A new debate is emerging across the world about the future agriculture at a time of food, fuel and financial crises. We need to foster an agriculture that is inclusive, multifunctional, and built on principles of resilience that are crucial in the process of adapting to climate change (LEISA 2008). We need farming systems that will increase food security, decrease environmental impact, respond to climate change and provide management alternatives that will enhance natural resource use, provide stable and high returns to the farmer. There is trade-off between agricultural benefits and environmental costs (see also Palmer 2008 and Box I).
The majority of agroforestry systems and practices are presently still found in the tropics and subtropics, although they are spreading into other regions. Existing larger areas of agroforestry are certainly not of a homogeneity comparable with non-tropical forests and the distance between trees is generally larger in agroforestry practices. Where that is not the case, the system is of a multi-storey agroforestry nature (Stigter 1988; 1994).
Trees hold nutrients in an efficient, closed cycling system. Their deeper roots tap nutrients from lower soil levels that are inaccessible to forage species, and nitrogen fixing trees can raise the nutrient levels of pasture soils. Well-managed forage production provides improved nutrition for livestock growth and production. There may be supplemental nutrient-rich fodder production in the tree leaves. Fertilizer applied for forage is also used by trees, livestock manure recycles nutrients to trees and forage, grazing controls weeds, grass competition for moisture, nutrients and sunlight. Shade and reduced wind speed raise the moisture level of soils by reducing evaporation. There is reduced soil compaction due to root development. Organic matter input is increased, erosion is reduced, thereby improving soil structure and soil biological activity. Increased wildlife diversity may be expected and forage or understorey litter protects the soil from water and wind erosion.
From the micrometeorological point of view, in agroforestry three manipulations or management areas should be distinguished. For the first one regarding radiation, we have the cover effects of shading from solar radiation in daytime and for long wave radiation loss at night. This means that we can manipulate in- or decrease of surface radiation absorption in daytime and surface loss of radiation at night.
The second manipulation or management area is the flow of heat and/or moisture and cultural measures like tillage and mulching. Influencing speed and direction of air flow are of effect here. Surface modifications (of soil surfaces but also of plant, tree and animal surfaces) influence these flow patterns and so does any modification near these surfaces.
The third manipulation and management area in relation to agroforestry is that of mechanical impact of wind, rain and hail (see for agroforestry also Baldy and Stigter 1997). Any measure hiding what has to be protected (from wind, rain or hail) should be considered. For that purpose the damaging impact can be (fully or partly) deflected or this impact can be taken (fully or partly) by bodies that are less vulnerable. The flow patterns of wind and water near/on the surface and the consequences of wind, rain/water and hail impact that come into being after the impact sharing and/or deflection must be well understood.
These microclimatic patterns and their consequences of radiation behaviour and flows, as carriers of heat, moisture, momentum and whatever is taken/lifted from the surfaces, are studied in microclimatology. For agroforestry its architecture makes these radiation and flow
patterns more complicated but the management and manipulation potential much richer.
Microclimate modification patterns are about the mimicking of natural systems to control the environment to achieve several benefits. They are all connected in one way or the other with multiple cropping to make them effective. These are microclimate modifications to:
Overhead shade has a major effect on the microclimate conditions under which pests and disease organisms, their natural enemies and the crop themselves develop and its optimization is a highly efficient control strategy for many pests and parasites.
The role of traditional knowledge and indigenous technology in sustainable natural resource management is substantial (Mohamed and Ventura 2000). These are location specific often age-old practices that are efficient in delivering desired goals, preserving the resource base without much degradation (e.g. Stigter et al. 2005). These technologies are effective in making the best use of scarce agro-climatic resources in a sustainable manner. The use of indigenous technology is common in agroforestry (Breman and Kessler 1995; Ong and Huxley 1996; Baldy and Stigter 1997) and other multiple cropping systems (Stigter and Baldy 1993; Baldy and Stigter 1997), agro-ecology (Altieri 1983), wind erosion control (Sivakumar et al. 1998), soil water balances (Sivakumar et al. 1991), integrated pest management (LEISA 1997), climatic risk in crop production (Muchow and Bellamy, 1991) and post harvest systems (FAO 1987). See also BOX II. Monocropping often needs protection from strong winds and their consequences with agroforestry interventions (Stigter et al. 2002).
