Aircraft-based inversions quantify the importance of wetlands and livestock for Upper Midwest methane emissions.

We apply airborne measurements across three seasons (summer, winter and spring 2017-2018) in a multi-inversion framework to quantify methane emissions from the US Corn Belt and Upper Midwest, a key agricultural and wetland source region. Combing our seasonal results with prior fall values we find that wetlands are the largest regional methane source (32 %, 20 [16-23] Gg/d), while livestock (enteric/manure; 25 %, 15 [14-17] Gg/d) are the largest anthropogenic source. Natural gas/petroleum, waste/landfills, and coal mines collectively make up the remainder. Optimized fluxes improve model agreement with independent datasets within and beyond the study timeframe. Inversions reveal coherent and seasonally dependent spatial errors in the WetCHARTs ensemble mean wetland emissions, with an underestimate for the Prairie Pothole region but an overestimate for Great Lakes coastal wetlands. Wetland extent and emission temperature dependence have the largest influence on prediction accuracy; better representation of coupled soil temperature-hydrology effects is therefore needed. Our optimized regional livestock emissions agree well with the Gridded EPA estimates during spring (to within 7 %) but are ∼25 % higher during summer and winter. Spatial analysis further shows good top-down and bottom-up agreement for beef facilities (with mainly enteric emissions) but larger (∼30 %) seasonal discrepancies for dairies and hog farms (with >40 % manure emissions). Findings thus support bottom-up enteric emission estimates but suggest errors for manure; we propose that the latter reflects inadequate treatment of management factors including field application. Overall, our results confirm the importance of intensive animal agriculture for regional methane emissions, implying substantial mitigation opportunities through improved management.

Climate Sensitivity of Peatland Methane Emissions Mediated by Seasonal Hydrologic Dynamics.

Peatlands are among the largest natural sources of atmospheric methane (CH4) worldwide. Peatland emissions are projected to increase under climate change, as rising temperatures and shifting precipitation accelerate microbial metabolic pathways favorable for CH4 production. However, how these changing environmental factors will impact peatland emissions over the long term remains unknown. Here, we investigate a novel data set spanning an exceptionally long 11 years to analyze the influence of soil temperature and water table elevation on peatland CH4 emissions. We show that higher water tables dampen the springtime increases in CH4 emissions as well as their subsequent decreases during late summer to fall. These results imply that any hydroclimatological changes in northern peatlands that shift seasonal water availability from winter to summer will increase annual CH4 emissions, even if temperature remains unchanged. Therefore, advancing hydrological understanding in peatland watersheds will be crucial for improving predictions of CH4 emissions.

Error characterization of methane fluxes and budgets derived from a long-term comparison of open- and closed-path eddy covariance systems.

Wetlands represent the dominant natural source of methane (CH4) to the atmosphere. Thus, substantial effort has been spent examining the CH4 budgets of global wetlands via continuous ecosystem-scale measurements using the eddy covariance (EC) technique. Robust error characterization for such measurements, however, remains a major challenge. Here, we quantify systematic, random and gap-filling errors and the resulting uncertainty in CH4 fluxes using a 3.5 year time series of simultaneous open- and closed path CH4 flux measurements over a sub-boreal wetland. After correcting for high- and low frequency flux attenuation, the magnitude of systematic frequency response errors were negligible relative to other uncertainties. Based on three different random flux error estimations, we found that errors of the CH4 flux measurement systems were smaller in magnitude than errors associated with the turbulent transport and flux footprint heterogeneity. Errors on individual half-hourly CH4 fluxes were typically 6%-41%, but not normally distributed (leptokurtic), and thus need to be appropriately characterized when fluxes are compared to chamber-derived or modeled CH4 fluxes. Integrated annual fluxes were only moderately sensitive to gap-filling, based on an evaluation of 4 different methods. Calculated budgets agreed on average to within 7% (≤ 1.5 g - CH4 m-2 yr-1). Marginal distribution sampling using open source code was among the best-performing of all the evaluated gap-filling approaches and it is therefore recommended given its transparency and reproducibility. Overall, estimates of annual CH4 emissions for both EC systems were in excellent agreement (within 0.6 g - CH4 m-2 yr-1) and averaged 18 g - CH4 m-2 yr-1. Total uncertainties on the annual fluxes were larger than the uncertainty of the flux measurement systems and estimated between 7-17%. Identifying trends and differences among sites or site years requires that the observed variability exceeds these uncertainties.

Linking variation in intrinsic water-use efficiency to isohydricity: a comparison at multiple spatiotemporal scales.

Species-specific responses of plant intrinsic water-use efficiency (iWUE) to multiple environmental drivers associated with climate change, including soil moisture (θ), vapor pressure deficit (D), and atmospheric CO2 concentration (ca ), are poorly understood. We assessed how the iWUE and growth of several species of deciduous trees that span a gradient of isohydric to anisohydric water-use strategies respond to key environmental drivers (θ, D and ca ). iWUE was calculated for individual tree species using leaf-level gas exchange and tree-ring δ13 C in wood measurements, and for the whole forest using the eddy covariance method. The iWUE of the isohydric species was generally more sensitive to environmental change than the anisohydric species was, and increased significantly with rising D during the periods of water stress. At longer timescales, the influence of ca was pronounced for isohydric tulip poplar but not for others. Trees' physiological responses to changing environmental drivers can be interpreted differently depending on the observational scale. Care should be also taken in interpreting observed or modeled trends in iWUE that do not explicitly account for the influence of D.

Dynamics of stem water uptake among isohydric and anisohydric species experiencing a severe drought.

Predicting the impact of drought on forest ecosystem processes requires an understanding of trees' species-specific responses to drought, especially in the Eastern USA, where species composition is highly dynamic due to historical changes in land use and fire regime. Here, we adapted a framework that classifies trees' water-use strategy along the spectrum of isohydric to anisohydric behavior to determine the responses of three canopy-dominant species to drought. We used a collection of leaf-level gas exchange, tree-level sap flux and stand-level eddy covariance data collected in south-central Indiana from 2011 to 2013, which included an unusually severe drought in the summer of 2012. Our goal was to assess how patterns in the radial profile of sap flux and reliance on hydraulic capacitance differed among species of contrasting water-use strategies. In isohydric species, which included sugar maple (Acer saccharum Marsh.) and tulip poplar (Liriodendron tulipifera L.), we found that the sap flux in the outer xylem experienced dramatic declines during drought, but sap flux at inner xylem was buffered from reductions in water availability. In contrast, for anisohydric oak species (Quercus alba L. and Quercus rubra L.), we observed relatively smaller variations in sap flux during drought in both inner and outer xylem, and higher nighttime refilling when compared with isohydric species. This reliance on nocturnal refilling, which occurred coincident with a decoupling between leaf- and tree-level water-use dynamics, suggests that anisohydric species may benefit from a reliance on hydraulic capacitance to mitigate the risk of hydraulic failure associated with maintaining high transpiration rates during drought. In the case of both isohydric and anisohydric species, our work demonstrates that failure to account for shifts in the radial profile of sap flux during drought could introduce substantial bias in estimates of tree water use during both drought and non-drought periods.