BIOLOGICAL ADVANTAGES OF AGROFORESTRY

· Increased space utilization. Increased utilization of the above and below ground environment should result in an increase in total biomass production potential. The planting of crops that differ in light requirements, root development, and height allows for more efficient use of solar radiation, soil moisture, and nutrients. Trees are generally thought to able to extract nutrients from greater volumes of soil than do annual crops. However, this assumption is based on little quantitative information for tropical trees species. A number of studies suggest that most tropical tree species have the highest concentration of roots in the uppermost soil horizons.

· Improvement of soil chemical, physical, and biological characteristics. Much has been written about soil improvement in agroforestry and the benefits of enhanced nutrient cycling or pumping in agroforestry practices that use deep-rooted tree species. The concept of nutrient pumping in agroforestry is that tree roots extend into portions of the soil profile (B and C horizon) that may not be accessible to annual crop root systems and that tree crops extract nutrients from this portions of the profile. These nutrients are then translocated to above ground plant parts (i.e., leaves, branches, stem, etc.) and to a much larger root mass in the surface horizons (A and B horizons). Litter fall completes the nutrient translocation from lower soil horizons to the soil surface.

· Increased productivity. Aggregate production from agroforestry mixtures is often perceived to be greater than that from monocultures. This idea has been well established in the multiple cropping literature for many combinations of annual crops. Experiments comparing production from agroforestry mixtures with monocultures generally show improvements in overall productivity.

· Potential reduction in soil erosion. Soil erosion risks can be reduced by a reduction in rainfall erosivity by a multistoried canopy. Agroforestry practices can include three or more canopy layers, which can contribute to reduced soil erosion risk : (1) tree canopy, (2) ground cover of annual crops or pasture grasses, legumes, and shrubs, (3) a surface litter layer. The time of initiation, the duration, and the density of the canopy or ground cover are critical factors in the effectiveness of agroforestry practices in reducing erosion risks. Stems and surface roots also play a significant role in erosion reduction by reducing the velocity and therefore the erosivity of surface runoff. Agroforestry practices can enhance above-ground canopy and soil surface conditions that reduce erosion risks.

· Reduction in microclimate extremes. Temperature and moisture extremes are modified under tree canopies. The tree crowns shield the soil surface from solar radiation during daylight hours and serve to reduce heat losses at night, thereby narrowing the amplitude of daily temperature variation.

· Reduced risk of complete crop failure. Plant diversity can reduce the risk of total crop failure because the risk of losses from pest infestation or climatic stress is spread among many species. There is considerable evidence that the combinations of crops in intercropping systems provides an insurance factor in which one crop may compensate for the loss in a pest-damages crop. How common or important this is under field conditions is not known. Yet one of the advantages perceived in traditional shifting cultivation is the stability and reduced risk of a mixture of crops.

· Physical support for herbaceous climbers. The tree component can provide support for climbers such as black pepper, vanilla, yams, and the like. This can be of significant economic value where tree stems are substituted for expensive poles or supports that need to be replaced periodically.

· Positive use of shade. Some crops, such as coffee and cacao, benefit from the provision of shade. This is particularly true under soil conditions that are not very favorable, when rainfall is excessive, and when temperatures are too extreme. Multistoried systems with nitrogen-fixing tree species are used in many parts of the world to provide shade for plantation crops.

ORIGINS OF AGROFORESTRY

Trees have been used in cropping system since the beginning of agriculture. In an account of the origins of life, the Bible (Gen.2:8-9) describes gardens where all kinds of trees grew, providing both beauty and food. This is perhaps one of the first descriptions of multipurpose tree use in an agroforestry system. Brownrigg (1985) reviews literature that documents paintings, papyrus illustration, and textual descriptions of home gardens in the Near East dating back to 300 B.C., suggesting that these home gardens in originated as early as 7000 B.C. Ancient Indians scriptures (ca. 1000 B.C.) also mention the multipurpose tree species Prosopis cineraria Mac Pride as a fodder source. Agricultural writers og the Roman era described a wide variety of agroforestry systems that included livestock and use of tree crops for food and fodder.

