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.     

A strategy for tree-perennial crop productivity: nursery phase nutrient additions in cocoa-shade agroforestry systems

Shade tolerant species response to nutrient additions and light regulation by canopy trees in perennial agroforestry systems has been well documented. However, accelerated early growth, particularly in cocoa-shade systems, may be offset by competition for limited resources on nutrient poor sites. To date, few agroforestry management strategies focus on nutrient manipulation of the shade tree component or strategies for precision nutrient application. Our research objective was to diagnose interactions between nutrient supplied shade trees intercropped with cocoa. We established greenhouse trials in Kwadaso, Ghana cultivating Terminalia superba seedlings with four fertility treatment levels: conventional rate (current practices) under linear additions, and half, full and double conventional rate under exponential additions (steady-state nutrition) to determine maximal growth and nutrient uptake. After 4 months of additions in the nursery, pre-fertilized T. superba seedlings were then out-planted into field trials with cocoa seedlings. After 4 months of intercropping, cocoa associated with half rate exponentially supplied T. superba had significantly larger leaf area, greater leaf number, and higher nutrition (N and P uptake) than cocoa associated with full rate conventionally supplied T. superba. This may be attributed to (1) more favorable light conditions under these taller shade seedlings and (2) the internal use of nutrients associated with exponentially supplied T. superba seedlings, which lowered stress on native soil resources. The latter is corroborated with our findings on soil fertility status. This strategy focused on reducing fertilizer inputs and developing precise plant nutrition technology for on-farm use. Our findings suggest that shade seedlings under steady state nutrition can mitigate early growth competition in the field.     

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.     

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.     

Traditional agroforestry practices in Zimbabwe

Traditional agroforestry systems in the communal areas of Zimbabwe are described. There are systems centered on main fields, on home gardens, on homesites and on grazing areas. In the main fields, the major tree-related management practice is the conservation of preferred indigenous fruit trees. Fruit trees are also the focus of forestry activities around the gardens and the homesite; but here it is the planting of exotic species. In a localized area of Zimbabwe Acacia albida is important in fields. There is almost no use of tree fallows in Zimbabwe. Trees in grazing areas have numerous roles, but at present there is little knowledge about traditional management practices in these areas. In the development of agroforestry systems in Zimbabwe it is suggested that those systems designed to improve fodder production will make a significant contribution to farm productivity because of the importance of cattle in the farming system and the present fodder shortage. Interventions involving the planting of fruit trees are likely to be very successful, as there is much interest in such planting. Another area that needs to be developed is that of tree plantings to improve soil fertility.     

Agroforestry in Europe: a review of the disappearance of traditional systems and development of modern agroforestry practices, with emphasis on experiences in Germany

Agroforestry is a new name for a rather old practice. From a historical point of view, various agroforestry systems existed in Europe, of which the wood pastures (Neolithicum), the Dehesas in Spain (~4,500 years old) and the Hauberg of the Siegerland (established in the Middle Age) are the most prominent. Other widespread systems in Europe were hedgerows, windbreaks and Streuobst (orchard intercropping). Due to mechanisation and intensification of agriculture, trees have been progressively removed from agricultural fields and traditional agroforestry systems slowly disappeared. Today, agroforestry systems are again increasing in interest as they offer the potential to solve important ecological and, especially, biodiversity problems, while at the same time enabling the production of food, wood products and fodder for cattle. Although agroforestry systems offer many advantages, many farmers are sceptical of these systems and are critical and risk-averse with regard to adopting new practices. However, in comparison to traditional systems, modern agroforestry systems can be adapted to current farming practices. By selecting suitable trees and appropriate tree management, high-quality timber can be produced without influencing agricultural crops excessively. In future, agroforestry systems will become increasingly important as they offer the prospect of producing woody perennials for bioenergy on the same land area as food and/or fodder plants, while enhancing overall biodiversity.     

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.     

