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CD-12 Abstract

A Pilot Study for Assessing the Carbon and Energy Balance of a Transitional Tropical Forest in Southwest Amazonia

Nicolau Priante Filho — UFMT - Universidade Federal do Mato Grosso (SA-PI)
George L. Vourlitis — California State University (US-PI)

Objective



The objective of this pilot study is to quantify the mass (CO2 and H2O vapor) and energy exchange of a mature transitional tropical forest stand over portions of the wet and dry season and to determine the meteorological and seasonal controls on these mass and energy fluxes. As a pilot study, our goal is to collect preliminary field data over a 6-month period that extends over portions of the wet and dry seasons. Measurements were initiated on 17 August, 1999 and will be continued (at a minimum) until 2 February, 2000.



Location and Description of the Study Site



The study area is located near the city of Sinop in NW Mato Grosso, Brazil (11°24.75'S: 55°19.50'W). This region is located near the ecotone of two major ecosystem types of South America (wet evergreen rainforest and

cerrado), and thus, represents the mesic boundary of cerrado and the xeric boundary of the rainforest. Vegetation consists of transitional broadleaf evergreen forest

(cerradćo) with a canopy height of ca. 30 m. The mean annual temperature near Sinop is 24.1°C, and rainfall is ca. 2 m/year with a 4-month dry season. Soils near the study site are predominantly ultisols (Brazilian nomenclature: Podzolico vermelho-amarelo

distrofico).



Eddy Covariance Measurements of Net CO2 Exchange



Net CO2 exchange, evapotranspiration (ET), and convective (sensible) heat flux (H) of the mature cerradćo stand are currently measured using tower-based eddy covariance. The

emddy covariance system consists of a 3-dimensional sonic anemometer-thermometer (Applied Technologies, Inc., K-probe) for measuring the mean and fluctuating quantities of wind speed (u, v, and w ) and temperature and an open-path infrared gas analyzer

(NOAA-ATDD) for measuring the mean and fluctuating quantities of CO2 and H2O vapor. The eddy covariance sensors are mounted at a height of 42 m above ground level, which corresponds to a height of ca. 12 m above the canopy. Raw CO2 and H2O vapor fluctuations are output as mean voltages and converted to densities by multiplying by the requisite calibration constants. Net CO2 flux, ET, and H are computed using NOAA-ATDD software following a coordinate rotation of the wind vectors, and fast response (10 Hz) fluxes are calculated and stored on a laptop computer as 30-minute averages using a 200 s running mean and digital recursive filtering technique. Raw (10 Hz) data are also archived for spectral analysis to assess the frequency response of the sensors and the data acquisition system.



The relatively low mean wind speed of the Sinop area (1961-1994 average = 1.5 m/s) is likely to result in low turbulent transport and high atmospheric stability, which in turn, can cause CO2 to be stored in the canopy. Because canopy CO2 storage can lead to errors in the measurement of surface-atmosphere CO2 exchange, canopy CO2 storage (ĘS) is quantified by determining the rate of change of CO2 in the air column between the ground surface and the eddy covariance sensors. The vertical CO2 concentration profile is measured every 30 seconds at 1, 4, 12, 20, and 28 m above ground level using a closed-path CO2 analyzer (LI-6251). Samples are drawn from each height using a diaphragm pump and plastic (3-5 mm diameter) tubing, and air samples are switched using a solenoid switching manifold system controlled by a datalogger (Campbell Scientific Inc., Model CR-10X).



Net ecosystem CO2 flux (Fc) is calculated as Fc = Fm + ĘS, where Fm is the net CO2 flux measured at the top of the tower and

ĘS is the CO2 storage in the canopy. Fm is measured from the eddy covariance array.

ĘS s calculated from measurements of the vertical CO2 concentration profile. Measured fluxes are corrected for the simultaneous fluctuations in heat and H2O vapor.



The measurement system is powered using 12 V batteries charged by solar panels, with occasional use of a gasoline-powered generator to rapidly re-charge the battery bank.





Micrometeorological Measurements



Measurements of net radiation are made above the canopy using a ventilated net radiometer

(REBS, model Q*7.1). Soil heat flux is measured using 2 heat flux plates installed at a depth of 0.03 m below the soil surface

(REBS, model HFT-3.1). Above-canopy incident photosynthetic photon flux density

(PPFD) and solar radiation are measured using a quantum sensor (LI-COR, model LI-190SA) and a pyranometer (LI-200SA), respectively. Air temperature and relative humidity are measured using above the canopy using a shielded combination thermistor and relative humidity sensor

(Vaisala, HMP45C). We are currently planning to install ventilated wet- and dry-bulb psychrometers throughout the canopy (1, 4, 12, 20, and 28 m above ground level) to measure the vertical temperature and vapor pressure profile. Precipitation is measured above the canopy using a tipping-bucket rainfall gauge. Micrometeorological data are averaged over 30-minute intervals from observations made every 1 s, and stored using a multiplexed datalogger (Campbell Scientific, model CR 21X).





Data Analysis



Although data reduction and analysis are ongoing, modeling and other statistical analyses will be conducted after completion of the field study (February-May 2000). Regression-based, non-linear physiological models will be used to assess the effects of seasonality (rainfall) and meteorology (radiation, humidity, and temperature) on stand-scale physiology.



Last Updated: August 26, 1999

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