In the much of tropics, human underwent a transition from hunting/gathering to the use of domesticated plants and livestock. As a part of this process they cut down trees, cleared the debris by burning, and sowes crops in the ash-enriched soil. Thus was born slash-and-burn agriculture, a primary forerunner of present-day agroforestry and practice that may have originated in the Neolithic period, around 7000 B.C. The combination of soil impoverishment arising from soil erosion and nutrient extraction through crop harvests and the invasion of aggressive and hardy weeds forces farmers to move to new sites to repeat the process in a system widely known as shifting cultivation.

Shifting cultivation and its complexity has been well described in many parts of the world. Where population densities are low, long forest fallows are still possible, and the system may remain sustainable. However, population pressures usually forces the shortening of fallows, resulting in continued decline in productivity and site degradation. This is one of the major reasons for the current widespread governmental abhorrence of shifting cultivation.

Shifting cultivation was a common practice in Europe untul at least the Middle Ages. In some cases tree species were sown or planted before, along with, or after the sowing of the agricultural crop. This system was widely used in Finland up to the end of the nineteenth century and was practiced in a few areas of Germany as late as 1920s.

Mixed gardens have also been a form of agroforestry practiced for centuries in Central America. In Asia, the home garden was mentioned in the Javanese charter of 860 A.D.

Near the end of the nineteenth century, a system of teak establishment was developed in Burma that was to be known as taungya, or literally, hill cultivation. This system spread to other parts of the British Empire and by the year 1896 had been introduced to what are now South Africa, India, and Bangladesh.

The late 1800s saw a recognition of the difficulties that face foresters and forest administrators. The difficulties in raising saplings under grazing conditions in India provided enough cause for concern that schemes for the use of living fences were developed to protect valuable tree and food crops.

SEVERAL BASIC IDEAS OF AGROFORESTRY

1 1. Agroforestry is a distinct land-use system, which may include combinations of agricultural, forestry, horticultural, and animal husbandry subsystems and practices. Simply stated, agroforestry is a means of managing or using land that combines trees or shrubs with agricultural or horticultural crops or livestock.

2. 2. Agroforestry integrates trees with crop and/or animals, with the main objectives of reducing risk and increasing total productivity. Farmers have historically used indigenous mixed cropping practices to minimize the risk of total crop failure by growing a variety of products on the same piece of land. Although increases in total productivity are presumed in much of the promotional literature on agroforestry, few side-by side comparisons of agroforestry and monoculture systems exist. Increased productivity is a major goal for rural development planners but may not be the most important benefit of agroforestry for most farmers. Social science research suggests that farmers are often at least as interested in diversification and reduction of risk as in increased yields.

3. In their ideal forms, agroforestry systems are both stable and sustainable. Agroforestry practices have greater diversity than do monoculture practices and can distribute production over a longer period of time. This more regular income can provide increased cash flow stability to farmers, particularly those who may have difficulty storing or marketing farm produce. Sustainability, or the ability to maintain productivity over time, is also common in definitions of agroforestry, but sustainabilityis perhaps more a statement of what is desirable rather than what is attainable. Sanchez (1987) suggests that the hypotheses that appropriate agroforestry systems improve soil physical properties, maintain soil organic matter, and promote nutrient cycling are strongly implied in agroforestry literature, but are probably not universally correct.

4. Integrations of trees into agricultural systems may result in more efficient use of sunlight, moisture, and plant nutrients than is generally possible by monocropping of either agricultural or forestry crops. One of the biological reasons for interest in agroforestry is that trees use portions of the biosphere that annual crops or animals generally do not, resulting in increased aggregate biomass production. Trees do compete with crops for sunlight, moisture, and nutrients. However, the assumption inherent in most definitions is that the net effect of trees in agroforestry is positive.