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
2.1. Site characteristics The experimental site Le Bray is located in the
Landes forest, about 20 km south-west of Bordeaux latitude 44
◦
42
′
N, longitude 0
◦
46
′
W, altitude 62 m, in the center of a flat plot of maritime pine Pinus
pinaster Ait., covering about 16 ha. In the measure- ment field, the trees were 28 years old at the time of
E. Lamaud et al. Agricultural and Forest Meteorology 106 2001 187–203 189
the experiment March 1998. They are distributed in parallel rows along a NE–SW axis. The inter-row dis-
tance is 4 m and the stand density about 500 treesha. The canopy height was 18 m and the mean leaf area
index 2.8. It generally reaches 3 or more in summer. The site has a fetch larger than 600 m for the prevail-
ing wind directions N and W and the ground surface is flat slope less than 0.2
◦
in all directions. The forest canopy exhibits three layers. The upper
one, between 12 and 18 m at the time of this experi- ment, consists of a dense vegetation layer of branches
and needles. The medium one 1–12 m corresponds to the trunks. The understorey, forming the lowest one
0 to 1 m, mainly consists of grass Molinia coerulea L. Moench. The soil is a hydromorphic podzol with
sand agglomerate at a depth of about 0.5 m. It is cov- ered by a litter, with an average thickness of 0.05 m,
formed by dead needles, dead grass, dead branches and decayed organic matter.
2.2. Eddy flux measurements Since 1996 the experiments conducted at the Bray
site have been part of the EUROFLUX programme Aubinet et al., 1999. Continuous measurements
of CO
2
, water vapor and sensible heat fluxes have been performed at canopy scale from July 1996 to
June 1998, using a 25 m high instrumented tower. No measurements were performed inside the forest
on a regular basis, but during a few periods of in- tensive measurements a complete set of turbulence
measurements was added above the understorey at a height of 6 m. Thus, in March and April 1998, un-
derstorey momentum, sensible heat, water vapor and CO
2
fluxes could be estimated by the eddy covariance method. The fluctuations in wind velocity compo-
nents and temperature were measured by a 3D sonic anemometer Solent R3, Gill Instruments, Lymington,
UK. The measurements of CO
2
and H
2
O concen- tration fluctuations were made using an open-path
infrared absorption spectrometer E009, Advanet Inc., Aero-Laser, Garmisch–Partenkirchen, Germany.
The signals from the sonic anemometer and the infrared spectrometer were collected at 20 Hz. The
computation of eddy covariances was performed us- ing the EdiSol software Moncrieff et al., 1997 over
30 min period. This system uses a recursive digital filter to approximate a running mean, in order to
calculate the real-time fluctuations in the measured components. The time constant of the digital filter
was 200 s. The software performs coordinate rotations on the raw wind speed data. As we used open-path
sensors, CO
2
and water vapor fluxes were corrected for density fluctuations resulting from rapid changes
in temperature and humidity Webb et al., 1980. 2.3. Additional flux and energy storage measurements
The “transmitted” net radiation net radiation just above the understorey, hereafter referred to as R
n,b
, is estimated using a radiative transfer model based
on Beer’s law, that has previously been calibrated at this site Berbigier and Bonnefond, 1995. This model
calculates R
n,b
from net and solar radiation above the canopy at 25 m, measured with a Q-7 net ra-
diometer Radiation Energy Balance System, Seattle, WA and a CE 180 pyranometer Cimel Electron-
ique, Paris, respectively. The original model assumes that the ground, the canopy and the air all are at the
same temperature. In this study, the difference between ground, canopy and air temperatures is accounted for
and a term has been added to the longwave component of the model Ogée et al., 2000.
Soil heat flux G is estimated by a two-step ver- sion of the null-alignment method using soil temper-
ature, water content and bulk density measurements between the soil surface and a depth of 1 m Ogée
et al., 2000. For temperature measurements, 32 ther- mocouples were installed at four different locations
and eight depths. Soil moisture was measured with a time domain reflectometer TRASE System, Soil
Moisture Equipment Corp., Santa Barbara, CA using 20 cm long buriable waveguides placed horizontally at
three different locations and four different depths in the soil. Lastly, soil bulk density was measured gravi-
metrically from samples collected at different depths and three locations in the vicinity of the other soil
measurements.
