ALBERT

Description: Seasonal dynamics of atmospheric carbonyl sulfide (OCS) at regional and continental scales and plant OCS exchange at the leaf level have shown a close relationship with those for CO2. CO2 has both sinks and sources within terrestrial ecosystems, but the primary terrestrial exchange for OCS is thought to be leaf uptake, suggesting potential for OCS uptake as a proxy for gross primary production (GPP). We explored the utility of OCS uptake as a GPP proxy in micrometeorological studies of biosphere-atmosphere CO2 exchange by applying theoretical concepts from earlier OCS studies to estimate GPP. We partitioned measured net ecosystem exchange (NEE) using the ratio of measured vertical mole fraction gradients of OCS and CO2. At the Harvard Forest AmeriFlux site, measured CO2 and OCS vertical gradients were correlated and were related to NEE and GPP, respectively. Estimates of GPP from OCS-based NEE partitioning were similar to those from established environmental regression techniques, providing evidence that OCS uptake can potentially serve as a GPP proxy. Measured vertical CO2 mole fraction gradients at five other AmeriFlux sites were used to project anticipated vertical OCS mole fraction gradients to provide indication of potential OCS signal magnitudes at sites where no OCS measurements were made. Projected OCS gradients at sites with short canopies were greater than those in forests, including measured OCS gradients at Harvard Forest, indicating greater potential for OCS uptake as a GPP proxy at these sites. This exploratory study suggests that continued investigation of linkages between OCS and GPP is warranted.

Description: Global vegetation models require the photosynthetic parameters, maximum carboxylation capacity (Vcm), and quantum yield (α) to parameterize their plant functional types (PFTs). The purpose of this work is to determine how much the scaling of the parameters from leaf to ecosystem level through a seasonally varying leaf area index (LAI) explains the parameter variation within and between PFTs. Using Fluxnet data, we simulate a seasonally variable LAIF for a large range of sites, comparable to the LAIM derived from MODIS. There are discrepancies when LAIF reach zero levels and LAIM still provides a small positive value. We find that temperature is the most common constraint for LAIF in 55% of the simulations, while global radiation and vapor pressure deficit are the key constraints for 18% and 27% of the simulations, respectively, while large differences in this forcing still exist when looking at specific PFTs. Despite these differences, the annual photosynthesis simulations are comparable when using LAIF or LAIM (r2 = 0.89). We investigated further the seasonal variation of ecosystem-scale parameters derived with LAIF. Vcm has the largest seasonal variation. This holds for all vegetation types and climates. The parameter α is less variable. By including ecosystem-scale parameter seasonality we can explain a considerable part of the ecosystem-scale parameter variation between PFTs. The remaining unexplained leaf-scale PFT variation still needs further work, including elucidating the precise role of leaf and soil level nitrogen.

Description: [1]  Similarities between the Atacama Desert (Chile) and Mars include extreme aridity, highly oxidizing chemistry, and intense ultraviolet radiation that promoted the photochemical production of perchlorates and nitrates. Concentration of these ions under hyperarid conditions led to the formation of nitrate- and perchlorate-bearing deposits in ephemeral lakes, followed by later deposition of chlorides and sulfates. At some locations, such as the Salar Grande, hypersaline deposits have remained unaltered for millions of years. We conducted a drilling campaign in deposits of the Salar to characterize the preservation state of biological molecules. A 5-meter deep discontinuous core was recovered, and subjected to multi-technique analysis including the antibody microarray-based biosensor LDChip300 and the SOLID (Signs Of Life Detector) instrument, complimented by geophysical, mineralogical, geochemical, and molecular analysis. We identified two units based on the mineralogy: the upper one, from the surface to ~320 cm depth characterized by a predominance of halite and anhydrite; and the lower one, from 320 to 520 cm, with a drop in halite and anhydrite and an enrichment in nitrate and perchlorate. Organic compounds including biomolecules were detected in association with the different depositional and mineralogical units, demonstrating the high capacity for molecular preservation. Hypersaline environments preserve biomolecules over geologically significant timescales; therefore, salt-bearing materials should be high-priority targets for the search for evidence of life on Mars.

