The concept of agroforestry is based on the premise that land-use systems that are structurally and functionally more complex than either crop or tree monocultures result in greater efficiency of resource (nutrients, light, and water) capture and utilization, and greater structural diversity that entails tighter nutrient cycles. While the above- and belowground diversity provides more system stability and resilience at the site level, the systems provide connectivity with forests and other landscape features at the landscape and watershed levels.
A common thread found in the many historical definitions of agroforestry is the reference to the systems nature of this multifaceted land-use system. However, the multitude of ways in which trees may be incorporated into agricultural production systems - intercropping as opposed to silvopastoralism, for example - is problematic in terms of conceptualizing all agroforestry systems in one standardized system model. Nonetheless, there is great merit in doing so because it allows comparison with natural forested or agroecosystems as to the relative extent that ecological properties are maintained or relinquished by agroforestry systems. For example, compared with the net primary productivity (NPP) of 2-6 Mg dry matter (biomass) ha-1 yr-1 (depending upon species) for temperate coniferous forest plantations, certain agrofor-estry systems in the tropics such as the multistrata (vertically stratified) homegardens (Figure 2) and shaded perennial systems (Figure 3) can exhibit in excess of 15Mgha- yr- . Indeed, the ecological indexes for species similarity, diversity, and richness (Sorenson's, Shannon-Wiener, and Margalef, respectively) of multi-species homegardens are similar to those of nearby primary forests. These similarities with natural ecosystems are strong indicators of ecological sustainability of agroforestry systems - assuming, of course, that natural ecosystems are ecologically sustainable.
The forced integration of trees into agricultural production systems adds considerable interspecific competition to whatever existing intraspecific competition is present for water, nutrients light, and CO2, thereby creating a more complex agroecosystem. Both positive (e.g., enhanced productivity, cycling of nutrients, soil fertility, and microclimate) and negative (e.g., allelo-pathic, pest, and disease vectors) may be created and ironically, single management activities undertaken in agroforestry systems may change both negative and positive interactions simultaneously. For example, the pruning of tree branches to add value to the tree component of the system, especially in temperate situations, will decrease shading of the crop component (a negative interaction) but will also decrease litterfall inputs (a positive interaction) to crop alleys, an important pathway for increasing soil carbon. The knowledge of interspecific interactions in agroforestry systems is considered currently inadequate in the temperate zone as compared to the tropics where considerably more is known. Competition for light in both regions by understory crops is a productivity-limiting factor, even in silvopas-toral systems where forage quality and quantity can suffer under extreme shade. In the tropics, nutrient and water availability is often as important, and competition for these resources has actually severely limited broader adoption of improved agroforestry technologies in these regions.
Four major ecological properties recently identified as critical to the understanding of agroforestry system design, development, management, and evaluation are discussed as follows.
1. Spatial and temporal heterogeneity. Considerable variation exists in agroforestry systems where system components can vary considerably in size, lifespan, and phenology.
2. Disturbance. In many agroforestry systems, some components occupying specific spatial territory are continually being returned to earlier stages of ecosystem succession (crops) while others are developing along much longer successional trajectories (trees).
3. Perennialism. The perennial nature of certain system components contributes to feedback systems which minimize nutrient losses and provide stability to soil chemical, physical, and biological properties.
4. Structural and functional diversity. Agroforestry systems increase vertical heterogeneity, providing niches for a multitude of organisms not normally associated with monocropped agricultural systems.
This will generally make energy flow more complex within agroforestry systems, although the overall understanding of this process remains low in both temperate and tropical systems.
An understanding of biogeochemical cycles in many tropical and temperate agroforestry systems is emerging globally; this, in concert with an understanding of NPP, carbon sequestration, and aspects of biodiversity, including genetic diversity, is key to embracing the aforementioned ecological properties, thus allowing for the evaluation of the numerous environmental benefits and services that will result from efficiently designed agroforestry systems.
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