Talk Abstracts – 2019 AmeriFlux Annual Meeting

Session Theme #1:  Mountain Systems/Complex Terrain (Niwot Ridge turns 21)

Characteristics of Airflow and Turbulence Just-Above and Within a Subalpine Forest in Complex Terrain
Burns, Sean
Winds at the the long-term Niwot Ridge Subalpine Forest AmeriFlux tower (US-NR1) have typically been measured at 3-4 levels between the primary flux-measurement level (21.5 m) and the ground. The US-NR1 site is located in complex mountainous terrain approximately 8 km east of the Continental Divide below Niwot Ridge, Colorado. The topography at the site has a slope angle that ranges from 4-7 degrees with much steeper terrain within several miles of the tower. The mixed-conifer subalpine forest surrounding the tower has a leaf-area index (LAI) of approximately 3.8-4.2. In order to enhance the vertical resolution of the wind profile at the site, seven CSAT3 sonic anemometers were added to the US-NR1 main tower in Fall of 2014 and operated for a full year. In addition to the US-NR1 data, measurements from the 2004 Carbon in the Mountains Experiment (CME04) are used to better understand the horizontal variations of the airflow—during CME04 five towers were placed near the US-NR1 tower (and within 400 m of each other) and loosely arranged along the drainage of a small creek to investigate the larger scale features of the flow along the drainage. One tower was in a relatively open area while the other four towers were in either aspen or a mixed-conifer subalpine forest. The CME04 towers in the forest all extended above the canopy by at least two times the canopy height. We present several aspects of air flow in complex terrain related to: (1) how a forest and stability affect the vertical profile of wind speed, wind direction, and turbulence at the site, and (2) the general characteristics of the downslope nocturnal drainage flow.

Evidence for non-steady-state carbon emissions from snow-scoured alpine tundra
Knowles, John
High-latitude warming is capable of accelerating permafrost degradation and the decomposition of previously frozen carbon. The existence of an analogous high-altitude feedback, however, has yet to be directly evaluated. We address this knowledge gap by coupling a radiocarbon-based model to seven years (2008-2014) of continuous eddy covariance data from a snow-scoured alpine tundra meadow in Colorado, USA, where solifluction lobes are associated with discontinuous permafrost. On average, the ecosystem was a net annual source of 232 ± 54 g C m-2 (mean ± 1 standard deviation) to the atmosphere, and respiration of relatively radiocarbon-depleted (i.e., older) substrate contributes to carbon emissions during the winter. Given that alpine soils with permafrost occupy 3.6 x 106 km2 land area and are estimated to contain 66.3 Pg of soil organic carbon (4.5% of the global pool), this scenario has global implications for the mountain carbon balance and corresponding resource allocation to lower elevations.

Twenty-nine towers, twenty-nine stories: The ChEAS core site cluster journey
Desai, Ankur
Will give an overview of both our short and long running sites in Wisconsin and Michigan advancing science of scaling carbon, water, and energy fluxes from landscape to globe, including first-cut results from the CHEESEHEAD 2019 field campaign.

Session Theme #2:  Year of Methane

Managing Marsh Hydrology to reduce Methane Emissions
Valach, Alex
Freshwater wetlands provide a range of ecological benefits such as wildlife habitats to support biodiversity, water quality improvements, flood protection, and carbon sequestration. They remove carbon from the atmosphere and act as a long-term carbon store making them a natural climate change mitigation solution. This service is offset by the high methane emissions, which results in the ecosystem becoming greenhouse gas source in the geological short-term. Studies have shown that methane emissions are temporarily inhibited by dry-out periods, which is maintained for several weeks following re-flooding. We show that this can present a significant reduction in methane emissions on annual timescales if occurring during peak emission periods. Managed intermittent water drawdowns do not impact the high carbon uptake through vegetation in freshwater systems, which have some of the highest carbon uptake potentials. This increases economic incentives for wetland restoration and monitoring through carbon cap and trade schemes. Here we present data from past water table draw-down events showing significant reductions of methane emissions at a restored managed freshwater marsh, as well as a water table manipulation experiment in California. Our aim is to understand the optimal conditions to maximize climate benefits and, where possible, to outline wetland management techniques.

