181 research outputs found
Application of an alternative method to derive reliable estimates of nighttime respiration from eddy covariance measurements in moderately complex topography
No full textEven moderately complex topography can lead to significant horizontal and vertical advection and a consequent underestimation of nocturnal CO2 effluxes derived from eddy covariance measurements on a single tower. The standard approach to select nighttime eddy flux data uses a threshold in friction velocity to exclude periods when advection is important but this is problematic in situations where turbulence is intermittent. van Gorsel et al. [van Gorsel, E., Leuning, R., Cleugh, H.A., Keith, H., Suni, T., 2007. Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u*-threshold filtering technique. Tellus 59B, 397–403] have developed an alternative method that estimates the CO2 flux from the maximum of the eddy flux plus change in storage term in the period after sunset when stable stratification develops. During this time the advection terms do not contribute significantly to the mass balance of the air layer below the eddy flux instruments at the Tumbarumba flux station (SE Australia). Advection dominates only later in the night, following the development of large horizontal and vertical gradients of CO2. As net nighttime and daytime fluxes are often of similar magnitude but opposite in sign, underestimation of respiration can lead to large errors in annual ecosystem carbon budgets.No Full Tex
Positieve veiligheid: Safety I en Safety II combineren
No full textNiet zo heel lang geleden stormde een veiligheidsprofessional na afloop van een praatje op mij af om mij te bezweren dat het vasthouden van de leuning eenvoortreffelijke maatregel is. In mijn ogen is het eerder betuttelend. Negatieve veiligheid, we moeten er nu écht vanaf.Safety and Security Scienc
Air and biomass heat storage fluxes in a forest canopy: Calculation within a soil vegetation atmosphere transfer model
No full textAn analytical method for calculating the sub-diurnal change in heat storage in tree trunks is presented and incorporated in a soil vegetation atmosphere transfer (SVAT) model. The modelled change in biomass heat storage (J(tr)) is driven by radial heat diffusion within the trunks and surface heat exchange by convection, insolation and longwave radiation. The calculation requires only variables from the previous and current time step and is independent of measured biomass temperature. The model was applied to a 40 m tall Australian temperate Eucalyptus forest at the Tumbarumba Ozflux site. A comparison between modelled and measured trunk temperatures showed agreement to within 1 degrees C, providing confidence in the model. Hourly values of J(tr) peaked at 61 W m(-2) for this site. Similar values of J(tr) were obtained using an adaptation of the force-restore method. Additional calculations for a range of leaf area indices and trunk radii enable a quick estimate of the maximum hourly value of J(tr) for any forest with given leaf area index, quadratic mean trunk radius (at breast height) and biomass. Inclusion of heat storage fluxes in the hourly available energy budget for the forest improved agreement between available energy and measured heat fluxes above the canopy, with energy closure rising from 90 to 101 %. Accounting for J(tr) in the SVAT model also improved agreement between measured and modelled fluxes of sensible and latent heat. (c) 2007 Elsevier B.V All rights reserved
Suitability of four stomatal conductance models in agro-pastoral ecotone in North China: A case study for potato and oil sunflower
No full text© 2016, Science Press. All right reserved. The suitability of four popular empirical and semi-empirical stomatal conductance models (Jarvis model, Ball-Berry model, Leuning model and Medlyn model) was evaluated based on parallel observation data of leaf stomatal conductance, leaf net photosynthetic rate and meteorological factors during the vigorous growing period of potato and oil sunflower at Wuchuan experimental station in agro-pastoral ecotone in North China. It was found that there was a significant linear relationship between leaf stomatal conductance and leaf net photosynthetic rate for potato, whereas the linear relationship appeared weaker for oil sunflower. The results of model evaluation showed that Ball-Berry model performed best in simulating leaf stomatal conductance of potato, followed by Leuning model and Medlyn model, while Jarvis model was the last in the performance rating. The root-mean-square error (RMSE) was 0.0331, 0.0371, 0.0456 and 0.0794 mol·m-2·s-1, the normalized root-mean-square error (NRMSE) was 26.8%, 30.0%, 36.9% and 64.3%, and R-squared (R2 ) was 0.96, 0.61, 0.91 and 0.88 between simulated and observed leaf stomatal conductance of potato for Ball-Berry model, Leuning model, Medlyn model and Jarvis model, respectively. For leaf stomatal conductance of oil sunflower, Jarvis model performed slightly better than Leuning model, Ball-Berry model and Medlyn model. RMSE was 0.2221, 0.2534, 0.2547 and 0.2758 mol·m-2 ·s-1, NRMSE was 40.3%, 46.0%, 46.2% and 50.1%, and R2 was 0.38, 0.22, 0.23 and 0.20 between simulated and observed leaf stomatal conductance of oil sunflower for Jarvis model, Leuning model, Ball-Berry model and Medlyn model, respectively. The path analysis was conducted to identify effects of specific meteorological factors on leaf stomatal conductance. The diurnal variation of leaf stomatal conductance was principally affected by vapour pressure saturation deficit for both potato and oil sunflower. The model evaluation suggested that the stomatal conductance models for oil sunflower are to be improved in further research
Measurement of horizontal and vertical advection of CO2 within a forest canopy
