A Large Daylight Geodesic Dome for Quantification of Whole-Ecosystem Carbon Dioxide and Water Vapor Fluxes

We developed, tested, and ultimately used a large-scale static chamber - a 4 meter diameter geodesic dome - as a way to (1) overcome the problems of scaling leaf-level gas exchange to estimate ecosystem-level CO₂ and water vapor fluxes, (2) maintain the ability to quantify plot level responses to experimental treatments that would be impossible to assess with eddy correlation methods which require relatively large footprints, and (3) avoid the problems of chamber over-pressurization commonly associated with open flow gas exchange systems. Our primary scientific objective was to quantify the consequences of post-fire alien plant succession on CO₂ and water vapor fluxes in native intact sagebrush steppe and adjacent post-fire successional areas over the course of a year. By quickly sealing the 12.3 m² dome (16.4 m³) over individual experimental plots established in each area (six plots in burned and six plots in adjacent intact sagebrush communities) onto wooden baseplates we were able to accurately measure net ecosystem CO₂ fluxes (NEE) as small as 0.1 μ mol CO₂ m^-2land area s^-1 and water vapor fluxes of 0.1 mmol H₂O m^-2land area s^-1 within two minutes of placing the dome on a plot. Changes in dome CO₂ and water vapor concentrations were measured using an open-patch IRGA (LICOR 7500), and two window fans were used to mix the air. The translucent dome with its skin of reinforced polyethylene allowed transmission of more than 70% of photosynthetically active radiation. Its large area allowed accurate representation of key ecosystem elements (shrubs and inter-shrub spaces). The change in air temperature within the dome during the 2 minute measurement was small and physiologically inconsequential, ranging from zero at night to +2° C at midday in the summer. One year after the wildfire leaf areas index in the burned plots with successional herbaceous vegetation at peak biomass averaged 0.1 while in intact sagebrush communities about 3. Surprisingly, however, NEE and water vapor fluxes (weighted average of five measurements on each plot over 24 hours) were similar in the two communities. The similarity appears to be due to low photosynthetic CO₂ uptake and low respiratory CO₂ emissions in successional communities and relatively high photosynthetic CO₂ uptake balanced by high plant and soil respiratory losses in intact sagebrush communities. Daytime NEEs in the late spring and summer in both plant communities ranged from 1 to 4 μ mol CO₂ m^-2 s^-1, while nighttime values (ecosystem respiration) ranged from -0.1 to -2 μ mol CO₂ m^-2 s^-1. Water vapor fluxes over the same period ranged from 0 to 1.0 mmol H₂O m^-2 s^-1 during the daytime and 0 to -0.1 mmol H₂O m^-2 s^-1 at night. Both NEE and water vapor fluxes declined in late summer and fall months with relatively large loss of leaf area and less strong decreases in plant and soil respiration. Thus, our inexpensive extension of standard static chamber techniques to a very large cuvette - geodesic dome - permits us to directly and accurately measure ecosystem level gas fluxes in two adjacent communities which can be integrated over time to calculate annual net ecosystem productivity and evapotranspiration.