The storage term J in the forest layer below the level of turbulent measurements 6 m is the sum of four
terms Diawara et al., 1991: J = J
h
+ J
w
+ J
p
+ J
v
. J
h
is the sensible heat storage in the canopy air space up to z
r
= 6 m
J
h
= Z
z
r
ρC
p
δT
a
δt dz
190 E. Lamaud et al. Agricultural and Forest Meteorology 106 2001 187–203
where ρ, C
p
and T
a
are the air density, specific heat and temperature, respectively. Air temperature was mea-
sured at 0.2, 0.7, 1.2 and 7 m. J
w
is the latent heat storage in the canopy air space up to z
r
J
w
= Z
z
r
ρC
p
Γ δe
δt dz
where Γ and e are the psychrometric constant and vapor pressure, respectively.
J
p
is the energy fixed by photosynthesis. It is de- duced from CO
2
flux measurements in the lower part of the forest by the relation J
p
= − L
p
F
CO2
, where L
p
is the specific energy equivalent of CO
2
fixation. In the understorey, this term may be neglected for the
following reasons. At canopy scale J
p
represents only 1 to 3 of the incident net radiation and, the ratio
between the net CO
2
flux and the photosynthetically active radiation is generally 2 to 3 times lower for the
understorey than for the trees. On top of this, J
p
in the understorey is particularly small at this time of year
March–April since there is virtually no grass. J
v
is the sensible heat storage in the vegetation up to z
r
J
v
= Z
z
r
3
X
i=1
ρ
vi
C
vi
δT
vi
δt dz
where ρ
vi
, C
vi
and T
vi
are the density, specific heat and temperature of each component of the vegetation,
i.e. trunks below the level of turbulent flux measure- ments, litter and grass. To measure the heat storage
in the trunks, ten probes were installed at 1.5 m, on two representative trees and at three depths one in the
center of the trunk, two under the bark and two in the middle of the sapwood oriented north and south. Lit-
ter temperatures were measured at five locations with four levels, 0, 0.02, 0.04 and 0.05 m from the soil to
the top of the litter. The radiative temperature of the grass was measured with an Everest 4000 GL infrared
radiometer. Thus, the experimental device for the mea- surement of heat storage in the air and the vegetation
of the understorey required 35 probes, out of a total of 66 for the whole canopy. The storage in the grass can
be neglected, as compared to the other two terms. In- deed, even in summer when the grass reaches its max-
imum LAI, its contribution to heat storage remains small about 2–3 W m
− 2
, which is less than 20 of J
v
. At this time of the year, the storage in the grass is around 1 W m
− 2
. Therefore, only heat storage in the trunks J
t
and the litter J
l
will be considered in this work.
Thus, the heat storage in the canopy layer up to z
r
finally reduces to J = J
h
+ J
w
+ J
t
+ J
l
2.4. Climatic conditions The climatic conditions during the measurement
campaign are illustrated in Fig. 1, showing the trans- mitted net radiation, the top soil surface temperature,
the air temperature at 7 m and rainfall. The 24 days of the data set, from 14 March to 6 April, can be split
up in two periods. Up to 27 March sunny conditions are prevalent, except on 14 March overcast and ex-
cept for some passing clouds in the mornings of 15, 16 and 24 March. From 28 March, conditions start
to change. Clouds appear on 28 March. 29 March is overcast from 10 a.m. and large rainfall occurs dur-
ing the following night. On 30 March net radiation remains lower than 20 W m
− 2
. After this first rain the weather remains unstable during the last 8 days. Most
days are affected by passing clouds and rainfall fre- quently occurs. During the first period, the Bowen ra-
tio is generally between 1 and 1.5. It remains around 1 during the 3 cloudy days from 28 to 30 March and
drops to 0.5 or less from 31 March to the end of the experiment.
The period of measurement is also characterized by a large range of variation in air temperature. From 14
to 23 March, the weather is stable, with a repetitive daily cycle. Air temperature at 7 m varies between
maximum values of 14–17
◦
C around 3 p.m. and min- imum values of 3–5
◦
C around 7 a.m. After a short period of cooling on 24 and 25 March maximum:
10–12
◦
C; minimum: −2−0
◦
C, air temperatures in- creases again to reach the maximum values for the
whole period of measurement on 27 March maxi- mum: 22
◦
C; minimum: 12
◦
C. From 28 to 30 March, nocturnal minimum values remain high 10–12
◦
C, but diurnal values decrease again because of the sig-
nificant cloudiness. During the last days of the exper- iment, the daily cycle of temperature is less marked,
as a consequence of frequent passing clouds during the day and rain events mostly at night.
E. Lamaud et al. Agricultural and Forest Meteorology 106 2001 187–203 191
Fig. 1. Climatic conditions during the March–April 1998 experiment in the Landes forest.
3. Validation of eddy flux measurements