Methane efflux from an American bison herd
Stoy, Paul
American bison (Bison bison L.) have recovered from the brink of extinction over the past century and offer potential environmental benefits as they re-occupy their native range. Bison produced some 2 Tg year-1 of the estimated 9-15 Tg year-1 of pre-industrial enteric methane emissions, but few contemporary measurements have been made due to their mobile grazing habits and safety issues. Here, we measure methane and carbon dioxide fluxes from a bison herd on an enclosed pasture during daytime periods in winter using the eddy covariance technique. Methane emissions were negligible in the absence of bison (mean ± standard deviation = 0.002 ± 0.010 μmol m-2 s-1) but averaged 0.041 ± 0.046 μmol m-2 s-1 when bison were present, similar in magnitude to eddy covariance measurements of sheep and cattle grazing systems. We coupled a flux footprint analysis coupled with bison location estimates from automated cameras to calculate a mean methane flux of 91.6 μmol s-1 per animal, or 127 g CH4 day-1, approximately 75% of measured emission rates for range cattle. Eddy covariance is a promising way to measure greenhouse gas fluxes from managed and natural ruminant grazing systems, and more studies should be undertaken to identify best management practices.

Process-scale observations of methane fluxes in a mineral soil marsh
Bohrer, Gil
Difficulties in modeling methane emissions are driven by the spatially heterogeneous nature of wetland ecosystems and methane fluxes. This variability in fluxes is a result of the complex and interconnected belowground and aboveground processes of methane production, consumption, and transport. Flux measurements are routinely used to parameterize and improve land-surface models for methane fluxes. However, model optimizations are typically done against observations of the net site-level flux and do not independently address within-wetland heterogeneity or the various processes that add up to the net flux. Therefore, even models that make accurate CH4 emission predictions may be right for the wrong reasons. At the footprint of US-OWC, we conducted process-level observations of components of the web of methane processes. Including chamber observations at the vegetation patch scale, the concentration and dynamics of methane and CO2 in the porewater, flux through vegetation, plant species conductivity to methane flux, and ebullition methane flux. We identify species-specific differences in plant control of methane transport, and quantify the resulting relationships between CO2 and methane fluxes. Our observations provide a unique opportunity to constrain model parameters and develop a patch-level parameterization for wetland methane flux.

Session Theme #3:  Beyond Forests:  Other systems such as Lakes, Reservoirs, Ag, Urban

Evaluating emissions inventory using atmospheric CO2 measurements and source partitioning in a suburban environment
Wu, Kai
The quantification of anthropogenic CO2 emissions from urban areas is important for political and scientific interest. Evaluation of the effectiveness of emission reduction policies requires accurate assessments of urban CO2 emissions. Atmospheric observations provide a valuable independent assessment to verify emissions inventories. Multiple integrated atmospheric measurements in the Indianapolis Flux Experiment (INFLUX) are being used to partition anthropogenic and biogenic CO2 fluxes. The partitioned fluxes are used to evaluate the accuracy and precision of a high-resolution emissions inventory (Hestia). We use tower-based measurements of CO2 and CO mole fractions and eddy covariance flux data to partition net CO2 fluxes into anthropogenic and biogenic flux components. The measurement of 14C is used to estimate emission ratio between CO and fossil fuel CO2. We use a flux footprint model to match with Hestia at a tower location in a suburban area. The high-resolution spatial coverage of Hestia and the high-accuracy of the flux data are complementary. We evaluate temporal-spatial structures of the discrepancies between flux measurements and Hestia. The consistent results in different seasons demonstrate the effectiveness of this disaggregation method that can be applied in future studies on urban CO2 flux measurement.