Description: Wetlands and flooded peatlands can sequester large amounts of carbon (C) and have high greenhouse gas mitigation potential. There is growing interest in financing wetland restoration using C markets, however this requires careful accounting of both CO 2 and CH 4 exchange at the ecosystem scale. Here we present a new model, the PEPRMT model (Peatland Ecosystem Photosynthesis Respiration and Methane Transport), which consists of a hierarchy of biogeochemical models designed to estimate CO 2 and CH 4 exchange in restored managed wetlands. Empirical models using temperature and/or photosynthesis to predict respiration and CH 4 production were contrasted with a more process-based model that simulated substrate-limited respiration and CH 4 production using multiple carbon pools. Models were parameterized using a model-data fusion approach with multiple years of eddy covariance data collected in a recently restored wetland and a mature restored wetland. A third recently restored wetland site was used for model validation. During model validation, the process-based model explained 70% of the variance in net ecosystem exchange of CO 2 (NEE) and 50% of the variance in CH 4 exchange. Not accounting for high respiration following restoration led to empirical models overestimating annual NEE by 33-51%. By employing a model-data fusion approach we provide rigorous estimates of uncertainty in model predictions, accounting for uncertainty in data, model parameters and model structure. The PEPRMT model is a valuable tool for understanding carbon cycling in restored wetlands and for application in carbon market-funded wetland restoration, thereby advancing opportunity to counteract the vast degradation of wetlands and flooded peatlands.

Description: The ratio of key elements such as nitrogen, phosphorus, and silica determine nutrient limitations that are important to regulating primary productivity and species composition in aquatic ecosystems. The flux of these nutrients in streams, as dissolved constituents or as particulate matter, is sensitive to variability in flow conditions. Most previous research on nutrient flux and hydrologic variability has focused on the response of individual elements, especially nitrogen, to changes in flow over time. This study examines how the ratios of total nitrogen to total phosphorus (N:P) and total nitrogen to dissolved silica (N:Si) respond to hydrologic variability in the Mississippi-Atchafalaya River Basin. A doubling of the discharge by the Mississippi and Atchafalaya rivers to the Gulf of Mexico is found to increase the N:P by 10% and the N:Si by 4%. Analysis of data from upstream stations indicates that the N:P increases with discharge in sub-basins with intensive row crop agriculture and high fertilizer application rates, but is less predictable in other sub-basins. Conversely, the response of N:Si to discharge does not vary predictably with the land-use characteristics of the sub-basin. The response of the nutrient ratios to variability in flow may be linked to the different sources and sinks of each nutrient, as well as the difference between the dominant transport pathways of each nutrient. High-resolution data and models that describe the dissolved and particulate nutrient cycling are needed to assess the relative contribution of different drivers to these observed patterns, and to identify the response of nutrient ratios to hydrologic variability under future land use and climate change.

Description: [1]  The strength of feedbacks between a changing climate and future CO 2 concentrations are uncertain and difficult to predict using Earth System Models (ESMs). We analyzed emission-driven simulations—in which atmospheric CO 2 levels were computed prognostically—for historical (1850–2005) and future periods (RCP 8.5 for 2006–2100) produced by 15 ESMs for the Fifth Phase of the Coupled Model Intercomparison Project (CMIP5). Comparison of ESM prognostic atmospheric CO 2 over the historical period with observations indicated that ESMs, on average, had a small positive bias in predictions of contemporary atmospheric CO 2 . Weak ocean carbon uptake in many ESMs contributed to this bias, based on comparisons with observations of ocean and atmospheric anthropogenic carbon inventories. We found a significant linear relationship between contemporary atmospheric CO 2 biases and future CO 2 levels for the multi-model ensemble. We used this relationship to create a contemporary CO 2 tuned model (CCTM) estimate of the atmospheric CO 2 trajectory for the 21 st century. The CCTM yielded CO 2 estimates of 600 ± 14 ppm at 2060 and 947 ± 35 ppm at 2100, which were 21 ppm and 32 ppm below the multi-model mean during these two time periods. Using this emergent constraint approach, the likely ranges of future atmospheric CO 2 , CO 2 -induced radiative forcing, and CO 2 -induced temperature increases for the RCP 8.5 scenario were considerably narrowed compared to estimates from the full ESM ensemble. Our analysis provided evidence that much of the model-to-model variation in projected CO 2 during the 21 st century was tied to biases that existed during the observational era, and that model differences in the representation of concentration-carbon feedbacks and other slowly changing carbon cycle processes appear to be the primary driver of this variability. By improving models to more closely match the long-term time series of CO 2 from Mauna Loa, our analysis suggests uncertainties in future climate projections can be reduced.

Description: Water relations in plant communities are influenced both by contrasting functional groups (grasses, shrubs) and by climate change via complex effects on interception, uptake and transpiration. We modelled the effects of functional group replacement and biomass increase, both of which can be outcomes of invasion and vegetation management, and climate change on ecological drought (soil water potential below which photosynthesis stops) in 340 semiarid grassland sites over 30-year periods. Relative to control vegetation (climate and site-determined mixes of functional groups), the frequency and duration of drought were increased by shrubs and decreased by annual grasses. The rankings of shrubs, control vegetation, and annual grasses in terms of drought effects were generally consistent in current and future climates, suggesting that current differences among functional groups on drought effects predict future differences. Climate change accompanied by experimentally-increased biomass (i.e. the effects of invasions that increase community biomass, or management that increases productivity through fertilization or respite from grazing) increased drought frequency and duration, and advanced drought onset. Our results suggest that the replacement of perennial temperate semiarid grasslands by shrubs, or increased biomass, can increase ecological drought both in current and future climates.

