11
Canopy Fluxes
John B. Moncrieff, Paul G. Jarvis, and Ricardo Valentini
Introduction
One of the foremost challenges for ecologists is to
integrate observations made at a range of scales so
that they become useful to others working at a different scale. An example of this would be scaling
observations of carbon and water exchange made
at leaf or plot scale to be of direct use to global
climate modelers. Micrometeorological techniques,
which operate at intermediate scales to these two
extremes, are being used increasingly to validate
and parameterize such models (Baldocchi and
Meyers 1998). In tum, micrometeorological techniques can be validated against suitably scaled observations at a smaller scale. Progress in closing the
carbon budget for example, must rely on such an
interdisciplinary approach. When made in combination with detailed biophysical field experiments,
such observations can reveal the atmospheric and
biophysical variables that control carbon and water
exchange. In this review, we describe briefly the
most common micrometeorological methods used
to measure fluxes of carbon and water at the scale
of the canopy, focusing in particular on the direct
techniques of eddy covariance and eddy accumulation. We also describe flux measurements at related scales that are commonly used to give added
value to canopy-scale fluxes.
This review comprises material available up until the end
of 1998. Readers interested in the eddy covariance methodology may wish also to consult Aubinet et al. (2000).
The Canopy Scale
In general, micrometeorological methods are usually reported over a time period of about half an
hour and integrate fluxes over a spatial scale of
about 1 km 2 . The space scale is influenced by the
height of the measuring point above the top of the
canopy and the length scales of atmospheric turbulence, which are a result of both mechanical (surface friction) and thermal effects that influence atmospheric stability. The reporting time for surface
fluxes of about 30 minutes is related to the need to
make observations over a suitably long period such
that the majority of the spectrum of flux-carrying
eddies are sampled, yet not too long so that natural
diurnal variability in scalar concentrations or forcing functions such as solar radiation are included.
Micrometeorological methods are nondestructive
in that they only sample the air as it advects past
the sensor.
The Surface Boundary Layer
All micrometeorological measurements are made
within the atmospheric boundary layer (ABL) and
the depth and structure of this layer present opportunities and challenges (Fig. ILl). The ABL as defined by Lenschow (1995) is "the lower part of the
atmosphere that interacts with the biosphere and is
closely coupled to the surface by turbulent exchange processes." The depth of the ABL depends
on the degree of mechanical (caused by surface
friction) and buoyant mixing (thermals rising from
the warmed surface) and its depth can also be dic161
Canopy Fluxes
John B. Moncrieff, Paul G. Jarvis, and Ricardo Valentini
Introduction
One of the foremost challenges for ecologists is to
integrate observations made at a range of scales so
that they become useful to others working at a different scale. An example of this would be scaling
observations of carbon and water exchange made
at leaf or plot scale to be of direct use to global
climate modelers. Micrometeorological techniques,
which operate at intermediate scales to these two
extremes, are being used increasingly to validate
and parameterize such models (Baldocchi and
Meyers 1998). In tum, micrometeorological techniques can be validated against suitably scaled observations at a smaller scale. Progress in closing the
carbon budget for example, must rely on such an
interdisciplinary approach. When made in combination with detailed biophysical field experiments,
such observations can reveal the atmospheric and
biophysical variables that control carbon and water
exchange. In this review, we describe briefly the
most common micrometeorological methods used
to measure fluxes of carbon and water at the scale
of the canopy, focusing in particular on the direct
techniques of eddy covariance and eddy accumulation. We also describe flux measurements at related scales that are commonly used to give added
value to canopy-scale fluxes.
This review comprises material available up until the end
of 1998. Readers interested in the eddy covariance methodology may wish also to consult Aubinet et al. (2000).
The Canopy Scale
In general, micrometeorological methods are usually reported over a time period of about half an
hour and integrate fluxes over a spatial scale of
about 1 km 2 . The space scale is influenced by the
height of the measuring point above the top of the
canopy and the length scales of atmospheric turbulence, which are a result of both mechanical (surface friction) and thermal effects that influence atmospheric stability. The reporting time for surface
fluxes of about 30 minutes is related to the need to
make observations over a suitably long period such
that the majority of the spectrum of flux-carrying
eddies are sampled, yet not too long so that natural
diurnal variability in scalar concentrations or forcing functions such as solar radiation are included.
Micrometeorological methods are nondestructive
in that they only sample the air as it advects past
the sensor.
The Surface Boundary Layer
All micrometeorological measurements are made
within the atmospheric boundary layer (ABL) and
the depth and structure of this layer present opportunities and challenges (Fig. ILl). The ABL as defined by Lenschow (1995) is "the lower part of the
atmosphere that interacts with the biosphere and is
closely coupled to the surface by turbulent exchange processes." The depth of the ABL depends
on the degree of mechanical (caused by surface
friction) and buoyant mixing (thermals rising from
the warmed surface) and its depth can also be dic161