Field-scale mapping and forecasting of water budgets in intensively irrigated agricultural regions through an advanced ensemble modeling framework
Talib, Ammara
In our ability to sustain food production, it is important to know how much water our crops need. Current drought forecasts cover large regions and are not specific to individual farms. Here, we seek to improve forecasting of how crops drought stress changes over time during different stages of growth through advanced mapping and modeling of evapotranspiration (ET) with NASA space borne sensors. We developed an algorithm that can accurately predict and forecast farm-scale regional daily out to 3 days. We use these forecasts to determine what varieties of a crops use less water or what varieties are best performing under water stress conditions. We hypothesized that thermal products from Moderate Resolution Imaging Spectroradiometer (MODIS) and meteorological information provide the resolution and temporal repeat frequency necessary to develop ET products. These goals were accomplished by forecasting water demand (ET loss) by crops with help of ensemble machine learning algorithms with use of meteorological data, MODIS products and ground observation from flux towers. From these products, we developed, calibrated and evaluated a new crop ET product against field measurements of ET from a network of crop eddy covariance flux towers in Midwest USA. These towers are currently operating in irrigated and rain fed field corn, soybean, wheat, clover and potato farms under a range of water management strategies. Daily ET forecast (3 days) model based on random forest (RF) has R2 and RMSE of 0.72 mm and 0.76mm respectively while recurrent neural network (RNN) ensemble forecast model was able to forecast 3 days ET with R2 and RMSE of 0.71 mm and 0.78 mm respectively. New field-scale ET maps can provide deep insights into how farmers could use limited water resources efficiently to maintain productivity, while sustaining surface water resources in the face of climate change.

Session Theme #4:  Data synthesis ideas, such as drought and other disturbances

A century of reforestation in the Southeastern United States Caused Widespread near-surface Cooling
Barnes, Mallory
Forests impact the earth’s climate system through chemical, biological, and physical processes including exchanges of energy, water, and carbon dioxide between the land and the atmosphere. Recent studies underscore the climate mitigation potential of widespread reforestation in terms of carbon dioxide uptake, however, at least in the temperate zone, the potential cooling impacts of reforestation via changes in surface energy balance remain largely unknown. Here, we show that widespread reforestation in the Southeast United states since the early 20th century is linked to the anomalous lack of warming in the region known as the “warming hole”. We use data from three independent data sources (satellites, flux towers, and historical weather stations) to show that forests have a substantial surface or near-surface cooling effect of 0.5 – 2℃. Our synthesis of paired grassland:forest flux towers found that increased turbulent sensible and latent heat fluxes outweighed the warming impacts of lower albedo in forests, resulting in cooler near-surface temperatures in forests than grasslands. Our results demonstrate that biophysical processes that link forest cover and local temperature can explain the lack of expected warming in the Southeast United States. Moreover, our results highlight the climate mitigation potential of forests through changes in energy balance that lead to surface and regional cooling.

Decreasing trend of ecosystem water use efficiency in an old-growth forest and an anthropogenic peatland in southern Chile
Perez-Quezada, Jorge
We studied the monthly and interannual variations of gross primary productivity (GPP), evapotranspiration (ET) and ecosystem water use efficiency (WUE: GPP/ET) and their relationships with meteorological factors, in an old–growth temperate rainforest and an anthropogenic peatland located in southern Chile, using eddy covariance (EC) data for the period 2014 – 2017. Average annual cumulative GPP and ET (±SE) were significantly higher in the forest (2282 ± 66 g C m-2 y-1 and 513.5 ± 74.5 kg H2O m-2 y-1, respectively) compared to the anthropogenic peatland (1115 ± 48 g C m-2 y-1 and 370 ± 46 kg H2O m-2 y-1, respectively). Average annual WUE (±SE) at the old-growth forest (3.70 ± 0.43 g C kg-1 H2O) was also higher compared to the anthropogenic peatland (1.50 ± 0.54 g C kg-1 H2O). WUE was mainly controlled by GPP at the forest and by ET at the anthropogenic peatland. Global radiation showed a steeper positive relation with GPP in the forest, whereas it showed a steeper positive relation with ET in the peatland. Monthly WUE decreased across the 4-year study period at both sites, which was related to increasing water vapor deficit during this period.

