5. INTERACTION BETWEEN TREES AND CROPS

5.1 Introduction

Farmers have always grown trees on their land, often noting that this has beneficial effects for the soil and crop yields. This capacity of trees and other plants to restore soil fertility was utilized in African traditional agricultural systems based on shifting cultivation. It is also a well-known fact that the topsoil in forests is usually rich in nutrients and has a good structure. Scientists have concluded that the cycles of carbon and other nutrients under natural vegetation are relatively closed, i.e. there is little leakage out of the system. In spite of intensive research, however, much still remains to be discovered and verified with regard to the effects of trees on soil properties.

Soil status is one of the important factors that determine how a crop will perform on a certain site. It is, however, not the only one. Moisture content is another important factor, and trees growing with the crops will also have an impact on the moisture content of the soil. Above ground a tree will provide shade, which will have some effect on crop performance. Additionally trees may harbour organisms that are harmful to crops, e.g. a tree may contribute to a high nematode population in the soil or attract birds to nest in its crown.

These and other factors affect the nature of the tree's interaction with the crops surrounding it. The area where this interaction takes place is sometimes called the tree/crop interface. Agroforestry is only beneficial to the farmer if the net effect of all the factors involved in this interaction is positive. Trees planted with crops certainly do not always contribute to a higher output or to more sustainable land use. The benefits will only be obtained through a combination of the right tree species with the right crops in the right spatial arrangements and with the right management practices.

In this chapter we will discuss the most important factors in tree/crop interaction. It is important to understand these factors and their effects on the trees and crops before we can discuss and plan agroforestry interventions with farmers.