Agriculture Reference
In-Depth Information
favour wind pollination and fecundation. However, strong winds can produce
mechanical damage and, depending on velocity, can have severe, sometimes
even fatal, ef ects on crops, resulting in uprooting the whole tree, broken main
trunks, broken scaf old branches or dehydrated and burnt leaves to completely
defoliated trees. Hurricanes or cyclones during fl owering and fruiting periods
can cause a severe fl ower and fruit drop in many species. Bananas and
plantains are some of the most susceptible crops to wind damage. In Central
America, bananas and plantains are sometimes subjected to blow down
caused by gusts of wind that unexpectedly hit certain areas and break all tall
pseudostems, and a whole year's production is lost until the ratoon pseudostem
grows and harvesting can resume. Windbreaks are especially helpful when
normal prevailing winds become gusty with velocities exceeding 65 km/h.
Occasional high-velocity winds are damaging to orchard trees, but equally
harmful in the long run are the mild prevailing winds of 40-50 km/h. Trees
exposed constantly to such prevailing winds gradually develop a deformed,
lopsided shape, with all branches growing away from the winds.
Winds can also af ect sprinkler irrigation and spraying operations, causing
uneven water distribution of herbicides and other chemicals, which can drift
on to other plants. It is necessary to protect the plantation with a barrier that
should be ef ective ideally from planting. Wind barriers will reduce water loss
from the soil and leaves, produce a more uniform temperature in the orchard
and reduce wind erosion (Wilkinson and Elevitch, 2000). The windbreak can
reduce crop evaporation on the leeward side to 40% within twice the height
of the windbreak, being more ef ective at higher wind speeds (Fig. 3.8a).
The reduction in evaporation allows the trees in the orchard to develop the
higher humidity in the canopy that is important for crops such as rambutan,
mangosteen and durian. Sometimes a partial solution can be to plant the tree
rows following the same direction of the main winds, so that the fi rst trees in
the rows protect the rest of the row.
Two types of barriers are used: natural (Fig. 3.8b) and artifi cial (Fig.
3.8c). Both should fi lter, not stop, the wind and have about 50% permeability.
The windbreaks are perpendicular to the prevailing wind direction and
extend beyond the width of the orchard so that no wind enters from the
sides. Natural barriers consist of two or three rows of trees. The ef ectiveness
of the windbreak depends upon the height and lateral extent of the barrier,
its permeability and the angle of incidence of the wind to the barrier. Wind
velocity is reduced to 35% within a distance of four times the height of the
windbreak (Fig. 3.8a).
Many dif erent species are used as windbreaks, which complement each
other in size, growth form and canopy height. Sometimes a single species is
used. All trees used as a windbreak should compete as little as possible with
the crop; therefore deep-rooted and well-anchored, fl exible trees are preferred.
The trees should be adapted to the zone and should not harbour enemies of
the crop (Table 3.5). Ideally the barrier should be planted before the crop so
