Design and development of agroforestry systems for Illinois,USA: silvicultural and economic considerations

Recommended managerial inputs and associated outputs expected from practicing agroforestry on marginal farm lands in the central USA are discussed. Modeled management combinations are based on five timber species, three sites indices, three timber growth rates, five agricultural crops, all common crop rotations, and three tillage systems. Black walnut (Juglans nigra L.), combined with row crop production, is used to illustrate a specific multicropping alternative. Based on net present value, modeled agroforestry systems incorporating black walnut performed better on the better sites and at the medium to high timber growth rates whereas management systems using red oak (Quercus rubra L.) with row crops performed better on the poorer sites and at the lower growth rates. For agroforestry to be competitive with traditional agriculture, medium to high timber growth rates were necessary. Also, lower interest rates and the existing U.S. income tax structure favored agroforestry versus traditional agriculture. Substantial increases in net income may be possible through incorporating other income producing activities such as nut production within the agroforestry system.     

国外混农林业系统中林木与农作物的相互关系研究进展

总结了近年国外混农林业系统中林木和农作物在地上和地下部分的相互作用关系的研究进展。其中林木根系分布与管理研究内容是通过对林木根系类型的筛选,选择与农作物根系矛盾较小的树种;水分关系研究中探讨了林木与农作物的水分矛盾;养分关系中主要说明林木枝叶对农作物的有利作用和它们对养分的竞争作用;他感作用研究指出某些树种的枝叶虽然有一定的毒性,但对农作物生长影响不大;地上部分的研究说明,林木对作物的遮荫作用是主要的,而作物对林木的影响是次要的。     

An equation for the replacement value of agroforestry

This article examines some of the existing analytical tools which quantify both the ecological and economic aspects of intercropping decisions. The characteristics of tree crops are evaluated to determine how a specific tool, the replacement value of intercropping (RVI), could be modified to better interpret agroforestry improvements to bush fallow farming systems. The modified equation captures some of the potential production improvements associated with agroforestry by accounting for the fraction of time that a field is actually in production over the long run. The result is an improved estimate of the average annual difference between a tree/crop polyculture and a monoculture system which employs fallows.     

Plant-soil interactions in multistrata agroforestry in the humid tropicsa

Multistrata agroforestry systems with tree crops comprise a variety of land use systems ranging from plantations of coffee, cacao or tea with shade trees to highly diversified homegardens and multi-storey tree gardens. Research on plant-soil interactions has concentrated on the former. Tree crop-based land use systems are more efficient in maintaining soil fertility than annual cropping systems. Certain tree crop plantations have remained productive for many decades, whereas homegardens have existed in the same place for centuries. However, cases of fertility decline under tree crops, including multistrata agroforestry systems, have also been reported, and research on the causal factors (both socioeconomic and biophysical) is needed. Plantation establishment is a critical phase, during which the tree crops require inputs but do not provide economic outputs. In larger plantations, tree crops are often established together with a leguminous cover crop, whereas in smallholder agriculture, the initial association with food crops and short-lived cash crops can have both socioeconomic and biological advantages. Fertilizers applied to, and financed by, such crops can help to `recapitalize' soil fertility and improve the development conditions of the young tree crops. Favorable effects on soil fertility and crop nutrition have been observed in associations of tree crops with N<sub>2</sub>-fixing legume trees, especially under N-deficient conditions. Depending on site conditions, the substitution of legume `service' trees with fast-growing timber trees may lead to problems of competition for nutrients and water, which may be alleviated through appropriate planting designs. The reduction of nutrient leaching and the recycling of subsoil nutrients are ways to increase the availability of nutrients in multistrata systems, and at the same time, reduce negative environmental impacts. These processes are optimized through fuller occupation of the soil volume by roots, allowing a limited amount of competition between associated species. The analysis of temporal and spatial patterns of water and nutrient availability within a system helps to optimize the use of soil resources, e.g., by showing where more plants can be added or fertilizer rates reduced. Important research topics in multistrata agroforestry include plantation establishment, plant arrangement and management for maximum complementarity of resource use in space and time, and the optimization of soil biological processes, such as soil organic matter build-up and the stabilization and improvement of soil structure by roots, fauna and microflora. This revised version was published online in June 2006 with corrections to the Cover Date.     

