of 50 kg, the drag coefficient is reduced by half when wind velocity increased from
1 to 27 ms
−1 . Monteith and Unsworth (2013) cited experimental results showing
that, for conifer trees exposed to a wind tunnel, the relation between drag force and
windspeed was affected by the increase of streamlining by individual leaves and
whole branches, reflecting a decrease in the effective cross section of the crowns. In
the same way, the effects of higher streamlining in small trees and in the top of large
trees were reflected in a lower increase rate of drag with air velocity, by comparison
with rigid objects.
The elements within the canopy are also sources of small-scale horizontal turbulence, due to wakes formed downwind, not directly exposed to flow. Independently of turbulence and of the shelter effects, the influence of wind speed on drag
is very complex, because of interactions of aerodynamic forces on leaf elements and
resistant elastic forces. With increasing air velocity, leaf elements are shaken with
the formation of elastic forces and an increase of momentum transfer. So, drag
arises due to a lack of equilibrium between aerodynamic and these elastic forces.
In field conditions, the drag caused by the complex airflow velocity field causes
turning moments and forces acting in the bottom of stems, pulling the plants. In
these plants, rotation moments are thereafter formed by the mass and soil attached
to root systems. The forces generated by wind fields are opposed by resistance
tensions in root systems and by soil shear resistance. In canopies, energy dissipation
mechanisms and interference among adjacent plants can damp mechanical oscillations. Plants will fail when the resulting bending moment , in the bottom zone of
contact of stems with soil surfaces, overwhelms the strong resistance of stems and
soil root systems (Monteith and Unsworth 2013). Individual plant failure to drag
forces develops as the stem bow or root system rotates and is pulled out from the
soil.
As trees develop, they gradually adjust to dominant airflows by building a root,
stem, and branch systems that can withstand wind forces along their lifetimes. The
likelihood of stem buckling is higher under conditions such as weakening of lower
parts of the plant system by diseases, decreasing soil shearing strength close to roots
by water accumulation; weakening of higher parts of plants by excessive water
interception or excessive nitrogen fertilization. Isolated trees, due, e.g., to harvesting or mortality, are also less resistant to external forces. Practices aiming to
reduce stem lengths, and thus bow tendencies, can be handicapped by greater force
transmission to roots, which can result in higher root lodging. Results with wheat,
reported in Ennos (1991), and cited by Monteith and Unsworth (2013), showed that
the pulling resistance of stems was 30% higher than that of root systems, insofar
that the latter would collapse firstly. In a tunnel laboratory, this author found that
the moment stresses needed to brake stems was around 0.2 Nm. The axial pulling
resistance of roots was found to change with distinct soil shear strengths.
In rice plants, Tani (1963) found that in laboratory wind tunnel plant stems
broke under moment forces of around 0.2 Nm, which were higher by a fourfold
order of magnitude than the equivalent rotational forces recorded on the field. This
difference in rotary moments was attributed to factors such as: (i) the much larger
forces exerted in the field under strong turbulent gust events when the instantaneous
4.2 Aerodynamic Characterization and Stability in the Rough Sublayer
109
1 to 27 ms
−1 . Monteith and Unsworth (2013) cited experimental results showing
that, for conifer trees exposed to a wind tunnel, the relation between drag force and
windspeed was affected by the increase of streamlining by individual leaves and
whole branches, reflecting a decrease in the effective cross section of the crowns. In
the same way, the effects of higher streamlining in small trees and in the top of large
trees were reflected in a lower increase rate of drag with air velocity, by comparison
with rigid objects.
The elements within the canopy are also sources of small-scale horizontal turbulence, due to wakes formed downwind, not directly exposed to flow. Independently of turbulence and of the shelter effects, the influence of wind speed on drag
is very complex, because of interactions of aerodynamic forces on leaf elements and
resistant elastic forces. With increasing air velocity, leaf elements are shaken with
the formation of elastic forces and an increase of momentum transfer. So, drag
arises due to a lack of equilibrium between aerodynamic and these elastic forces.
In field conditions, the drag caused by the complex airflow velocity field causes
turning moments and forces acting in the bottom of stems, pulling the plants. In
these plants, rotation moments are thereafter formed by the mass and soil attached
to root systems. The forces generated by wind fields are opposed by resistance
tensions in root systems and by soil shear resistance. In canopies, energy dissipation
mechanisms and interference among adjacent plants can damp mechanical oscillations. Plants will fail when the resulting bending moment , in the bottom zone of
contact of stems with soil surfaces, overwhelms the strong resistance of stems and
soil root systems (Monteith and Unsworth 2013). Individual plant failure to drag
forces develops as the stem bow or root system rotates and is pulled out from the
soil.
As trees develop, they gradually adjust to dominant airflows by building a root,
stem, and branch systems that can withstand wind forces along their lifetimes. The
likelihood of stem buckling is higher under conditions such as weakening of lower
parts of the plant system by diseases, decreasing soil shearing strength close to roots
by water accumulation; weakening of higher parts of plants by excessive water
interception or excessive nitrogen fertilization. Isolated trees, due, e.g., to harvesting or mortality, are also less resistant to external forces. Practices aiming to
reduce stem lengths, and thus bow tendencies, can be handicapped by greater force
transmission to roots, which can result in higher root lodging. Results with wheat,
reported in Ennos (1991), and cited by Monteith and Unsworth (2013), showed that
the pulling resistance of stems was 30% higher than that of root systems, insofar
that the latter would collapse firstly. In a tunnel laboratory, this author found that
the moment stresses needed to brake stems was around 0.2 Nm. The axial pulling
resistance of roots was found to change with distinct soil shear strengths.
In rice plants, Tani (1963) found that in laboratory wind tunnel plant stems
broke under moment forces of around 0.2 Nm, which were higher by a fourfold
order of magnitude than the equivalent rotational forces recorded on the field. This
difference in rotary moments was attributed to factors such as: (i) the much larger
forces exerted in the field under strong turbulent gust events when the instantaneous
4.2 Aerodynamic Characterization and Stability in the Rough Sublayer
109