SpecNet: adding spectral measurements to flux tower data

Type of optical sampling being conducted PP System spectrometer, 1 meter above canopy, every 5 meters for 100 meters out from the tower in all 8 major and minor compass directions. See Stylinski, C.D., J.A. Gamon, and W.C. Oechel, Seasonal patterns of reflectance indices, carotenoid pigments and photosynthesis of evergreen chaparral species, Oecologia, 131(3), 366-374, 2002. for detailed description of instrumentation, foreoptics, etc.

Coordinates (lat-lon) of site 24°08' N, 110° 26' W

Type of ecosystem Vegetation of the area is considered sarcocaulescent scrub (thick trunks and or stems; Shreve and Wiggins, 1964) and lies within the Arid-Tropical Region (Wiggins, 1980). Classified as the gulf coast desert (Coyle and Roberts, 1975), it is a transition between the Sonoran Desert of the Northern Peninsula and the tropical dry forest of the Sierra de La Laguna to the south and west. The major plant species include Prosopis articulata, Fouguieria diguetti, Bursera microphylla, Cyrtocarpa edulis, Jatropha cinerea and Jatropha cuneata. which make up more than 80% of the vegetation on site (Maya and Arriaga, 1996). A general description of the reserve describing the objectives, flora and fauna can be found at: http://www.cibnor.mx/colecciones/reserva/ereserva.html.

Eddy Tower (courtesy of Steve Hastings)

Site Information:

The principle site for the joint SDSU-CIBNOR activities centers around a natural scrub community approximately 2 kilometers south of the Bay of La Paz and the facilities of CIBNOR and 12 km west of the city of La Paz within a natural reserve (Reserva de El Comitan) administered by CIBNOR (24°08' N, 110° 26' W). The area is an alluvial plain formed by granite deposits from the nearby Sierra de La Laguna and is traversed in a number of places by superficial streams or arroyos. Soils are deep, sandy and well drained, light colored and classified as Yermosols, Xerosols, and Regosols (Maya and Arriaga, 1996). Elevation is approximately 2 meters.

The climate is characterized as very dry, warm and extreme (Garcia, 1973) with a highly variable, bimodal annual precipitation (Maya and Arriaga, 1996). Major rains take place in late summer (often as a result of tropical cyclones; Salinas-Zavala et al. 1991) with a peak in September while a much smaller and more variable peak is observed in December (Comision Nacional Del Agua, La Paz, B.C.S., Mexico). Total annual precipitation and average temperature is 173.6 mm and 23.8°C respectively (INEGRI, 1981 a,b).

Vegetation of the area is considered sarcocaulescent scrub (thick trunks and or stems; Shreve and Wiggins, 1964) and lies within the Arid-Tropical Region (Wiggins, 1980). Classified as the gulf coast desert (Coyle and Roberts, 1975), it is a transition between the Sonoran Desert of the Northern Peninsula and the tropical dry forest of the Sierra de La Laguna to the south and west. The major plant species include Prosopis articulata, Fouguieria diguetti, Bursera microphylla, Cyrtocarpa edulis, Jatropha cinerea and Jatropha cuneata. which make up more than 80% of the vegetation on site (Maya and Arriaga, 1996).

Instrumentation and facilities on site include a small storage building with four 70 water solar panel located on the roof and a 12 meter radio tower with meteorogological equipment attached. Within the building are six, six volt deep cycle storage batteries, a power regulator to control the battery bank charge, and a transformer to convert the 12 volts dc to 120 volts AC that supplies power to instruments in the building as well as 20 meters away to the instruments on the tower. Data from the tower are linked to fiber optics and a data storage computer within an office at CIBNOR that is directly connected to the internet.

The central function of the tower is to quantify net ecosystem CO2 exchange using tower based eddy covariance technique (Baldocchi, Hicks and Meyers, 1988; Verma 1990; Vourlitis and Oechel, 1997; 1999; Vourlitis et al. 2001. Instruments on the tower include a fast response (10Hz) three-dimensional sonic anemometer-thermometer (WindMaster Pro, Gill Instruments, Lymington, England), and a fast response (10 Hz) open-path infrared gas analyzer (LI-7500, LI-COR, Inc. Lincoln, NE, USA). The sonic anemometer provides the mean and fluctuating quantities of wind speed and temperature while the open path analyzer the mean and fluctuating quantities of CO2 and H20. Both instruments are located at 12 meters, approximately 6 meters above the average vegetation height, and oriented into the direction of the mean wind and slightly upwind to minimize potential flow distortionfrom the tower.

