Trace gas exchange above the floor of a deciduous forest: 1. Evaporation and CO₂ efflux

Abstract

The eddy correlation method has great potential for directly measuring trace gas fluxes at the floor of a forest canopy, but a thorough validation study has not been yet conducted. Another appeal of the eddy correlation method is its ability to study processes that regulate and modulate gas exchange between the soil/litter complex and the atmosphere that cannot be probed with chambers. In this paper we report on eddy correlation measurements of water vapor, sensible heat, and carbon dioxide exchange that were made at the floor of a deciduous forest. The validity of the eddy correlation method to measure the emission of water vapor and CO2 from a deciduous forest floor is demonstrated by our ability to close the surface energy budget during periods that meet the requirements of the technique. Water vapor fluxes from a dry forest floor are strongly influenced by large-scale turbulent events that penetrate deep into the canopy. The frequency of these turbulent events prevents equilibrium evaporation rates from being achieved because the dynamic time constant for water vapor exchange is longer. Consequently, maximal evaporation rates are capped to rates defined by the product of the driving potential of the atmosphere and the surface conductance. On the other hand, evaporation from a wet forest floor proceeds at rates reaching or exceeding equilibrium evaporation and are highly correlated with static pressure fluctuations. CO2 effiux rates are governed by litter and soil temperature, as expected. But we also find a significant correlation between static pressure fluctuations and soil/litter CO2 exchange rates. , 1967; Kanemasu et al., 1979; Businger, 1986; Baldocchi et al., 1988]. mised or violated [see TannerIn this paper we describe micrometeorological studies of water vapor and CO2 gas exchange from the soil/litter complex below a deciduous forest. The goals of this work are (1) to demonstrate the validity and applicability of the eddy correlation method below a deciduous forest canopy to measure forest floor gas exchange rates, (2) to examine the processes governing the emission of water vapor and CO2 from a deciduous forest floor, and (3) to document diurnal and seasonal trends in the exchange rates of these gases. The deposition of ozone and SO2 at the forest floor were also measured and are discussed in a companion paper

Carbon dioxide and energy flux partitioning between the understorey and the overstorey of a maritime pine forest during a year with reduced soil water availability

“…This often leads to an underestimation of understorey CO 2 efflux when compared to chamber measurements (Norman et al, 1997;Kabwe et al, 2005), although some authors (Law et al, 1999) found a good agreement between both methods. However, averaging half-hourly values over several days should decrease the errors caused by the presence of heterogeneities under forest canopies (Baldocchi and Meyers, 1991).…”

Ecosystem fluxes of hydrogen: a comparison of flux-gradient methods

“…For the methods to work, trace gas species should not have significantly different vertical distributions of sources and sinks. Furthermore, the trace gas in question should be inert over the timescale of the flux-gradient measurement, meaning that the timescale of turbulence (200-300 s in such ecosystems) should not exceed the timescale of chemical reactions Baldocchi and Meyers, 1991).…”

“…Counter-gradient transport of heat, momentum, and trace gases has been documented beneath plant canopies and may severely compromise fluxgradient methods (Shaw, 1977;Raupach and Thom, 1981;Amiro 1990;Baldocchi and Meyers, 1991). On the other hand, flux-gradient methods have been preferred over eddy covariance techniques for measuring surface-atmosphere fluxes within a few meters of the surface layer (Fitzjarrald and Lenschow, 1983;Gao et al, 1991;de Arellano and Duynkerke, 1992;Wagner-Riddle et al, 1996;Taylor et al, 1999;Dunn et al, 2009).…”

“…The integrating volumes avoided sampling errors of around 10 % for our GC configuration that would have resulted from intermittent sampling with a single instrument (see Sect. 2.4.3) assuming τ T = 200-300 s (Baldocchi and Meyers, 1991). Sampling intervals interact systematically with the autocorrelation of the time series arising from the eddy structures (Woodruff, 1986).…”

“…Data were rejected during rainy periods with more than 0.2 mm of rain per 30 min (Baldocchi and Meyers, 1991). Periods with u * < 0.07 m s −1 and u * < 0.17 m s −1 were excluded for below-and above-canopy data, respectively, because of poorly developed turbulent conditions Liu and Foken, 2001;Bocquet et al, 2011) and potential for advective fluxes driven by drainage flows on sloping terrain (Yi et al, 2008).…”