ALBERT

Description: Soil CO 2 efflux ( F soil ) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO 2 ] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long-term effects of these factors on F soil are less clear. Expanding on previous studies at the Duke Free Air CO 2 Enrichment (FACE) site, we quantified the effects of elevated [CO 2 ] and N fertilization on F soil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient-unfertilized plots, annual F soil increased under elevated [CO 2 ] (~17%) and decreased with N (~21%). N fertilization under elevated [CO 2 ] reduced F soil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity but declined after productivity saturated. Despite treatment-induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Inter-annually, low soil water content decreased annual F soil from potential values – estimated based on temperature alone assuming non-limiting soil water content – by ~0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO 2 ]. Variability of soil N availability among plots accounted for the spatial variability of F soil , showing a decrease of ~114 g C m -2 y -1 per 1 g m -2 increase in soil N availability, with consistently higher F soil in elevated [CO 2 ] plots ~127 g C per 100 ppm [CO 2 ] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO 2 ] and N fertilization on F soil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production. This article is protected by copyright. All rights reserved.

Description: The Allen Telescope Array (ATA) at the Hat Creek Radio Observatory (HCRO) is a wide-field panchromatic radio telescope currently consisting of 42 offset-Gregorian antennas each with a 6 m aperture, with plans to expand the array to 350 antennas. Through unique back-end hardware, the ATA performs real-time wideband beamforming with independent subarray capabilities and customizable beam shaping. The beamformers enable science observations requiring the full gain of the array, time domain (nonintegrated) output, and interference excision or orthogonal beamsets. In this paper we report on the design of this beamformer, including architecture and experimental results. Furthermore, we address some practical considerations in large-N wideband beamformers implemented on field programmable gate array platforms, including device utilization, methods of calibration and control, and interchip synchronization.

Description: Predicted responses of transpiration to elevated atmospheric CO 2 concentration (eCO 2 ) are highly variable among process-based models. To better understand and constrain this variability among models, we conducted an intercomparison of 11 ecosystem models applied to data from two forest free-air CO 2 enrichment (FACE) experiments at Duke University and Oak Ridge National Laboratory. We analysed model structures in order to identify the key underlying assumptions causing differences in model predictions of transpiration and canopy water-use efficiency. We then compared the models against data to identify model assumptions that are incorrect or are large sources of uncertainty. We found that model-to-model and model-to-observations differences resulted from four key sets of assumptions, namely: (i) the nature of the stomatal response to elevated CO 2 (coupling between photosynthesis and stomata was supported by the data); (ii) the roles of the leaf and atmospheric boundary layer (models which assumed multiple conductance terms in series predicted more decoupled fluxes than observed at the broadleaf site); (iii) the treatment of canopy interception (large inter-model variability, 2-15 %); and (iv) the impact of soil moisture stress (process uncertainty in how models limit carbon and water fluxes during moisture stress). Overall, model predictions of the CO 2 effect on WUE were reasonable (inter-model μ = ~28 ± 10 %) compared to the observations (μ = ~30 ± 13 %) at the well-coupled coniferous site (Duke), but poor (inter-model μ = ~24 ± 6 %; observations μ = ~38 ± 7 %) at the broadleaf site (Oak Ridge). The study yields a framework for analysing and interpreting model predictions of transpiration responses to eCO 2 , and highlights key improvements to these types of models. © 2013 Blackwell Publishing Ltd

Description: [1]  The Dead Sea, located in the rift valley between Jordan and Israel, is a hypersaline lake, resulting in unique biogeochemistry and optical properties. In the spring of 2004 we conducted two days of physical and optical measurements in the lake. Because of the significant effect of dissolved salts onthe optical propertiesof water,our analysis required a novel processing approach to obtain dissolved and total inherent optical properties from the measurements. In addition, we show that the lake's salinity can be estimated from measurements of hyper-spectral absorption or attenuation spectra in the red and infrared parts of the spectrum, using published values of specific absorption of dissolved NaCl, despite the fact that the lake's salt chemistry is complex. In situ observations demonstrated that the lake has a two-layer structure with a warm and more turbid layer at the top 20-30 m and a clearer colder layer below. Both the particulate and dissolved absorption are well approximated by exponentially decreasing functions with the spectral slope of the particulate absorption about half that of the dissolved fraction and consistent with other aquatic environments. Both have relatively low and similar magnitudes in the blue (O(0.15 m -1 )). Mean particle size was observed to increase with depth consistent with precipitating salt crystals (observed in past campaigns) shown here toplay a major role in the lake's optical properties.

Description: The role of evapotranspiration (ET) in the global, continental, regional, and local water cycles is reviewed. Elevated atmospheric CO2, air temperature, vapor pressure deficit (D), turbulent transport, radiative transfer, and reduced soil moisture all impact biotic and abiotic processes controlling ET that must be extrapolated to large scales. Suggesting a blueprint to achieve this link is the main compass of this review. Leaf-scale transpiration (fe) as governed by the plant biochemical demand for CO2 is first considered. When this biochemical demand is combined with mass transfer formulations, the problem remains mathematically intractable, requiring additional assumptions. A mathematical “closure” that assumes stomatal aperture is autonomously regulated so as to maximize the leaf carbon gain while minimizing water loss is proposed, which leads to analytical expressions for leaf-scale transpiration. This formulation predicts well the effects of elevated atmospheric CO2 and increases in D on fe. The case of soil moisture stress is then considered using extensive gas exchange measurements collected in drought studies. Upscaling the fe to the canopy is then discussed at multiple time scales. The impact of limited soil water availability within the rooting zone on the upscaled ET as well as some plant strategies to cope with prolonged soil moisture stress are briefly presented. Moving further up in direction and scale, the soil-plant system is then embedded within the atmospheric boundary layer, where the influence of soil moisture on rainfall is outlined. The review concludes by discussing outstanding challenges and how to tackle them by means of novel theoretical, numerical, and experimental approaches.

