecological, and biogeochemical processes related to ecosystem, regional, and global carbon cycles and provide information at a variety of temporal and spatial scales. Much is known about the processes that regulate the carbon isotopic composition (delta(13)C) of leaf, plant, and ecosystem carbon pools and of photosynthetic and respiratory carbon dioxide (CO(2)) fluxes. In this review, systematic patterns and mechanisms underlying variation in delta(13)C of plant and ecosystem carbon pools and fluxes are described. We examine the hypothesis that the delta(13)C of leaf biomass can be used as a reference point for other carbon pools and fluxes, which differ from the leaf in delta(13)C in a systematic fashion. Plant organs are typically enriched in (13)C relative to leaves, and most ecosystem pools and respiratory fluxes are enriched relative to sun leaves of dominant plants, with the notable exception of root respiration. Analysis of the chemical and isotopic composition of leaves and leaf respiration suggests that growth respiration has the potential to contribute substantially to the observed offset between the delta(13)C values of ecosystem respiration and the bulk leaf. We discuss the implications of systematic variations in delta(13)C of ecosystem pools and CO(2) fluxes for studies of carbon cycling within ecosystems, as well as for studies that use the delta(13)C of atmospheric CO(2) to diagnose changes in the terrestrial biosphere over annual to millennial time scales.


Asunto(s)
Dióxido de Carbono/metabolismo , Isótopos de Carbono/metabolismo , Ecosistema , Plantas/metabolismo , Procesos Autotróficos/fisiología , Respiración de la Célula/fisiología
18.
Isotopes Environ Health Stud ; 42(1): 21-35, 2006 Mar.
Artículo en Inglés | MEDLINE | ID: mdl-16500752

RESUMEN

Recent measurements of carbon isotopes in carbon dioxide using near-infrared, diode-laser-based cavity ring-down spectroscopy (CRDS) are presented. The CRDS system achieved good precision, often better than 0.2 per thousand, for 4% CO2 concentrations, and also achieved 0.15-0.25 per thousand precision in a 78 min measurement time with cryotrap-based pre-concentration of ambient CO2 concentrations (360 ppmv). These results were obtained with a CRDS system possessing a data rate of 40 ring-downs per second and a loss measurement of 4.0 x 10(-11) cm(-1) Hz(-1/2). Subsequently, the measurement time has been reduced to under 10 min. This standard of performance would enable a variety of high concentration (3-10%) isotopic measurements, such as medical human breath analysis or animal breath experiments. The extension of this ring-down to the 2 microm region would enable isotopic analysis at ambient concentrations, which, combined with the small size, robust design, and potential for frequent measurements at a remote site, make CRDS technology attractive for remote atmospheric measurement applications.


Asunto(s)
Aire/análisis , Dióxido de Carbono/química , Isótopos de Carbono/química , Espectroscopía Infrarroja por Transformada de Fourier , Humanos , Reproducibilidad de los Resultados , Espectroscopía Infrarroja por Transformada de Fourier/instrumentación , Espectroscopía Infrarroja por Transformada de Fourier/métodos
19.
Oecologia ; 131(1): 113-124, 2002 Mar.
Artículo en Inglés | MEDLINE | ID: mdl-28547501

RESUMEN

Variation in the carbon isotopic composition of ecosystem respiration (δ13CR) was studied for 3  years along a precipitation gradient in western Oregon, USA, using the Keeling plot approach. Study sites included six coniferous forests, dominated by Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Juniperus occidentalis, and ranged in location from the Pacific coast to the eastern side of the Cascade Mountains (a 250-km transect). Mean annual precipitation across these sites ranged from 227 to 2,760 mm. Overall δ13CR varied from -23.1 to -33.1‰, and within a single forest, it varied in magnitude by 3.5-8.5‰. Mean annual δ13CR differed significantly in the forests and was strongly correlated with mean annual precipitation. The carbon isotope ratio of carbon stocks (leaves, fine roots, litter, and soil organic matter) varied similarly with mean precipitation (more positive at the drier sites). There was a strong link between δ13CR and the vapor saturation deficit of air (vpd) 5-10 days earlier, both across and within sites. This relationship is consistent with stomatal regulation of gas exchange and associated changes in photosynthetic carbon isotope discrimination. Recent freeze events caused significant deviation from the δ13CR versus vpd relationship, resulting in higher than expected δ13CR values.

20.
Tree Physiol ; 19(14): 917-924, 1999 Dec.
Artículo en Inglés | MEDLINE | ID: mdl-12651303

RESUMEN

Because the rate of isoprene (2-methyl-1,3-butadiene) emission from plants is highly temperature-dependent, we investigated natural fluctuations in leaf temperature and effects of rapid temperature change on isoprene emission of red oak (Quercus rubra L.) leaves at the top of the canopy at Harvard Forest. Throughout the day, leaves often reached temperatures as much as 15 degrees C above air temperature. The highest temperatures were reached for only a few seconds at a time. We compared isoprene emission rates measured when leaf temperature was changed rapidly with those measured when temperature was changed slowly. In all cases, isoprene emission rate increased with increasing leaf temperature up to about 32 degrees C and then decreased with higher temperatures. The temperature at which isoprene emission rates began to decrease depended on how quickly measurements were made. Isoprene emission rates peaked at 32.5 degrees C when measured hourly, whereas rates peaked at 39 degrees C when measurements were made every four minutes. This behavior reflected the rapid increase in isoprene emission rate that occurred immediately after an increase in leaf temperature, and the subsequent decrease in isoprene emission rate when leaf temperature was held steady for longer than 20 minutes. We concluded that the observed temperature response of isoprene emission rate is a function of measurement protocol. Omitting this parameter from isoprene emission models will not affect simulated isoprene emission rates at mild temperatures, but can increase isoprene emission rates at high temperatures.

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