农林业系统工程与农桐间作的结构模式

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Influence of latitude on the light availability for intercrops in an agroforestry alley-cropping system

Light competition by trees is often regarded as a major limiting factor for crops in alley-cropping agroforestry. Northern latitude farmers are usually reluctant to adopt agroforestry as they fear that light competition will be fiercer in their conditions. We questioned the light availability for crops in alley-cropping at different latitudes from the tropic circle to the polar circle with a process-based 3D model of alley-cropping agroforestry. Two tree densities and two tree line orientations were considered. The effect of the latitude was evaluated with same-sized trees. The relative irradiance of the crops was computed for the whole year or at specific times of the year when crops need more light. The heterogeneity of crop irradiance across the alley was also computed. Surprisingly, crop relative irradiance of summer crops at high latitudes is high, at odds with farmers’ fears. Best designs were highlighted for improving the crop irradiance: North–South tree lines are recommended at high latitudes and East–West tree lines at low latitudes. At medium latitudes, North–South tree lines should be preferred to achieve an homogeneous irradiance of the crop in the alley. If we assume that trees at northern latitudes grow slower when compared to southern latitudes, then alley-cropping agroforestry is highly advisable even at high latitudes with summer crops.     

Soil water content and infiltration in agroforestry buffer strips

Agroforestry practices are receiving increased attention in temperate zones due to their environmental and economic benefits. To test the hypothesis that agroforestry buffers reduce runoff by increased infiltration, water use, and water storage; profile water content and soil water infiltration were measured for a Putnam soil (fine, smectitic, mesic Vertic Albaqualf). The watershed was under no-till management with a corn (Zea mays L.)-soybean (Glycine max L.) rotation since 1991. Agroforestry buffer strips, 4.5 m wide and 36.5 m apart, were planted with redtop (Agrostis gigantea Roth), brome (Bromus spp.), and birdsfoot trefoil (Lotus corniculatus L.). Pin oak (Quercus palustris Muenchh.), swamp white oak (Q. bicolor Willd.) and bur oak (Q. macrocarpa Michx.) trees were planted at 3-m intervals in the center of the agroforestry buffers in 1997. Ponded water infiltration was measured in agroforestry and grass buffers and row crop areas. Water content in agroforestry and row crop areas at 5, 10, 20, and 40 cm depths were measured throughout the year. Quasi-steady infiltration rates were not different (P > 0.05) among the treatments. Agroforestry had lower soil water content than row crop areas (P < 0.05) during the growing season. Higher water content after the principal recharge event in the agroforestry treatment was attributed to better infiltration through the root system. Results show that agroforestry buffer strips reduce soil water content during critical times such as fallow periods, and increase water infiltration and water storage. Therefore, adoption of agroforestry buffer practices may reduce runoff and soil loss from watersheds in row crop management.     

Insect pest problems in tropical agroforestry systems: Contributory factors and strategies for management

Agroforestry trees are attacked by a wide spectrum of insects at all stages of their growth just like other annual and perennial crops. Pest management in agroforestry has not received much attention so far, but recent emphasis on producing high value tree products in agroforestry and using improved germplasm in traditional systems, and emergence of serious pest problems in some promising agroforestry systems have increased awareness on risks posed by pests. Insects may attack one or more species within a system and across systems in the landscape, so pest management strategies should depend on the nature of the insect and magnitude of its damage. Although greater plant diversity in agroforestry is expected to increase beneficial arthropods, diversity by itself may not reduce pests. Introduction of tree germplasm from a narrow genetic base and intensive use of trees may lead to pest outbreaks. In simultaneous agroforestry systems, a number of factors governing tree—crop—environment interactions, such as diversity of plant species, host range of the pests, microclimate, spatial arrangement and tree management modify pest infestations by affecting populations of both herbivores and natural enemies. Trees also affect pest infestations by acting as barriers to movement of insects, masking the odours emitted by other components of the system and sheltering herbivores and natural enemies. In sequential agroforestry systems, it is mostly the soil-borne and diapausing insects that cause and perpetuate damage to the common hosts in tree—crop rotations over seasons or years. An integrated approach combining host-plant resistance to pests, exploiting alternative tree species, measures that prevent pest build up but favour natural enemies and biological control is suggested for managing pests in agroforestry. Species substitution to avoid pests is feasible only if trees are grown for ecological services such as soil conservation and low value products such as fuelwood, but not for trees yielding specific and high value products. For exploiting biological control as a potent, low cost and environmentally safe tool for pest management in agroforestry, research should focus on understanding the influence of ecological and management factors on the dynamics of insect pest-natural enemy populations. Scientists and policy makers in national and international institutions, and donors are urged to pay more attention to pest problems in agroforestry to harness the potential benefits of agroforestry.This revised version was published online in November 2005 with corrections to the Cover Date.     