In alley cropping, an intercropping system with trees in alleys, shade of the trees is important to suppress weeds and decrease soil moisture loss, such as in the “Inga alley cropping design”. This shade can be seen as an agrometeorological service to the farmers concerned. A system was designed in which the conditions found in virgin tropical forests were mimicked: minimize weed growth – first by tree shading then by leaf mulches – and recycle nutrients, including phosphorus, by using thick leaved nitrogen fixing trees providing sufficient biomass under the local sub-humid to humid climate conditions, without too limiting competition. After this system worked well with maize crops in Costa Rica, Honduran slash and burn farmers further developed the alley cropping of Inga edulis with maize and beans and with pepper as well as vanilla (Stigter 2007, 2008; 2011). Moreover, the trees provide an important amount of fuelwood (e.g., Walker et al. 2011).
Use of trees combined with livestock
Shade, wind protection, water erosion protection, food, fodder and timber/fuel wood are (individually and in combinations, with or without influences on pastures) the main uses of trees (scattered, in belts or live fences) in silvopastoral agroforestry (WCA 2004). Sooner or later many agroforestry systems will become silvopastoral (e.g. Exconde and Castillo 2004) and under arid rainfed conditions the latter systems appear almost the only viable agroforestry (e.g. Benzarti and Ben Youssef 2004). Often with protective properties, agroforestry serves food self-sufficiency and feed self-sufficiency better than any monocropping (Jiru 2004), even when yields of crops associated to trees are lower (WCA 2004). Alley cropping, forest farming, riparian buffers, silvopasture and windbreaks are also the subjects of today’s temperate agroforestry (AfTA 2007).
Tree pattern is an important factor for any successful silvopastoral system. Trees can be evenly distributed over the area, either in rows or clusters, and pruning or thinning must be managed. Shelter for livestock must be planned for severe weather events. Stress to livestock can be reduced through moderation of pasture microclimate that a well-planned silvopastoral system provides. There is indeed a climate stabilizing effect of trees that reduce wind and provide shade to reduce heat stress and wind chill on livestock (Klopfenstein et al. 2007).
Before a new silvopastoral system is established as preparedness, implications of merging forestry and agricultural systems should be explored thoroughly for economic and development considerations, along with local land use, zoning and cost-sharing programmes (see also BOX III).
A complex form of agroforestry, silvopastoral systems are also common. Some have various grasses and/or other fodder crops/trees, which when low and dense may be considered a kind of live mulches, exemplified by the three strata forage system (pasture, tree legumes, large trees) in Bali, Indonesia (Gutteridge and Shelton 1998-2004), and the date palm oases (Baldy and Stigter 1991; 1997; Stigter and Baldy 1989; 1991). Other silvopastoral systems have fruit and timber trees associated with food and fodder crops, contributing to soil and water conservation, and livestock (e.g. Reddy and Ramakrishna 2004), while homegardens and multipurpose trees play their own many roles, the latter within or out of associations (WCA 2004).
The combinations of trees, crops and/or animals should be intentionally designed as services and managed to achieve the desired benefits whilst maintaining the soil resource base. A well-planned system must have (Walker et al. 2011):
(i) considerations of spacing. The number and size of trees should be managed for continuous even spacing that optimizes light, growing space for tree crop and fodder production. Spacing must keep in mind how it effects tree growth and competition with one another;
(ii) plant selection considerations. When considering tree and forage crop selection, consider potential markets, soil type, climate conditions, and species compatibility;
(iii) environmental considerations. How the pattern of trees affects wildlife habitat, ease of livestock handling, forage and tree growth and competition, and microclimate.
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