Raw CO2 and H2O vapour fluctuations are output as mean voltages and converted to densities by the appropriate calibration constants (Leuning and Moncrieff, 1990). Mass (CO2 and H2O vapour), energy and momentum fluxes are computed from the 10 Hz data using a 200 second running mean and filtering technique following a co-ordinate rotation of the wind vectors and stored as 30 min average on the computer linked by fiber optics (McMillen 1986; McMillen, 1988). Initial post processing of the data, including correcting carbon and water fluxes for the simultaneous fluctuations in heat and H20 vapour (Webb, Pearman and Leuning, 1980) are made on the data and provided for public viewing at http://gcrg.sdsu.edu/data.php/, "On Line Data Pages".

Initial Results:

Maximum carbon flux takes place within two weeks after the onset of rainy season in September accumulating up to 1.5 gC day-1 . m-2. Rates decline through December to approximately 0.7 gC day-1 . m-2, in part due to the shorter day length. A gradual decline from January till just prior to the first major rains of late summer was observed with a carbon loss on the order of 0.25 gC day-1 . m-2. The decline was primarily due to decreasing soil water availability. The observed seasonal pattern and net annual carbon exchange by the system is greatly modified by small, frequent rains where soil water is maintained near the surface such that a significant amount of carbon is loss via soil respiration and. Large, infrequent rains tend to reach deeper soil depths, allowing the soil surface to dry out, minimizing soil respiration, but moderating the degree of water stress in August and resulting in a larger annual carbon uptake. Initial comparisons of the eddy covariance data with remote sensing measurements from satellites and ecosystem models show good agreement. This indicates that the prospects for extending the measurements to other areas of Baja California and the western coast of Mexico our within reach in the not to distant future.

Literature Cited:

Baldocchi, D.D., Hicks, B.B. and Meyers, T.P. 1988. Measuring biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology 69, 1331-1340.

Coyle, J., and Roberts, N.C. 1975. A field guide to the common and interesting plants of Baja California. Natural History Publishing Company, La Jolla, USA. pp. 206

Leuning, R. and Moncrieff, J. 1990. Eddy covariance CO2 flux measurements using open- and closed-path CO2 analyzers: corrections for analyzer water vapor sensitivity and damping fluctuations in air sampling tubes. Boundary Layer Meteorology 53, 63-76.

Maya, Y. and Arriaga, L. 1996. Litterfall and phonological patterns of the dominant overstorey species of a desert scrub community in north-western Mexico. Journal of Arid Environments, 34:23-35.

McMillen, R. T. 1986. A BASIC program for eddy correlation in non-simple terrain. NOAA Technical Memorandum, ERL ARL-147, NOAA Environmental Research Laboratories, Silver Spring, MD, 32 pages.
McMillen, R. T. 1988. An eddy correlation technique with extended applicability to non-simple terrain. Boundary Layer Meteorology 43: 231-245.

Salinas-Zavala, C., Coria, R. and Díaz, E. (1991). Climatología y meteorología. In Ortega, R. and Arriaga, L. (eds), La Reserva de la Biósfera El Vizcaíno en la Península de Baja California. Public. No. 4 del CIB-BCS, La Paz, B.C.S., Mexico.

Shreve, F. and Wiggins, I.L. (1964). Vegetation and Flora of the Sonoran Desert. Stanford University Press, Stanford, Calif., U.S.A. 2 vols., 1740 pp.

Verma, S.B. 1990. Micrometeorological methods for measuring surface fluxes of mass and energy. Remote Sensing Reviews 5, 99-115.

Vourlitis, G.L. and Oechel W.C. 1997. Landscape-scale CO2, H2O vapor, and energy flux of moist-wet coastal tundra ecosystems over two growing-seasons. Journal of Ecology 85: 575-590.
Vourlitis, G.L. and W.C. Oechel. 1999. Eddy covariance measurements of net CO2 flux and energy balance of an Alaskan moist-tussock tundra ecosystem. Ecology 80: 686-701.

Vourlitis, G.L., Priante-Filho, N., Hayashi, M.M.S., Nogueira, J.De S., Caseiro, F.T. and Campelo Jr, J.H. 2001. Seasonal variations in the net ecosystem CO2 exchange of a mature Amazonian transitional tropical forest (cerradão). Functional Ecology, 15:388-395.

Webb, E. K., G.I. Pearman, and R. Leuning. 1980. Corrections of flux measurements for density effects due to heat and water vapor transfer. Quarterly Journal of the Royal Meteorological Society 106: 85-100.

Wiggins, I.L. (1980). Flora of Baja California. Stanford University Press, Stanford, Calif., U.S.A. 1025 pp.

Contact Dr. Gamon