Overstory #56 - Integrating Understory and Tree Crops
Crops planted under forest, orchard, or other trees are called "understory crops." Understory crops can be vines, shrubs, herbaceous plants, or shorter trees. When integrated with tree crops, they can provide earlier returns and diversify farm yields. Understory cropping systems can also make more efficient use of land, labor, and resources, while increasing the total productivity of an area.
However, combining understory crops with tree crops is complex, and requires careful planning. The needs of the species to be planted should be well understood, as well as the effect each species will have on the other species in the system. Planning helps to avoid problems, minimize risks, and maximize benefits in crop combinations.
Key practical issues in planning understory cropping systems are introduced below.
The Understory Environment
The understory is a unique environment, involving more than just shade. Shade influences air temperature, humidity, soil temperature, soil moisture content, wind movement, and more. These factors impact plants.
The shady environment in the understory can have the following effects:
- Reduces evapotranspiration (evaporation of water through the leaves and branches of the plant), conserving moisture in the plants and reducing water use
- Buffers crops from temperature extremes and fluctuations
- Protects crops from winds
- Suppresses many invasive problem weeds, which tend to prefer open conditions and full sun
- Supports a range of beneficial soil microlife that do not thrive in the open (Nair 1993).
Overstory (Canopy) Species Selection
The upper strata of a multi-layered planting is called the overstory, or canopy. The trees that make up this layer play the key role in creating the understory environment. When planning an understory intercropping system, the overstory is a crucial element. The most influential factors are canopy shape/tree form, canopy foliage type, and tree spacing.
The shape and form of the overstory trees is an important consideration in planning an intercropped system. Some trees have very wide, umbrella-shaped canopies. In contrast, some trees have a very narrow, columnar form. The form and canopy shape of the overstory trees should be used to help determine appropriate spacing for the trees and understory crops. In some cases, the form of the trees can be altered by pruning.
Some types of tree foliage create dappled sunlight or light shade (e.g. coconut, sesbania, shower tree); others create a thick canopy with dense, heavy shade beneath (e.g. eucalyptus). Although understory crops are selected to tolerate some degree of shade, some light must be available in order for the crops to be productive. The type of foliage should be considered along with canopy shape, to determine the spacing needed to create an optimal understory environment for the crop.
The spacing of the overstory trees is important to creating the understory environment. If the standard, close spacing of single-species monocultures of forestry or orchard trees is used, usually the understory crops are phased out after few years due to competition for light and space. Compared to single-species plantings for timber or fruit trees, understory cropping systems normally involves a reduced number of trees per acre. The number of trees per acre is usually 25-75% less compared with timber or fruit trees planted alone. This wider spacing may be in a uniform pattern, or in a more random pattern of dispersed trees.
Spacing should be planned to provide the most optimal environment for the understory, and minimize competition for space, light, and nutrients. The species growth rate, rooting patterns, and other factors should be taken into account when planning.
Environmental Transition of Understory
One of the important considerations in planning for an integrated understory cropping system is the rate at which the understory environment changes. As the overstory species mature, the understory environment becomes shadier, cooler, and more humid. The understory environmental transition will influence decisions in overstory and understory plant selections and plant spacing. The challenge is to predict when the optimum environmental conditions will occur and how long they will last.
Determining when the optimum environmental conditions will occur affects the understory planting schedule. For example, if understory crops thrive in shade but cannot tolerate full sun, then they should not be planted until the overstory trees have grown enough to provide sufficient shade.
How long favorable environmental conditions will last influences expectations of optimum output from understory crops, thus influencing crop selection, spacing, and scheduling. As overstory trees continue to mature, the understory conditions can be maintained by pruning or thinning the overstory trees. The net benefits of maintaining a set environmental conditions by pruning or thinning will help make decisions in scheduling (Arakaki, 2000).
Because the understory environment changes over time, some farmers may choose to change understory crop species over time. This practice is sometimes called sequential cropping, when short-term crops are eventually replaced by longer-term crops. Sequential cropping of understory crops optimizes the productivity of the understory as the environmental conditions change.
Understory Crop Selection
Whether the understory crop is integrated for continuous yields or in a sequential cropping system, species selection is an important consideration. When selecting species, growth and rooting habits should be understood so plants are compatible and not overly competitive for nutrients and space. Understory crops should be integrated in a way that maximizes available light, space, and nutrients, while minimizing competition.
In relation to the overstory trees, understory crops should (after Wadsworth 1997):
- tolerate partial shade
- exploit, at least partially, different soil horizons than the overstory trees
- be shorter than the overstory trees when mature (although some trees may be planned to use the shade as seedlings, but eventually overtake and become part of the upper layer)
- be less susceptible than the overstory trees to diseases they may have in common
- not involve damage to the overstory trees during cultivation or harvest of understory crops.
Understanding the rooting habits, potential allopathic effects, and growth rates of the overstory species is part of good planning, to ensure that the trees chosen will create a beneficial environment for the understory crops. Some tree species may be too fast-growing or have allopathic (suppressant) effects on crops, making them inappropriate for this kind of system. For example, ironwood (Casuarina species) have been reported to have allopathic effects crops including sorghum, sunflower, and cowpea (Nair 1993). Other trees may simply have aggressive root systems or growth rates that are incompatible with most understory crops. Likewise, understory crops chosen should not be overly competitive with the overstory tree crops.
A Note on Varieties for Understory Crops
Much of modern agricultural research has focused on growing crops in fully exposed, monocultural (single-species) systems. In fact, many new varieties of crops have been especially selected for high light intensity tolerance and high productivity. For example, coffee trees evolved in the shady understory of tropical forests. Traditional varieties of coffee are grown under the shade of other trees, usually nitrogen-fixing trees that also add nitrogen, control erosion, recycle nutrients, and provide habitat for wildlife. Coffee has been grown intercropped with other trees for centuries (Wadsworth 1997). Agroforesters exploring potential understory crops may benefit from researching traditional varieties of crops, which may be better adapted to the understory environment than recent crop selections.
As with any farm practice, growers should also understand the potential limitations of understory intercropping. These include:
- Shortage of scientific study and information about tree/understory crop interactions
- Risk of unforeseen competition or allelopathic effects
- Greater complexity in management of multiple species and multiple products
- Potential damage to overstory from harvest of the understory, or vice-versa
- Increased challenges of marketing diversified products.
With adequate research and good planning, many of these limitations can be overcome to effectively integrate understory crops with tree crops.
Arakaki, A. 2000. Personal communication. University of Hawaii, Cooperative Extension Service, Molokai.
Nair, PKR. 1993. An Introduction to Agroforestry. Kluwer Academic Publisher. This comprehensive textbook bridges the gap between theoretical and practical knowledge in agroforestry. Order through ECHO, 17391 Durrance Rd. N. Ft. Myers, FL 33917 USA. Tel: 941-543-3246; Fax: 941-543-5317.
Wadsworth, FH. 1997. Forest Production for Tropical America. US Department of Agriculture, Forest Service, Agriculture Handbook 710
Related Editions to The Overstory
- The Overstory #72--Microenvironments (Part 1)
- The Overstory #71--Nontimber Forest Products (Temperate)
- The Overstory #64--Homegardens
- The Overstory #53--Nontimber Forest Products--An Introduction
- The Overstory #11--Understory Crops
- The Overstory #10--Sequential Planting