C. Lloyd

Spatial representativeness and the location bias of flux footprints over inhomogeneous areas

Turbulent fluxes over natural vegetation are spatially inhomogeneous up to a level called the physical blending height. Measurements below this blending height suffer from a location bias that varies with sensor height, wind direction and stability. The location bias is the degree, to which a flux measured at a given location differs from the aggregated ecosystem-scale flux. This location bias is analyzed using the source area/footprint approach, where the aggregated surface conditions contained in the footprint are compared to average surface conditions in the ecosystem. When a single observation event at a given location is considered, this method leads to a quantitative measure of the sensor location bias. It provides an objective estimate of how a given flux observation compares to the ecosystem scale flux. The expected sensor location bias is expressed as the fraction of surface variability not accounted for by measurements at a given sensor height, independent of its location within the ecosystem. It results from a convolution of the footprint filter function with a detailed surface condition database. These tools to examine the spatial representativeness of flux measurements are applied to an inhomogeneous savannah surface (Tiger bush) in the Sahel region of Niger, and to measurements of evaporation collected during the HAPEX-Sahel field campaign.

How do soil emissions of N2O, CH4 and CO2 from perennial bioenergy crops differ from arable annual crops?

It is important to demonstrate that replacing fossil fuel with bioenergy crops can reduce the national greenhouse gas (GHG) footprint. We compared field emissions of nitrous oxide (N2O), methane (CH4) and soil respiration rates from the C4 grass Miscanthus × giganteus and willow (salix) with emissions from annual arable crops grown for food production. The study was carried out in NE England on adjacent fields of willow, Miscanthus, wheat (Triticum aetivum) and oilseed rape (Brassica napus). N2O, CH4 fluxes and soil respiration rates were measured monthly using static chambers from June 2008 to November 2010. Net ecosystem exchange (NEE) of carbon dioxide (CO2) was measured by eddy covariance on Miscanthus from May 2008 and on willow from October 2009 until November 2010. The N2O fluxes were significantly smaller from the bioenergy crops than that of the annual crops. Average fluxes were 8 and 32 μg m−2 h−1 N2O‐N from wheat and oilseed rape, and 4 and 0.2 μg m−2 h−1 N2O‐N from Miscanthus and willow, respectively. Soil CH4 fluxes were negligible for all crops and soil respiration rates were similar for all crops. NEE of CO2 was larger for Miscanthus (−770 g C m−2 h−1) than willow (−602 g C m−2 h−1) in the growing season of 2010. N2O emissions from Miscanthus and willow were lower than for the wheat and oilseed rape which is most likely a result of regular fertilizer application and tillage in the annual arable cropping systems. Application of 15N‐labelled fertilizer to Miscanthus and oil seed rape resulted in a fertilizer‐induced increase in N2O emission in both crops. Denitrification rates (N2O + N2) were similar for soil under Miscanthus and oilseed rape. Thus, perennial bioenergy crops only emit less GHGs than annual crops when they receive no or very low rates of N fertilizer.

Net ecosystem exchange over heterogeneous Arctic tundra: Scaling between chamber and eddy covariance measurements

Net ecosystem exchange (NEE) was estimated for an area of tundra near Abisko using both eddy covariance (EC) data and chamber measurements. This area of tundra is heterogeneous with six principal elements forming a landscape mosaic. Chamber measurements in patches of the individual mosaic elements were used to model NEE as a function of irradiance and temperature. The area around the EC mast was mapped, and a footprint model was used to simulate the varying source fraction attributable to each mosaic element. Various upscaling approaches were used to estimate NEE for comparison with NEE calculated from the EC observations. The results showed that EC measurements made for such a heterogeneous site are robust to the variations in NEE between mosaic elements that also vary substantially in their source fractions. However, they also revealed a large (∼60%) bias in the absolute magnitude of the cumulative negative NEE for a 40-day study period simulated by various upscaling approaches when compared to the value calculated from the EC observations. The magnitude of this bias, if applied to estimates for the entire tundra region, is substantial in relation to other components of the global carbon budget. Various hypotheses to account for this bias are discussed and, where possible, evaluated. A need is identified for more systematic sampling strategies when performing chamber measurements in order to assess the extent to which subjectivity of chamber location may account for much of the observed bias. If this is the origin of the bias, then upscaling approaches using chamber measurements may generally overestimate CO2 uptake.

