J. M. Massheder

A system to measure surface fluxes of momentum, sensible heat, water vapour and carbon dioxide

An eddy covariance system is described which has been developed jointly at a number of European laboratories and which was used widely in HAPEX-Sahel. The system uses commercially available instrumentation: a three-axis sonic anemometer and an IR gas analyser which is used in a closed-path mode, i.e. air is brought to the optical bench by being ducted down a sampling tube from a point near the sonic anemometer. The system is controlled by specially written software which calculates the surface fluxes of momentum, sensible and latent heat and carbon dioxide, and displays them in real time. The raw turbulent records can be stored for post-processing. Up to five additional analogue instruments can be sampled at up to 10 Hz and digitised by the sonic anemometer. The instruments are described and details of their operation and connection are presented. The system has relatively low power consumption and can operate from appropriate solar cells or rechargeable batteries. Calibration of the gas analyser needs to be performed typically every 2 or 3 days, and, given that the system requires minimal maintenance and is weather insensitive, it can be operated for the routine collection of surface flux data for extended periods. There are a number of corrections which have to be applied in any eddy covariance system and we describe the system of transfer functions which define our system. Some representative results showing the potential of the system are presented.

Carbon Dioxide Uptake by an Undisturbed Tropical Rain Forest in Southwest Amazonia, 1992 to 1993

Measurements of carbon dioxide flux over undisturbed tropical rain forest in Brazil for 55 days in the wet and dry seasons of 1992 to 1993 show that this ecosystem is a net absorber of carbon dioxide. Photosynthetic gains of carbon dioxide exceeded respiratory losses irrespective of the season. These gains cannot be attributed to measurement error, nor to loss of carbon dioxide by drainage of cold air at night. A process-based model, fitted to the data, enabled estimation of the carbon absorbed by the ecosystem over the year as 8.5 ± 2.0 moles per square meter per year.

Seasonal variation of carbon dioxide, water vapor, and energy exchanges of a boreal black spruce forest

Measurements of the fluxes of latent heat λE, sensible heat H, and CO2 were made by eddy covariance in a boreal black spruce forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS) for 120 days through the growing season in 1994. BOREAS is a multiscale study in which satellite, airborne, stand-scale, and leaf-scale observations were made in relation to the major vegetation types [Sellers et al., 1995]. The eddy covariance system comprised a sonic anemometer mounted 27 m above the forest, a system for transferring air rapidly and coherently to a closed path, infrared gas analyzer and a computer with the Edinburgh EdiSol software. Over the measurement period, closure of the energy balance on a 24 hour basis was good: (H + λE)/(Rn - G - B - S) = 0.97. The midday Bowen ratio was typically in the range 1.0–2.5, with an average value of ∼1.9 in the first Intensive Field Campaign (IFCl) and 1.3–1.4 in IFC2 and IFC3. Daily ecosystem evapotranspiration from moss, understory, and trees followed daily net radiation. Mean half-hourly net ecosystem flux followed photosynthetic photon flux density (PPFD) closely, reaching −9 μmol m−2 s−1 in June and August. The mean respiratory efflux on nights during which the atmosphere was well mixed (u*>0.4 m s−1) reached 6 μmol m−2s−1. The PPFD-saturated biotic CO2 assimilation reached 20 μmol; m−2 s−1 and showed little response to air temperature or vapor pressure deficit (VPD). Storage of CO2 in the air column at night did not account adequately for respiration on stable nights, so nighttime efflux was modeled for periods when u*<0.4 m s−1. There was a net gain of CO2 on most of the 120 days, but on 31 days of high temperature or low PPFD there was a net carbon loss. High PPFD promoted influx of CO2 by the foliage, whereas high temperatures reduced net CO2 influx through high respiration rates by the roots and soil microorganisms, leading to lower net uptake at high PPFD. Over the 120 day period, 95 g m−2 of C were stored (an average of 0.8 g m−2 d−1), and 237 mm of water evaporated (an average of 2 mm d−1).