Tree root characteristics as criteria for species selection and systems design in agroforestry

This literature review presents information about the role of tree root systems for the functioning of agroforestry associations and rotations and attempts to identify root-related criteria for the selection of agroforestry tree species and the design of agroforestry systems. Tree roots are expected to enrich soil with organic matter, feed soil biomass, reduce nutrient leaching, recycle nutrients from the subsoil below the crop rooting zone and improve soil physical properties, among other functions. On the other hand, they can depress crop yields in tree-crop associations through root competition. After a brief review of favourable tree root effects in agroforestry, four strategies are discussed as potential solutions to the dilemma of the simultaneous occurrence of desirable and undesirable tree root functions: 1) the selection of tree species with low root competitiveness, eventually supplemented by shoot pruning; 2) the identification of trees with a root distribution complementary to that of the crops; 3) the reduction of tree root length density by trenching or tillage; and 4) the use of agroforestry rotations instead of tree-crop associations. The potential and limitations of these strategies are discussed, and deficits in current understanding of tree root ecology in agroforestry are identified. In addition to the selection of tree species and provenances according to root-related criteria, the development of management techniques that allow the manipulation of tree root systems to maximize benefit and minimize competition are proposed as important tasks for future agroforestry research.     

Adequate design of control treatments in long term agroforeestry experiments with multiple objectives

Agroforestry experiments usually include control plots of either pure crop or pure tree stands. A clear distinction should be made between intensively managed biophysical controls and farming system controls with realistic labour input and management regimes. Trying to draw biophysical conclusions from farming system controls (or the reverse) is often not justifiable. The design and management of these elusive control plots is a complicated issue which is often overlooked. Many factors beyond the control of the experiment manager can disturb long term field agroforestry experiments. Some examples from French agroforestry experiments illustrate how uncontrolled factors may bias the results, including the proportion of harvested to planted trees, the weeding regimes, and the use of tree- shelters. The analysis of agroforestry data could be more efficient when considering a continuum of tree – crop mixture management options between the agroforestry plot and the non agroforestry plot. The concept of biophysical control plots becomes then less essential. A relevant modelling approach of interactions between trees and crops should 1) perform correctly for any tree/crop proportion and even for pure stands, when setting the parameters of the other component to zero, 2) provide for the inclusion of new, uncontrolled factors that could emerge through time. The biological efficiency of agroforestry systems may however be a subordinate criterion for agroforestry adoption, as observed at the moment in France. Agroforestry systems with poor biological outcomes can even be very attractive in some ecological or sociological conditions, and only farming system controls may bring this aspect to light. This revised version was published online in June 2006 with corrections to the Cover Date.     

Biophysical interactions in tropical agroforestry systems

The rate and extent to which biophysical resources are captured and utilized by the components of an agroforestry system are determined by the nature and intensity of interactions between the components. The net effect of these interactions is often determined by the influence of the tree component on the other component(s) and/or on the overall system, and is expressed in terms of such quantifiable responses as soil fertility changes, microclimate modification, resource (water, nutrients, and light) availability and utilization, pest and disease incidence, and allelopathy. The paper reviews such manifestations of biophysical interactions in major simultaneous (e.g., hedgerow intercropping and trees on croplands) and sequential (e.g., planted tree fallows) agroforestry systems. In hedgerow intercropping (HI), the hedge/crop interactions are dominated by soil fertility improvement and competition for growth resources. Higher crop yields in HI than in sole cropping are noted mostly in inherently fertile soils in humid and subhumid tropics, and are caused by large fertility improvement relative to the effects of competition. But, yield increases are rare in semiarid tropics and infertile acid soils because fertility improvement does not offset the large competitive effect of hedgerows with crops for water and/or nutrients. Whereas improved soil fertility and microclimate positively influence crop yields underneath the canopies of scattered trees in semiarid climates, intense shading caused by large, evergreen trees negatively affects the yields. Trees in boundary plantings compete with crops for above- and belowground resources, with belowground competition of trees often extending beyond their crown areas. The major biophysical interactions in improved planted fallows are improvement of soil nitrogen status and reduction of weeds in the fallow phase, and increased crop yields in the subsequent cropping phase. In such systems, the negative effects of competition and micro-climate modification are avoided in the absence of direct tree–crop interactions. Future research on biophysical interactions should concentrate on (1) exploiting the diversity that exists within and between species of trees, (2) determining interactions between systems at different spatial (farm and landscape) and temporal scales, (3) improving understanding of belowground interactions, (4) assessing the environmental implications of agroforestry, particularly in the humid tropics, and (5) devising management schedules for agroforestry components in order to maximize benefits. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil organic carbon under a linear simultaneous agroforestry system in Uganda