Semi-arid biome vulnerability to nearly two decades of uncharacteristic drought in the Southwestern US
Litvak, Marcy 
Semi-arid biomes provide a wide range of ecosystem services, including carbon sequestration and climate regulation. The quantity and quality of these services is dependent on ecosystem structure and function, both of which are sensitive to climate driven disturbances such as drought and fire. To better understand the impact the last two decades of uncharacteristic drought and drought related disturbances have had on the structure/function of semi-arid biomes in the Southwestern US, we leveraged 12 years of net ecosystem exchange of carbon and energy across the New Mexico Elevation Gradient, and 35-year time series of both satellite imagery and climate. I will discuss recent efforts to: 1) combine these datasets to show how a disturbance index calculated from the satellite imagery is able to pick out an increase in the frequency of land surface anomalies in the last two decades, 2) link the increased frequency of these disturbance anomalies to drought and drought related disturbance, and 3) quantify the carbon and energy balance consequences of these anomalies. These results provide a better understanding of which biomes are more resilient/vulnerable to observed drought, and improve our efforts to understand and forecast how future climates will affect carbon/energy dynamics of southwestern landscapes.

Session Theme #5:  New Tools and products:  Remote Sensing (eg SIF), gap-filling, partitioning, data infrastructure, and other new instrumentation, products, and tools

Integration of atmospheric observing networks to evaluate regional ecosystem flux models
Davis, Ken 
Regional fluxes cannot be fully diagnosed with any individual atmospheric observation system. Networks of tower- and aircraft-based observations, merged with numerical weather models, present a powerful approach for evaluating hypotheses concerning regional carbon fluxes. I will present examples of such integrative analyses, and highlight the opportunities presented by the Atmospheric Carbon and Transport (ACT) – America flight campaigns for regional analyses that can be complemented by AmeriFlux observations, and the potential application of these results to improved atmospheric inverse flux estimates.

Impact of standardization on eddy covariance measurements
Sabbatini, Simone
The continuous increase in the number of eddy covariance (EC) monitoring stations worldwide, and of those part of research networks, is fueling the discussion in the EC community on the need for standardization of measurement protocols. Some networks invested in the standardization of sensors and processing (e.g. ICOS, NEON), while others gave emphasis to the need for flexibility (e.g. AmeriFlux). At our knowledge, a quantification of the impact of standardization on the final EC fluxes was never accomplished. Hence, our analysis aims at answering the following two questions: 1. Does standardization affect the calculation of EC fluxes, and how? 2. Which component of the standardization weighs more in terms of fluxes variability? To answer this, we applied the official ICOS processing scheme to high-frequency datasets from each of eight ICOS stations that ran, for a period of minimum two months, an additional, non-ICOS EC setup parallel to the ICOS one. Results from a non-ICOS processing scheme relative to the same two datasets were also collected. We then compared the resulting fluxes of CO2, sensible and latent heat (ICOS against non-ICOS setups and processing), checking also how fluxes from standardized and non-standardized systems compare to the overall flux variability.

Research priorities in integrated SIF/EC studies
Gu, Lianhong
Sun-induced chlorophyll fluorescence (SIF) provides critical information on the dynamics of gross primary productivity, a unique role not readily achievable using other methods. Long-term continuous SIF observations have the potential to advance terrestrial ecosystem science. Realizing this potential, however, requires synergistic implementation of SIF measurements within eddy covariance (EC) flux networks and deep engagement with plant scientists. Here we outline some research priorities that will likely have to be pursued before SIF can be used as a powerful ecosystem research tool. continuous increase in the number of eddy covariance (EC) monitoring stations worldwide, and of those part of research networks, is fueling the discussion in the EC community on the need for standardization of measurement protocols. Some networks invested in the standardization of sensors and processing (e.g. ICOS, NEON), while others gave emphasis to the need for flexibility (e.g. AmeriFlux). At our knowledge, a quantification of the impact of standardization on the final EC fluxes was never accomplished. Hence, our analysis aims at answering the following two questions: 1. Does standardization affect the calculation of EC fluxes, and how? 2. Which component of the standardization weighs more in terms of fluxes variability? To answer this, we applied the official ICOS processing scheme to high-frequency datasets from each of eight ICOS stations that ran, for a period of minimum two months, an additional, non-ICOS EC setup parallel to the ICOS one. Results from a non-ICOS processing scheme relative to the same two datasets were also collected. We then compared the resulting fluxes of CO2, sensible and latent heat (ICOS against non-ICOS setups and processing), checking also how fluxes from standardized and non-standardized systems compare to the overall flux variability.