The Simile visual modelling environment

Abstract Simile is a visual modelling environment that has been developed to overcome the problems involved in implementing agro-ecological simulation models using conventional programming languages: problems such as the effort and skill needed to program the models, the lack of transparency in models implemented as programs, and the lack of re-useability of models and submodels. It combines the familiar System Dynamics (compartment-flow) paradigm with an object-based paradigm, allowing many forms of disaggregation to be handled, as well as spatial modelling and individual-based modelling. Its visual modelling interface makes it accessible to non-programmers, at the same time allowing models to be largely self-documenting. Models can be run very efficiently as compiled C++ programs, and users can develop new visualisation tools for displaying model results. Simile has been used in international research programmes, including the modelling of Mediterranean vegetation dynamics and modelling the interaction between households and land at the forest margin in developing countries. Simile has been developed in a spirit of open standards for model sharing. Models are saved as a text file in a structured format, with a view to enable model sharing with other modelling environments and to encourage others to develop additional tools for working with models.

Controls on carbon and energy exchange by a black spruce - moss ecosystem : Testing the mathematical model ecosys with data from the BOREAS experiment

Stomatal limitations to mass and energy exchange over boreal black spruce forests may be caused by low needle N concentrations that limit CO2 fixation rates. These low concentrations may be caused by low N uptake rates from cold boreal soils with high soil C:N ratios and by low N deposition rates from boreal atmospheres. A mathematical model of terrestrial ecosystems ecosys was used to examine the likelihood that slow N cycling could account for the low rates of mass and energy exchange measured over a 115-year old boreal spruce/moss forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). In the model, net N mineralization was slowed by the high C:N ratios measured in the forest floor and by high lignin contents in spruce litterfall. Slow mineralization caused low N uptake rates and hence high C:N ratios in spruce and moss leaves that reduced specific activities and areal densities of rubisco and chlorophyll. Consequent low CO2 fixation rates caused low stomatal conductances and transpiration rates which in turn caused high soil water contents. Wet soils, in conjunction with large accumulations of surface detritus generated by slow litter mineralization, caused low soil temperatures that further slowed mineralization rates. Model outputs for ecosystem N status were corroborated by low needle N concentrations (< 10 mg g−1), stomatal conductances (< 0.05 mol m−2 s−1) and CO2 fixation rates (< 6 μmol m−2 s−1), and by high canopy Bowen ratios (1.5–2.0) and low canopy net CO2 exchange rates (< 10 μmol m−2 s−1) measured over the black spruce/moss forest at the BOREAS site. Modeled C accumulation rates of 60 (wood) + 10 (soil) = 70 g C m−2 yr−1 were consistent with estimates from aggregated CO2 fluxes measured over the spruce canopy and from allometric equations developed for black spruce in Canadian boreal forests. Model projections under IS92a climate change indicate that rates of wood C accumulation would rise and those of soil C accumulation would decline from those under current climate. Because these rates are N-limited, they would be raised by increases in atmospheric N deposition.

CO2 fluxes at leaf and canopy scale in millet, fallow and tiger bush vegetation at the HAPEX-Sahel southern super-site

Measurements of canopy and leaf scale CO2 flux from the three sub-sites at the HAPEX-Sahel Southern supersite are presented. These are analysed in relation to biological and environmental variables. At leaf scale, the flux is most strongly influenced by photosynthetic photon flux density (PPFD) and stomatal conductance. Together with measurements of canopy structure at each site, the measurements of leaf photosynthesis, stomatal conductance and stem respiration were used to parameterise sub-models within the canopy model MAESTRO, which predicts canopy net CO2 flux. Comparison of the independent canopy flux measurements with predictions is informative, as the model represents an integration of our knowledge of the system, and so differences highlight weak points in our understanding as well as measurement artefacts. These differences are largest in tiger bush and smallest in millet, and are attributed to the effect of canopy heterogeneity on measurements rather than biological processes. Generally, good agreement was found at all three sites and the model can be regarded as validated. The model was used to extrapolate measurements in time, and, using a years weather data, predicted a value for carbon sequestration at the millet site over the growing season very close to harvest measurements