The objective of this study was to quantify and compare the amount and distribution of soil organic carbon (SOC) under a linear simultaneous agroforestry system with different tree species treatments. Field work was conducted at Kifu National Forestry Resources Research Institute in Mukono District, Central Uganda, in a linear agroforestry system established in 1995 with four different tree species and a crop only control treatment. Soil samples were collected in 2006 at three depths; 0–25, 25–50, and 50–100 cm, before planting and after harvesting a maize crop. The results indicate that an agroforestry system has significant potential to increase SOC as compared to the crop only control. There was no significant difference in the amount of SOC under exotic and indigenous tree species. Among the exotic species, Grevillea robusta had higher SOC than Casuarina equisetifolia across the entire depth sampled. There is significant difference in SOC among the indigenous species, where Maesopsis eminii has more SOC than Markhamia lutea. Distance from the tree row did not significantly influence SOC concentration under any of the tree species. In selecting a tree species to integrate with crops that will sequester reasonable quantities of carbon as well as boost the performance of the crops, a farmer can either plant an exotic species or an indigenous. In this study, the soil under Grevillea robusta and Maesopsis eminii have the highest potential to store organic carbon compared to soil under other tree species.     

Root architecture in relation to tree-soil-crop interactions and shoot pruning in agroforestry

Desirable root architecture for trees differs between sequential and simultaneous agroforestry systems. In sequential systems extensive tree root development may enhance nutrient capture and transfer to subsequent crops via organic pools. In simultaneous systems tree root development in the crop root zone leads to competition for resources.Fractal branching models provide relationships between proximal root diameter, close to the tree stem, and total root length or surface area. The main assumption is that a root branching proportionality factor is independent of root diameter. This was tested in a survey of 18 multipurpose trees growing on an acid soil in Lampung (Indonesia). The assumption appeared valid for all trees tested, for stems as well as roots. The proportionality factor showed a larger variability in roots than in stems and the effects of this variabilily should be further investigated. A simple index of tree root shallowness is proposed as indicator of tree root competitiveness, based on superficial roots and stem diameter.Pruning trees is a major way to benefit from tree products and at the same time reduce above-ground competition between trees and crops. It may have negative effects, however, on root distribution and enhance below-ground competition. In an experiment with five tree species, a lower height of stem pruning led to a larger number of superficial roots of smaller diameter, but had no effect on shoot:root ratios or the relative importance of the tap root.     

混农林业对石漠化地区土壤环境的影响及其应用

土壤环境变化是恢复生态学研究的重要内容之一,加强混农林的立体优化配置,有利于控制水土流失、改善土壤环境质量和提高生态脆弱区的生态和经济效益。文中系统总结混农林业对石漠化地区土壤水分、养分、生物特征的影响,阐明在石漠化极端脆弱生态环境条件下,林木与农作物存在的水肥竞争以及复合农田中树龄较小时对土壤生物活性的抑制作用,进一步从石漠化地区土壤环境角度对混农林业的水分利用策略、乡土树种的选择搭配、有益土壤生物的应用等方面提出建议,以期为喀斯特石漠化地区提高土地生产力、控制水土流失、发展立体生态农业等提供参考。     

Performance of maize, beans and ginger as intercrops in Paulownia plantations in China

Yield, morphology and specific leaf weight of summer crops maize, beans and ginger were studied in relation to tree proximity and PAR transmissivity in a 7-year old plantation of Paulownia elongata grown at a spacing of 15 m x 5 m in Eastern China. The yield of beans and maize was significantly reduced, compared to controls, at all positions relative to the trees. This yield depression was particularly marked at positions closest to the trees. Beans grown at 2 m from the trees had reduced height and specific leaf weight. These crops would yield more at wider tree row spacings. Ginger gave high yields when intercropped and is an ideal shade crop for these systems. Further research is required on optimisation using a bioeconomic approach. Further agronomic, physiological and biochemical work is required on ginger and other economic members of its family as these appear to have compatibility with dense agroforestry plantings. This revised version was published online in August 2006 with corrections to the Cover Date.     

