4
Exchange of Energy and Mass Over
Forest Canopies
Abstract
This chapter aimed to characterize the aerodynamic mass and energy fluxes over
forest canopies. These canopies are rough surfaces wherein flow-gradient
assumptions may not be valid, leading to exponential wind profiles in trunk
spaces and understory, superimposed by logarithmic profiles above the top of
canopies. This leads to airflow regimes wherein canopy stomatal resistance is
dependent on tree physiological factors and of short and long-term phenomena,
in interaction with aerodynamic variables. Forest canopies are also very prone to
intermittent events, such as gusts or ejections, and wake formation in trees
downstream, adding components to the usual kinetic energy equations and
influencing turbulence spectral dynamics and particle transport. The analysis of
drag processes in canopies, in wind tunnel and field, allows also to characterize
phenomena such as tree bending and pulling. The evapotranspiration regime of
these canopies is also coupled with the moisture of soil and atmosphere, in
comparison with lower canopies, with transient flow mechanisms particularly
relevant in trunk and understory spaces. Finally, forest canopies are relevant in
carbon balance dynamics, especially related to the net ecosystem exchange, on a
micro or global scale, and these dynamics reflect the influence of physical and
biological factors in carbon sinking.
4.1 Introduction
The magnitude of atmospheric mass and energy fluxes over forest canopies, for
example, in relation to water loss, soil erosion, or the microclimate, depends on
processes at the level of the biosphere, within the control volume between the
ground surface and the top of the surface boundary layer. The latter is divided, as
referred to in Chap. 2, into the inertial and roughness sublayers.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. Rodrigues et al., Fundamental Principles of Environmental Physics,
https://doi.org/10.1007/978-3-030-69025-0_4
105
Exchange of Energy and Mass Over
Forest Canopies
Abstract
This chapter aimed to characterize the aerodynamic mass and energy fluxes over
forest canopies. These canopies are rough surfaces wherein flow-gradient
assumptions may not be valid, leading to exponential wind profiles in trunk
spaces and understory, superimposed by logarithmic profiles above the top of
canopies. This leads to airflow regimes wherein canopy stomatal resistance is
dependent on tree physiological factors and of short and long-term phenomena,
in interaction with aerodynamic variables. Forest canopies are also very prone to
intermittent events, such as gusts or ejections, and wake formation in trees
downstream, adding components to the usual kinetic energy equations and
influencing turbulence spectral dynamics and particle transport. The analysis of
drag processes in canopies, in wind tunnel and field, allows also to characterize
phenomena such as tree bending and pulling. The evapotranspiration regime of
these canopies is also coupled with the moisture of soil and atmosphere, in
comparison with lower canopies, with transient flow mechanisms particularly
relevant in trunk and understory spaces. Finally, forest canopies are relevant in
carbon balance dynamics, especially related to the net ecosystem exchange, on a
micro or global scale, and these dynamics reflect the influence of physical and
biological factors in carbon sinking.
4.1 Introduction
The magnitude of atmospheric mass and energy fluxes over forest canopies, for
example, in relation to water loss, soil erosion, or the microclimate, depends on
processes at the level of the biosphere, within the control volume between the
ground surface and the top of the surface boundary layer. The latter is divided, as
referred to in Chap. 2, into the inertial and roughness sublayers.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. Rodrigues et al., Fundamental Principles of Environmental Physics,
https://doi.org/10.1007/978-3-030-69025-0_4
105