Prevention of wind and soil erosion
Tropical areas are subject to very heavy rainfall which can wash away the topsoil especially on sloping land. Tree roots help anchor the soil and trees can be planted on contours to prevent landslip. The tree canopy and leaves help lower the speed of the rainfall and wind and act as a windbreak. The branches and leaves also form a protective canopy for the crops below providing shade for young plants and protection from heavy rainfall. Tree litter created by fallen leaves, twigs, berries etc provide a mulch to protect against evaporation and extreme temperatures. Tree residues in the form of decomposing leaves, twigs and branches provide nutrients for surrounding crops.
Leguminous trees, like leguminous crops have the capacity to fix nitrogen in the soil. This can be used by the surrounding crops. The deep root system of trees help soil fertility by being able to make nutrients available for crops and animals that have been leached deep into the subsoil. Trees store water in their crowns which increases the availability of water to plants growing below.
Possible adverse affects of planting trees include:
This is caused by a high transpiration and excessive water uptake by the plant from the deeper soil layers. Salt gets deposited on the surface of the soil and can be visible as crusts. This mainly occurs in costal regions and arid or semi-arid regions.
This occurs when trees release acid substances. Toxic substances are released into the soil which can lead to crop failure. Some eucalyptus species have this effect
For livestock animals:
Trees provide essential shade for animals in the heat of the day
Animals can graze from the foliage of the trees and shrubs and so supplement their usual diet.
However, there are also some potentially negative interaction between trees and livestock:
- Some foliage, fruit and bark may contain toxic substances which are harmful to some animals causing skin diseases and blindness
- Trees can be host to pests which are harmful to animals. For example, the tetse fly which is very harmful to cattle and currently keeps cattle out of forested areas.
Leucaena is the most widely used tree in modern agroforestry, particularly for alley cropping and living fences.
Its attributes include:
- Easy to grow from seed
- Fast growing
- Fixes nitrogen in the soil
- Grows well almost everywhere (doesn’t like acid soil)
- Protein rich leaves suitable for cattle feed
- Trees provide firewood, poles and shelter for animals
This makes is ideal for agroforestry.
For other articles on agroforestry click here
Written by Fiona Johnson