Aboveground and underground relationship

Trees and agriculture crops compete with each others for light, space, moisture and nutrients. This competition can be grouped into two main classes, which are 1) aboveground and 2) underground competition.

1. Aboveground competition

The aboveground competition between trees and agriculture crops takes place for light and space. It is usually limited in areas of physical interaction. The aboveground physical possessions of trees that compete with crops are stem, clumps and foliage. The tree crown is mainly responsible for light competition.

The productivity of understory crops depends on its capacity to utilize solar energy. Low light intensity is one of the important constraints for higher yield in agroforestry system. Tanaka et al (1964) reported lower dry matter accumulation and decreased photosynthesis under shaded condition.

In low light intensity, etiolation occurs in understory plants, which is characterized by long, weak stems; smaller, sparser leaves. Janardhan and Murty (1980) reported that grain yield of tall and semi dwarf rice cultivars reduced under low light due to the low grain number per panicle and small grain size.

Tree canopy also disturbs rainfall pattern. For example, about 25-30% rainfall is intercepted by the tree crown, and 10-15% of the rainfall reaches the ground as stem flow. Thus, only about 50-60% rainfall comes to the ground directly, and therefore, soil directly under trees remains dry, which also creates moisture deficiency situation to understory crops.

2. Underground competition

Underground competition between trees and crops occurs mainly for moisture, nutrients and space, which is more important in agroforestry systems than above ground competition.

This competition depends on root system of both trees and agriculture crops. Generally, agriculture crops absorb nutrients from the top layer of a 30 cm soil depth, and to some extent up to 50 cm depth. A higher concentration of fine roots of trees in the soil layer up to 50 cm suggests that trees also obtain most of the nutrient requirements from the soil layer up to 50 cm. This suggests that there is huge competition between trees and agriculture crops for water and nutrient absorption in 50 cm soil depth. In addition, some trees like eucalyptus absorb and transpire more soil water.

 To avoid this kind of water and nutrient absorption completion, trees in agroforestry system should be deep rooted with few lateral roots, and agriculture crops should be with fibrous root systems concentrating roots in 30 cm soil depth. The desirable characteristics of trees and agriculture crops to avoid aboveground and underground competition are given below:

Desirable characteristics of trees

The main desirable characteristics of trees for reducing negative effects on agriculture crops are as follows:

• Tree species should have adequate shade regulation and upright stems.
• Trees should not interfere with agriculture crops with respect to soil moisture, nutrients, solar energy and agricultural operations.
• Trees should not attract birds, which damages the crops.
• Tree species should have ability to fix atmospheric nitrogen.
• Tree species should have deep rooting system with very few lateral roots.
• Tree species should not have any toxic effects on soil and on associated crop plants.
• Tree species should give multiple products.
• Tree species should have high yield potential and higher profitability.
• Tree species should be suitable to local climatic condition and widely acceptable by the local farmers.
• Trees which shed leaves during main cropping season such as Populus species shed leaves during wheat growing season.

Desirable characteristics of agriculture crops

The main desirable characteristics of agriculture crops for reducing negative effects on trees are as follows:

• Crops should be grown well under high density planting.
• Crops should be partially or completely shade tolerant.
• Crops should not compete with trees for water, nutrient, space and light.
• Crops should be of short duration with fast growing capacity.
• Crops should have nitrogen fixing capacity.
• Crops should have capacity to bear adverse condition like water stress or excessive water.
• Crops should be suitable for multiple cropping or mix cropping.
• Crops should return adequate organic matter into the soil through fallen leaves, roots and crop residues.
• Crops should be suitable for local climatic condition.
• Crops should be high yielding.
• Crops should be resistant to insects, pests and diseases.
• Cash crop for income generation.
• Crops should be of high demand and high market value.
• Products other than grain can be used as animal feed.