Description: Abstract Drought predisposes conifer forests to bark beetle attacks and mortality. Although plant hydraulic stress mechanistically links to tree mortality, its capacity to predict trees' susceptibility to beetle attacks has not been evaluated. Further, both tree size and water supply could influence plant hydraulic stress, but their relative importance remained unknown. In this study, we modeled plant hydraulic stress of individual trees in a mixed forest of Lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), and Subalpine fir (Abies lasiocarpa) in southern Wyoming, using an integrated model of plant hydraulics and hydrology, ground surveys of tree size as well as physiological and geophysical measurements. Based on the established link between plant hydraulic stress and tree mortality, we found interspecific differences in the relative importance of water availability and tree size. Pine mortality was best explained by the combination of tree size and water supply, and fir mortality was best explained by variations in water supply. We next compared the prediction of beetle attack by modeled plant hydraulic stress versus tree size and found tree size best explained beetle attack consistently for all three species. Taken together, our results suggested beetle attack was primarily influenced by beetle preference for large trees, potentially as food sources, rather than more hydraulically stressed trees. These findings highlighted the importance of integrated understanding of biotic/abiotic factors and their mechanistic pathways in order to accurately predict the sustainability of forests susceptible to drought and beetle outbreaks.

Description: We investigate the benefits of assimilating in situ and satellite data of the fraction of photosynthetically active radiation (FAPAR) relative to eddy-covariance flux measurements for the optimization of parameters of the ORCHIDEE biosphere model. We focus on model parameters related to carbon fixation, respiration and phenology. The study relies on two sites – Fontainebleau (deciduous broadleaf forest) and Puechabon (Mediterranean broadleaf evergreen forest) – where measurements of net carbon exchange (NEE) and latent heat (LE) fluxes are available at the same time as FAPAR products derived from ground measurements or derived from spaceborne observations at high (SPOT) and medium (MERIS) spatial resolutions. We compare the different FAPAR products, analyze their consistency with the in situ fluxes, and then evaluate the potential benefits of jointly assimilating flux and FAPAR data. The assimilation of FAPAR data leads to a degradation of the model-data agreement with respect to NEE at the two sites. It is caused by the change in leaf area required to fit the magnitude of the various FAPAR products. Assimilating daily NEE and LE fluxes however has a marginal impact on the simulated FAPAR. The results suggest that the main advantage of including FAPAR data is the ability to constrain the timing of leaf onset and senescence for deciduous ecosystems, which is best achieved by normalizing FAPAR time series. The joint assimilation of flux and FAPAR data lead to a similar model-data improvement across all variables than when each data-stream is used independently, corresponding however to different and likely improved parameter values.

Description: The nitrate (NO 3 − ) dual isotope approach was applied to snowmelt, tundra active layer pore waters, and underlying permafrost in Barrow, Alaska, USA, to distinguish between NO 3 − derived from atmospheric deposition versus that derived from microbial nitrification. Snowmelt had an atmospheric NO 3 − signal with δ 15 N averaging −4.8 ± 1.0 (standard error of the mean) ‰ and δ 18 O averaging 70.2 ± 1.7 ‰. In active layer pore waters, NO 3 − primarily occurred at concentrations suitable for isotopic analysis in the relatively dry and oxic centers of high-centered polygons. The average δ 15 N and δ 18 O of NO 3 − from high-centered polygons were 0.5 ± 1.1 ‰ and −4.1 ± 0.6 ‰, respectively. When compared to the δ 15 N of reduced nitrogen (N) sources, and the δ 18 O of soil pore waters, it was evident that NO 3 − in high-centered polygons was primarily from microbial nitrification. Permafrost NO 3 − had δ 15 N ranging from approximately −6 to 10 ‰, similar to atmospheric and microbial NO 3 − , and highly variable δ 18 O ranging from approximately −2 to 38 ‰. Permafrost ice wedges contained a significant atmospheric component of NO 3 − , while permafrost textural ice contained a greater proportion of microbially-derived NO 3 − . Large-scale permafrost thaw in this environment would release NO 3 − with a δ 18 O signature intermediate to that of atmospheric and microbial NO 3 . Consequently, while atmospheric and microbial sources can be readily distinguished by the NO 3 − dual isotope technique in tundra environments, attribution of NO 3 − from thawing permafrost will not be straightforward. The NO 3 − isotopic signature, however, appears useful in identifying NO 3 − sources in extant permafrost ice.