Description: In recent years, increased awareness of the potential interactions between rising atmospheric CO 2 concentrations ([CO 2 ]) and temperature has illustrated the importance of multi-factorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multi-factorial experiments, this paper synthesizes how ecosystem productivity and soil processes respond to combined warming and [CO 2 ] manipulation, and compare with those obtained in single factor [CO 2 ] and temperature manipulation experiments. Across all combined elevated [CO 2 ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [CO 2 ]-only treatment than to those in the warming-only treatment. In contrast to warming-only experiments, both the combined and the [CO 2 ]-only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [CO 2 ]-only treatment, possibly due to the warming-induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [CO 2 ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [CO 2 ] and warming, i.e . the response to the combined treatment was usually less-than-additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long-term) multifactor manipulation experiments. Because single factor CO 2 responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.

Description: Leaf responses to elevated atmospheric CO 2 concentration (C a ) are central to models of forest CO 2 exchange with the atmosphere, and constrain the magnitude of the future carbon sink. Estimating the magnitude of primary productivity enhancement of forests in elevated C a requires an understanding of how photosynthesis is regulated by diffusional and biochemical components and up-scaled to entire canopies. To test the sensitivity of leaf photosynthesis and stomatal conductance to elevated C a in time and space, we compiled a comprehensive dataset measured over ten years for a temperate pine forest of Pinus taeda but also including deciduous species, primarily Liquidambar styraciflua . We combined over one thousand controlled-response curves of photosynthesis as a function of environmental drivers (light, air C a and temperature) measured at canopy heights up to 20m over eleven years (1996-2006) to generate parameterisations for leaf-scale models for the Duke free-air CO 2 enrichment (FACE) experiment. The enhancement of leaf net photosynthesis ( A net ) in P. taeda by elevated C a of +200 μmol mol −1 was 67% for current-year needles in the upper crown in summer conditions over ten years. Photosynthetic enhancement of P. taeda at the leaf-scale increased by two-fold from the driest to wettest growing seasons. Current-year pine foliage A net was sensitive to temporal variation, while previous-year foliage A net was less responsive and overall showed less enhancement (+30%). Photosynthetic downregulation in overwintering upper canopy pine needles was small at average leaf N ( N area ), but statistically significant. In contrast, co-dominant and subcanopy L. styraciflua trees showed A net enhancement of 62% and no A net – N area adjustments. Various understory deciduous tree species showed an average A net enhancement of 42%. Differences in photosynthetic responses between overwintering pine needles and subcanopy deciduous leaves suggest that increased C a has the potential to enhance the mixed-species composition of planted pine stands and, by extension, naturally regenerating pine dominated stands.

Description: Free Air CO 2 Enrichment (FACE) experiments provide a remarkable wealth of data which can be used to evaluate and improve terrestrial ecosystem models (TEMs). In the FACE Model-Data Synthesis project (FACE-MDS), 11 TEMs were applied to two decade-long FACE experiments in temperate forests of the south eastern US—the evergreen Duke Forest and the deciduous Oak Ridge forest. In this baseline paper, we demonstrate our approach to model-data synthesis by evaluating the models' ability to reproduce observed Net Primary Productivity (NPP), transpiration and Leaf Area Index (LAI) in ambient CO 2 treatments. Model outputs were compared against observations using a range of goodness-of-fit statistics. Many models simulated annual NPP and transpiration within observed uncertainty. We demonstrate however, that high goodness-of-fit values do not necessarily indicate a successful model, because simulation accuracy may be achieved through compensating biases in component variables. For example, transpiration accuracy was sometimes achieved with compensating biases in leaf area index and transpiration per unit leaf area. Our approach to model-data synthesis therefore goes beyond goodness-of-fit to investigate the success of alternative representations of component processes. Here, we demonstrate this approach by comparing competing model hypotheses determining peak LAI. Of three alternative hypotheses—(1) optimisation to maximise carbon export, (2) increasing specific leaf area (SLA) with canopy depth and (3) the pipe model—the pipe model produced peak LAI closest to the observations. This example illustrates how datasets from intensive field experiments such as FACE can be used to reduce model uncertainty despite compensating biases, by evaluating individual model assumptions.

Description: Future carbon and water fluxes within terrestrial ecosystems will be determined by how stomatal conductance (gs) responds to rising atmospheric CO2 and air temperatures. While both short- and long-term CO2 effects on gs have been repeatedly studied, there are few studies on how gs acclimates to higher air temperatures. Six gs models were parameterized using leaf gas exchange data from black spruce (Picea mariana) seedlings grown from seed at ambient (22/16°C day/night) or elevated (30/24°C) air temperatures. Model performance was independently assessed by how well carbon gain from each model reproduced estimated carbon costs to close the seedlings' seasonal carbon budgets, a ‘long-term’ indicator of success. A model holding a constant intercellular to ambient CO2 ratio and the Ball-Berry model (based on stomatal responses to relative humidity) could not close the carbon balance for either treatment, while the Jarvis-Oren model (based on stomatal responses to vapor pressure deficit, D) and a model assuming a constant gs each closed the carbon balance for one treatment. Two models, both based on gs responses to D, performed best overall, estimating carbon uptake within 10% of carbon costs for both treatments: the Leuning model and a linear optimization model that maximizes carbon gain per unit water loss. Since gs responses in the optimization model are not a priori assumed, this approach can be used in modeling land-atmosphere exchange of CO2 and water in future climates.