No full textEddy covariance measurements often underestimate the net exchange of CO2 between forest canopies and the atmosphere under stable atmospheric conditions, when horizontal and vertical advection are significant. A novel experimental design was used to measure all terms in the mass balance of CO2 in a 50 m × 50 m wide, 6 m tall control volume (CV) located on the floor of a 40 m tall Eucalyptus forest to examine the contributions of the eddy flux, the change in storage and the horizontal and vertical advection terms. Horizontal flux divergences between the four vertical walls of the CV were determined using perforated tubing arranged parallel to the ground to measure CO2 mixing ratios. The change in storage was calculated using CO2 concentration profiles measured in the centre of the CV. Vertical advection was calculated using these profiles, combined with vertical velocities, wc, calculated using the mass continuity equation and horizontal velocities measured at the mid-point of each wall of the CV. Vertical and horizontal advection and the eddy flux terms all contributed significantly to the mass balance of the CV at night, while the eddy flux term was dominant and negative for a short period around noon when photosynthesis exceeded respiration. Large vertical gradients of CO2 at night cause estimates of vertical advection to be extremely sensitive to small errors in wc with standard errors of the mean flux exceeding 3 μmol CO2 m-2 s-1. Vertical velocities need be measured to an accuracy better than 1 mm s-1 to minimize errors in vertical advection when vertical gradients of CO2 ratios are very large at night. Calculated horizontal advection is sensitive to errors in the wind vectors through the faces of the CV when horizontal concentrations gradients are large. Errors in eddy fluxes and change in storage are smaller than for the advection terms and errors for all components are smaller during the day than at night. Crow
δ13C of organic matter transported from the leaves to the roots in Eucalyptus delegatensis: short-term variations and relation to respired CO2
No full textPost-photosynthetic carbon isotope fractionation might alter the isotopic signal imprinted on organic matter (OM) during primary carbon fixation by Rubisco. To characterise the influence of post-photosynthetic processes, we investigated the effect of starch storage and remobilisation on the stable carbon isotope signature (delta C-13) of different carbon pools in the Eucalyptus delegatensis R. T. Baker leaf and the potential carbon isotope fractionation associated with phloem transport and respiration. Twig phloem exudate and leaf water-soluble OM showed diel variations in delta C-13 of up to 2.5 and 2 parts per thousand, respectively, with C-13 enrichment during the night and depletion during the day. Damped diel variation was also evident in bulk lipids of the leaf and in the leaf wax fraction. delta C-13 of nocturnal phloem exudate OM corresponded with the delta C-13 of carbon released from starch. There was no change in delta C-13 of phloem carbon along the trunk. CO2 emitted from trunks and roots was C-13 enriched compared with the potential organic substrate, and depleted compared with soil-emitted CO2. The results are consistent with transitory starch accumulation and remobilisation governing the diel rhythm of delta C-13 in phloem-transported OM and fragmentation fractionation occurring during respiration. When using delta C-13 of OMor CO2 for assessing ecosystem processes or plant reactions towards environmental constraints, post- photosynthetic discrimination should be considered
Measurement of horizontal and vertical advection of CO2 within a forest canopy
No full textEddy covariance measurements often underestimate the net exchange of CO2 between forest canopies and the atmosphere under stable atmospheric conditions, when horizontal and vertical advection are significant. A novel experimental design was used to measure all terms in the mass balance of CO2 in a 50 m × 50 m wide, 6 m tall control volume (CV) located on the floor of a 40 m tall Eucalyptus forest to examine the contributions of the eddy flux, the change in storage and the horizontal and vertical advection terms. Horizontal flux divergences between the four vertical walls of the CV were determined using perforated tubing arranged parallel to the ground to measure CO2 mixing ratios. The change in storage was calculated using CO2 concentration profiles measured in the centre of the CV. Vertical advection was calculated using these profiles, combined with vertical velocities,No Full Tex
Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u.-threshold filtering technique
No full textMicrometeorological measurements made on single towers often underestimate nighttime respiration of terrestrial ecosystems because they cannot account for vertical and horizontal advection, thereby causing systematic errors in estimates of net ecosystem carbon exchange. We show that there is a maximum in the sum of the turbulent flux and change in storage of CO2 in the early evening, Rmax, that is in close agreement with concurrent and independent estimates of net carbon exchange from soil and plant chambers.We hypothesize that the peak occurs because there is a time delay between the onset of radiative cooling and the development of temperature gradients that are strong enough to initiate thermally-driven horizontal and vertical flows that remove the stored CO2. We propose taking advantage of this time delay to develop relationships between Rmax and soil temperature and moisture. The new parameterization leads to realistic values of nighttime respiration, and therefore to improved estimates of net ecosystem exchange.Full Tex
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