Agricultural and Forest Meteorology 106 2001 187–203
Validation of eddy flux measurements above the understorey of a pine forest
E. Lamaud
∗
, J. Ogée, Y. Brunet, P. Berbigier
INRA-Bioclimatologie, BP 81, 33833 Villenave d’Ornon Cedex, France Received 7 January 2000; received in revised form 21 August 2000; accepted 1 September 2000
Abstract
Measurements of turbulent exchange in the understorey of a forest canopy are necessary to understand and model both the functioning of the lower stratum and its contribution to turbulent exchange at the whole canopy scale. Eddy covariance
measurements of eddy fluxes just above the floor of a forest canopy must be thoroughly validated, given the particular conditions prevailing there. Our objective is two-fold: i check the overall quality of such eddy flux measurements through
the analysis of the understorey energy balance closure and ii define quality criteria for each half-hourly sample, based on the residual term of the energy balance. A subset of the EUROFLUX data base, collected within a pine forest canopy in south-west
France, was used for this purpose. During this experiment, all heat storage terms were carefully measured, which allowed the closure of the understorey energy balance to be rigorously tested. As in most experiments storage term measurements are
not available, we also developed a method to estimate them, in order to apply the above-mentioned data selection method. The energy balance closure was found to be quite satisfactory the slope of the sum of eddy fluxes and storage terms versus
transmitted net radiation is 0.99, the intercept is less than 1 W m
− 2
, r
2
is 0.94, there is no deviation from a linear trend. The data selection procedure allows a fair description of the daily and day-to-day variation of turbulent fluxes while rejecting the
most dubious data, whether experimental or estimated storage terms are used. This analysis proves the validity of eddy flux measurements in the lower part of the forest and offers tools for flux data selection, depending on the type of studies such
data are intended for. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Eddy flux; Energy balance; EUROFLUX; Forest canopy; Understorey; Validation
1. Introduction
An increasing number of studies on energy and mass exchange between forest canopies and the atmosphere
have been conducted over the past few years, in order to understand both forest functioning and the role of
forests as sinks or sources of atmospheric pollutants. Although most studies have focused on carbon dioxide
∗
Corresponding author. Tel.: +33-5-56843128; fax: +33-5-56843135.
E-mail address: lamaudbordeaux.inra.fr E. Lamaud.
and water vapor exchanges Grace et al., 1995, 1996; Jarvis et al., 1997; Malhi et al., 1998, the interest for
pollutants like ozone has grown during the past decade Lamaud et al., 1994; Coe et al., 1995; Munger et al.,
1996; Carrara et al., 2000.
One important difference between the forests and other vegetative surfaces is the presence of an under-
storey, which contributes to the overall exchange of mass and energy. Until recently, forests have
been considered as a whole and the specific role of the understorey was not taken into account.
Thus, forest-atmosphere exchanges have often been
0168-192301 – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 2 1 5 - X
188 E. Lamaud et al. Agricultural and Forest Meteorology 106 2001 187–203
modelled using a “big-leaf” approach. For a given site, this model may provide accurate results if the
processes occurring in the understorey and the over- storey are well coupled. This is not always the case.
Also, a comparison between two sites may lead to erroneous interpretations if existing differences be-
tween their understoreys are not taken into account. For example, Lamaud et al. 1996 showed that the
differences observed between two pine forest plots resulted both from differences in the leaf area index
of the overstoreys 3.5 versus 2.1 and from differ- ences in the understoreys graminae versus ferns.
Furthermore, as the processes involved in the transfer of the various scalars water vapor, CO
2
, ozone may be different, the ratio of the corresponding fluxes
between the overstorey and the understorey gener- ally differ. For example, more than 50 of day-time
ozone deposition was found to occur on the under- storey, whereas only about 25 of the total exchange
of water vapor resulted from understorey evaporation Carrara et al., 2000. Lastly, as the vegetative cycles
of the overstorey and understorey are different, es- pecially in coniferous forests, measurements cannot
be performed throughout the year without taking into account the two layers separately.
In the recent years an increasing number of articles have been specifically considering the understorey
Baldocchi and Vogel, 1996, 1997; Blanken et al., 1997. The eddy covariance method is generally used
to estimate the turbulent fluxes above the forest floor, as it is above the forest canopy itself. However, using
this method in the lower part of the forest should be subject to much caution because the underlying hy-
pothesis are generally not valid in the conditions pre- vailing there low wind speed, strong heterogeneity,
intermittent turbulence.
A thorough validation of turbulent flux measure- ments is therefore a necessary step in such studies.
For this, energy balance closure represents a power- ful test for eddy flux measurements since it may allow
the experimental data set to be validated on a mean basis, but also the most reliable data to be selected by
rejecting spurious half-hourly values that do not per- mit the energy balance to be closed in a satisfactory
way. The severity of this constraint must be modulated, depending on the objectives of the work.
Ecophysiological studies do not require as high a degree of accuracy in eddy flux measurements as
turbulence studies do. It is generally sufficient to be able to describe the relevant processes at the scale
of several days, in order to assess the evolution of energy and mass transfer resulting from changes in
environmental conditions. At daily scale, the data set must essentially allow one to describe the evolution
of the processes between day and night and through the day as a consequence of stomatal activity and
possible changes in dynamic wind velocity and di- rection and radiative clear or cloudy conditions.
In this case, a rejection of the most spurious data, like occasional peaks in eddy flux measurements, is
sufficient to provide an adequate data set. A severe selection may strongly reduce the use of the data set,
as only “ideal” conditions strong winds, clear days can be studied. However, for specific analysis of tur-
bulent transfer involving spectral analysis, analysis of turbulent moments. . . a higher degree of relia-
bility is required. In this case, it is essential to have the best possible accuracy on the measurement of
the eddy fluxes themselves, even at the expense of a drastic reduction in the amount of available data.
The present analysis was conducted on a data set collected during the EUROFLUX programme Aubi-
net et al., 1999, within and above a pine forest canopy in south-west France. As already mentioned, we only
consider here the understorey level. In a first step we show that intensive measurements of heat storage al-
low good closure of the understorey energy balance, thereby validating the turbulent flux measurements
deep within the forest. In a second step, an analysis of the residue of the energy balance permits flux values
to be selected with various degrees of confidence. We finally suggest an alternate selection method, based
upon an estimate of the storage terms which can be applied when no storage measurements are available.
2. Materials and methods