混农林业:一条发展林业的有效途径

本文将生态系统工程原理作为现代混农林业的理论基础,提出了规划设计现代混农林业生产体系的基本步骤和方法。作者分析了现代林业和传统林业的本质区别,认为把农、林和土地利用有机结合起来,因地制宜,建立生产力高、综合效益大、稳定持续的混农林业生产体系,是发展现代林业的一条有效途径。     

Crop commercialization and adoption of gum-arabic agroforestry and their effect on farming system in western Sudan

Increasing trend in the sole crops commercialization has questioned the sustainability of the traditional gum Arabic agroforestry (Bush-fallow) system in western Sudan. This shift in the farming system from a traditional sustainable system to more commercialized one has resulted in a drastic decline in soil fertility, decreased crop productivity and low gum Arabic production. Therefore, identification of alternative options and incentives are required to ameliorate the negative environmental effects of increasing commercialization trends. This paper takes this issue and empirically investigating the factors influencing the decision to adopt agroforestry using a binary probit model. The results show that farmers with less commercialization, access to credit, less fragmented land, more education, high gum Arabic gate price, located away from the markets, and with more years of experience in farming are more likely to practice the traditional gum Arabic agroforestry system. In contrast, the allocation of more working days for commercial sole crops production, more fragmented land, and higher commercialization index reduces the probability of gum Arabic agroforestry adoption. In conclusion, this dichotomy between sole crop commercialization and traditional gum agroforestry system may be solved through the encouragement of the adoption of the traditional intercropping system with gum tree (Acacia senegal). Production of gum tree (Acacia senegal) itself should be more commercialized and prioritized. Conservation programs should focus on more educated, experienced famers with less fragmentized land if the policy is to promote the sustainable farming system in the region to promote soil fertility and to improve the effect of sole crop commercialization on traditional gum Arabic agroforestry system.     

A review of belowground interactions in agroforestry, focussing on mechanisms and management options

This review summarises current knowledge on root interactions in agroforestry systems, discussing cases from temperate and tropical ecosystems and drawing on experiences from natural plant communities where data from agroforestry systems are lacking. There is an inherent conflict in agroforestry between expected favourable effects of tree root systems, e.g. on soil fertility and nutrient cycling, and competition between tree and crop roots. Root management attempts to optimise root functions and to stimulate facilitative and complementary interactions. It makes use of the plasticity of root systems to respond to environmental factors, including other root systems, with altered growth and physiology. Root management tools include species selection, spacing, nutrient distribution, and shoot pruning, among others. Root distribution determines potential zones of root interactions in the soil, but are also a result of such interactions. Plants tend to avoid excessive root competition both at the root system level and at the single-root level by spatial segregation. As a consequence, associated plant species develop vertically stratified root systems under certain conditions, leading to complementarity in the use of soil resources. Parameters of root competitiveness, such as root length density, mycorrhization and flexibility in response to water and nutrient patches in the soil, have to be considered for predicting the outcome of interspecific root interactions. The patterns of root activity around individual plants differ between species; knowing these may help to avoid excessive competition and unproductive nutrient losses in agroforestry systems through suitable spacing and fertiliser placement. The possibility of alleviating root competition by supplying limiting growth factors is critically assessed. A wide range of physical, chemical and biological interactions occurs not only in spatial agroforestry, but also in rotational systems. In a final part, the reviewed information is applied to different types of agroforestry systems: associations of trees with annual crops; associations of trees with grasses or perennial fodder and cover crops; associations of different tree and shrub species; and improved fallows. This revised version was published online in June 2006 with corrections to the Cover Date.     

Using sap flow gauges to quantify water uptake by tree roots from beneath the crop rooting zone in agroforestry systems

Grevillea (Grevillea robusta A. Cunn.; Proteaceae) is used in agroforestry in many areas of the highlands of East and Central Africa, and is reported to be mainly deep rooted, with few shallow roots and correspondingly low levels of competition with associated crops for water and nutrients. To examine the extent of below-ground complementarily in water use between grevillea and cowpea (Vigna unguiculata L.; Leguminosae), experiments were carried out at the International Centre for Research in Agroforestry (ICRAF) Field Centre at Machakos, Kenya. Sap flux was measured using heat balance gauges attached to the stems of young grevillea trees (10–18 months old), both before and after excavating the crop rooting zone (upper 60 cm of soil) around the stem base, in order to establish the capacity of the grevillea to extract water from below this zone. After excavation, the trees maintained sap fluxes of up to 85% of the unexcavated values, suggesting a high degree of below-ground complementarity.