Forest canopies are combinations of vegetation of considerable height and rough
surfaces, adjacent to the atmosphere, as compared to lower and smoother vegetable
canopies such as pastures or crops. The physical environment of forest stands is
determined by factors such as the penetration of solar radiation and the extent of
airflow within canopies. The configuration of the canopy dictates the development
of air velocity fields and radiation transmission regimes. Branches absorb the linear
momentum of flow through drag, while simultaneously absorbing and dispersing
solar radiation, minimizing its transmission to lower levels. In this way, the surface
boundary layer and the air layer within the canopy are aerodynamically coupled.
Air circulation regulates the microenvironment and plant growth via the
exchange of carbon dioxide, heat, and water vapor on leaf surfaces and through
diffusion of heat and mass between the area inside the canopy and the air layer
above it. Heat and mass exchanges at the level of the foliage occur by molecular
diffusion coupled with higher exchange resistance at the molecular boundary layers.
Turbulent exchanges of heat and mass occur outside the molecular boundary layer,
adjacent to the foliage surfaces.
Airflow also induces direct mechanical action on plants and other obstacles as
well as dispersion of particles, microorganisms, fungal spores, and pollen released
by plants (Shaw 1995). The forces exerted may be enough to damage branches and
trunks or even uproot trees. Such processes are mainly due to intermittent turbulent
phenomena. The dispersion rate of bacterial or fungal diseases is also dependent on
the velocity of the wind fields, like wise determined by the vegetative configuration.
The Penman–Monteith equation , based on the big-leaf principle and the Bowen
ratio , can be used to quantify vertical fluxes of mass and energy in forests, as an
aerodynamic and turbulent covariance method (Jarvis and McNoughton 1986;
Kelliher et al. 1990). The Penman–Monteith equation quantifies vertical flux of
water vapor and patterns of the evapotranspiration regime. This equation includes
aerodynamic and canopy resistance, which relate to the canopy physiology and
configuration. Characterization of flow within forest canopies, also addresses topics
such as parameterization of the rough sublayer and the interaction between carbon
and water vapor fluxes, which are of great importance, e.g., in the Mediterranean
region.
4.2 Aerodynamic Characterization and Stability
in the Rough Sublayer
The roughness sublayer (Fig. 2.1) is in the atmospheric zone, adjacent to the
canopy surface including the understory, where the dynamics of circulation are
influenced by the spacing between the elements with the development of circulation
wakes downwind.
In the canopy roughness sublayer, the main flux-gradients principles are not
normally followed, because anomalous phenomena occur. These run in the opposite
direction to the gradients and are mainly due to the influence of coherent,
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4 Exchange of Energy and Mass Over Forest Canopies
surfaces, adjacent to the atmosphere, as compared to lower and smoother vegetable
canopies such as pastures or crops. The physical environment of forest stands is
determined by factors such as the penetration of solar radiation and the extent of
airflow within canopies. The configuration of the canopy dictates the development
of air velocity fields and radiation transmission regimes. Branches absorb the linear
momentum of flow through drag, while simultaneously absorbing and dispersing
solar radiation, minimizing its transmission to lower levels. In this way, the surface
boundary layer and the air layer within the canopy are aerodynamically coupled.
Air circulation regulates the microenvironment and plant growth via the
exchange of carbon dioxide, heat, and water vapor on leaf surfaces and through
diffusion of heat and mass between the area inside the canopy and the air layer
above it. Heat and mass exchanges at the level of the foliage occur by molecular
diffusion coupled with higher exchange resistance at the molecular boundary layers.
Turbulent exchanges of heat and mass occur outside the molecular boundary layer,
adjacent to the foliage surfaces.
Airflow also induces direct mechanical action on plants and other obstacles as
well as dispersion of particles, microorganisms, fungal spores, and pollen released
by plants (Shaw 1995). The forces exerted may be enough to damage branches and
trunks or even uproot trees. Such processes are mainly due to intermittent turbulent
phenomena. The dispersion rate of bacterial or fungal diseases is also dependent on
the velocity of the wind fields, like wise determined by the vegetative configuration.
The Penman–Monteith equation , based on the big-leaf principle and the Bowen
ratio , can be used to quantify vertical fluxes of mass and energy in forests, as an
aerodynamic and turbulent covariance method (Jarvis and McNoughton 1986;
Kelliher et al. 1990). The Penman–Monteith equation quantifies vertical flux of
water vapor and patterns of the evapotranspiration regime. This equation includes
aerodynamic and canopy resistance, which relate to the canopy physiology and
configuration. Characterization of flow within forest canopies, also addresses topics
such as parameterization of the rough sublayer and the interaction between carbon
and water vapor fluxes, which are of great importance, e.g., in the Mediterranean
region.
4.2 Aerodynamic Characterization and Stability
in the Rough Sublayer
The roughness sublayer (Fig. 2.1) is in the atmospheric zone, adjacent to the
canopy surface including the understory, where the dynamics of circulation are
influenced by the spacing between the elements with the development of circulation
wakes downwind.
In the canopy roughness sublayer, the main flux-gradients principles are not
normally followed, because anomalous phenomena occur. These run in the opposite
direction to the gradients and are mainly due to the influence of coherent,
106
4 Exchange of Energy and Mass Over Forest Canopies