Trade-off in ecosystem services of the Somerset Levels and Moors wetlands

Abstract It is widely recognized that healthy ecosystems can provide considerable benefits to people, including food, timber, freshwater, protection from floods and much of what we call quality of life. A global review of these ecosystem services carried out as part of the Millennium Ecosystem Assessment (MEA) provided a framework for national and local studies. Using the MEA approach, this paper reviews the ecosystem services provided by the Somerset Levels and Moors wetland system in southwest England. This wetland provides a series of important services that are beneficial locally, regionally and globally, including grazing for cattle, carbon sequestration, flood water storage, recreation and archaeology. Some services are synergistic and reinforcing; for example, maintaining wet conditions supports wetland bird life that maintains biological diversity, attracts tourists, protects archaeological artefacts and reduces CO2 emissions; raising water levels to or above the ground leads to net greenhouse gas uptake by the wetland. Other services are potentially conflicting, for example raising water levels may reduce potential flood water storage and increase methane emissions. Comparison of the services of the wetland with those of drier habitats reveals for example that carbon sequestration, bird habitat provision and hay production is greater in wetlands, whilst grazing quality may decline and plant diversity may be reduced in the short term and distributions of disease vectors may be altered by wetland restoration through raising water levels. Management decisions affecting wetlands may necessitate a trade-off of ecosystem services. Editor Z.W. Kundzewicz Citation Acreman, M.C., Harding, R.J., Lloyd, C., McNamara, N.P., Mountford, J.O., Mould, D.J., Purse, B. V., Heard, M.S., Stratford, C.J. and Dury, S.J., 2011. Trade-off in ecosystem services of the Somerset Levels and Moors wetlands. Hydrological Sciences Journal, 56 (8), 1543–1565.

The Carbon Cycle of a Maritime Ancient Temperate Broadleaved Woodland at Seasonal and Annual Scales

This study compares different approaches to quantifying the carbon cycle in a temperate deciduous forest at Wytham Woods in England, which is unusual in its maritime climate and mixed age structure, reflecting low levels of past management. We tested whether eddy covariance and biometric measurements gave consistent estimates of woodland productivity and ecosystem respiration at monthly and annual timescales. Biometric methods estimated gross primary productivity (GPP) as 22.0xa0±xa01.6xa0Mgxa0Cxa0ha−1xa0y−1, close to the eddy covariance GPP value of 21.1xa0Mgxa0Cxa0ha−1xa0y−1. Annual ecosystem respiration (RECO) was similar, at 20.3xa0±xa01.5xa0Mgxa0Cxa0ha−1xa0y−1 for biometric and 19.8xa0Mgxa0Cxa0ha−1xa0y−1for eddy covariance. The seasonal cycle of monthly biometric and eddy covariance RECO estimates also closely matched. Net primary productivity (NPP) was 7.0xa0±xa00.8xa0Mgxa0Cxa0ha−1xa0y−1, 37% of which was allocated below ground. Leaf fluxes were the greatest component of NPP and RECO. Ecosystem carbon-use efficiency (CUExa0=xa0NPP/GPP) was 0.32xa0±xa00.04; low compared to many temperate broadleaved sites but close to values for old-growth sites. This may reflect the age of some trees, and/or the oceanic climate with relatively mild winters during which there can be substantial autotrophic maintenance respiration in winter but negligible growth. This study demonstrates that biometric measurements can provide robust estimates of site productivity and respiration and that eddy covariance and bottom-up measurements can be combined on seasonal and interannual timescales to enable a detailed understanding of the forest carbon cycle.

Providing low-budget estimations of carbon sequestration and greenhouse gas emissions in agricultural wetlands

The conversion of wetlands to agriculture through drainage and flooding, and the burning of wetland areas for agriculture have important implications for greenhouse gas (GHG) production and changing carbon stocks. However, the estimation of net GHG changes from mitigation practices in agricultural wetlands is complex compared to dryland crops. Agricultural wetlands have more complicated carbon and nitrogen cycles with both above- and below-ground processes and export of carbon via vertical and horizontal movement of water through the wetland. This letter reviews current research methodologies in estimating greenhouse gas production and provides guidance on the provision of robust estimates of carbon sequestration and greenhouse gas emissions in agricultural wetlands through the use of low cost reliable and sustainable measurement, modelling and remote sensing applications. The guidance is highly applicable to, and aimed at, wetlands such as those in the tropics and sub-tropics, where complex research infrastructure may not exist, or agricultural wetlands located in remote regions, where frequent visits by monitoring scientists prove difficult. In conclusion, the proposed measurement-modelling approach provides guidance on an affordable solution for mitigation and for investigating the consequences of wetland agricultural practice on GHG production, ecological resilience and possible changes to agricultural yields, variety choice and farming practice.