Agriculture Reference
In-Depth Information
(a)
(b)
100
80
60
Wind
direction
40
(c)
20
-10
-5 0 5
Distance - multiple of windbreak height
10
15
20
25
30
Fig. 3.8.
Windbreaks reduce tree damage by reducing the wind velocity on both
sides of the barrier (a), with a signi
fi
cant reduction on the leeward side up to ten
times the height of the windbreak. Trees can be hedged (b), as in this kiwi orchard,
or an arti
fi
cial barrier can be made with plastic netting (c), as in this carambola
orchard.
that it starts protecting it from the beginning. Barriers have to be maintained,
and yearly pruning is usually performed to avoid too much compactness; also
subsoiling hook should be passed 3 m away from the barrier to avoid excessive
root competition with the crop. Barriers of trees that produce fruit instead of
timber or fi rewood can be used. The main disadvantages of natural barriers
are that they take time to be ef ective, use up land and compete with the crop
for light, water and nutrients. They also need maintenance and can harbour
crop enemies.
Few windbreak trees can provide crop protection under hurricane
(typhoon) wind velocities. One of the most substantial windbreak trees is
Garcinia spicata
, called 'Fukugi' in Okinawa, Japan. Windbreaks composed of
this species have been observed to withstand hurricane winds of approximately
210 km/h in northern Okinawa, providing crop protection without itself being
damaged. This species is related to mangosteen and grows well in subtropical
and tropical climates. Its only drawback is the very slow growth rate.
Artifi cial barriers are usually plastic fabric with 30-70% permeability to
wind (Fig. 3.8c). They should be resistant to ultraviolet rays as well as to sun
and rain, and resist deformation. Good barrier materials should last 10-15
years. The main advantage is that they can be installed and start functioning
immediately. Additional advantages include that the permeability can be
chosen and it will remain the same over the lifetime; they do not use much
space or compete with the crop and do not harbour plant enemies; they
can be moved to another location if necessary, and they do not need any
maintenance. Their main disadvantage is the high initial installation cost.