Why tree-crop interactions in agroforestry appear at odds with tree-grass interactions in tropical savannahs

This paper describes recent research findings on resource sharing between trees and crops in the semiarid tropics and attempts to reconcile this information with current knowledge of the interactions between savannah trees and understorey vegetation by examining agroforestry systems from the perspective of succession. In general, productivity of natural vegetation under savannah trees increases as rainfall decreases, while the opposite occurs in agroforestry. One explanation is that in the savannah, the beneficial effects of microclimatic improvements (e.g. lower temperatures and evaporation losses) are greater in more xeric environments. Mature savannah trees have a high proportion of woody above-ground structure compared to foliage, so that the amount of water 'saved' (largely by reduction in soil evaporation) is greater than water 'lost' through transpiration by trees. By contrast, in agroforestry practices such as alley cropping where tree density is high, any beneficial effects of the trees on microclimate are negated by reductions in soil moisture due to increasing interception losses and tree transpiration. While investment in woody structure can improve the water economy beneath agroforestry trees, it inevitably reduces the growth rate of the trees and thus increases the time required for improved understorey productivity. Therefore, agroforesters prefer trees with more direct and immediate benefits to farmers. The greatest opportunity for simultaneous agroforestry practices is therefore to fill niches within the landscape where resources are currently under-utilised by crops. In this way, agroforestry can mimic the large scale patch dynamics and successional progression of a natural ecosystem. This revised version was published online in June 2006 with corrections to the Cover Date.     

Using vector analysis to understand temporal changes in understorey-tree competition in agroforestry systems

Techniques to determine the respective effects of resource limitation or over supply on tree productivity are important for developing effective forestry and agroforestry management strategies. There is a need for a simple integrative measure of the understorey effect on soil nutrient and moisture competition on tree productivity in agroforestry systems during the time period before trees dominate understorey vegetation. For the first time, the little used, but potentially valuable tool of vector analysis was evaluated over 5 years by investigating nutrient and water competition in a Pinus radiata silvopastoral experiment which had 5 pastures and a nil-understorey control. The study, from ages 2 to 6 years, was on fertile arable soils in a temperate, sub-humid climate. Establishing permanent crops on this arable site increased soil pH, C, N, organic-P and C:N ratio. Vector analysis, an analysis based on fascicle nutrient concentrations, dry weights and nutrient contents, predicted nutrient and moisture competition until the trees dominated the site after year 5. Foliar critical nutrient levels were helpful where one of the vectors was unable to distinguish between nutrient and moisture stresses. While moisture and N were found to be the main competitive factors, vector analysis also detected foliar nutrient accumulation, particularly for P and Mg. Lucerne and phalaris understoreys were the most competitive pastures, followed by cocksfoot and the two ryegrass treatments. Foliage vector analysis enabled the relative competitive effects of soil nutrients and moisture on tree productivity to be determined. Soil nutrient concentrations and soil moisture measurements and the effects of competition on tree growth were consistent with predictions from vector analysis.     

Soil andLeucaena leucocephala L. growth variability underFaidherbia albida Del. andZiziphus spina-christi L. Desf.

In order to obtain data which will facilitate the scientific development of multiple intercropping of trees in agroforestry systems, the effects ofFaidherbia albida Del. orZiziphus spina-christi L. Desf. on soil quality and on yield of interplantedLeucaena leucocephala L. were investigated. The distance to which bothFaidherbia albida andZiziphus spinachristi trees improved soil quality as well asLeucaena leucocephala andSorghum vulgare L. production around them was about two times the radius of the tree crown. Higher yields of plants underZiziphus spina-christi thanFaidherbia albida were explained by similarly higher concentrations of available phosphorus (P). Other soil parameters, including total nitrogen (N), were similar under both species; being higher under the tree canopies than away from them. The study identified optimum tree spacing for agroforestry to be twice the tree crown radius. It also demonstrated the advantages of multiple tree mixtures over pure stands in improving soil quality and productivity.     

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.     

Diversity, composition and density of trees and shrubs in agroforestry homegardens in Southern Ethiopia

Diversity of trees and shrubs in agricultural systems contributes to provision of wood and non-wood products, and protects the environment, thereby, enhancing socioeconomic and ecological sustainability of the systems. This study characterizes the diversity, density and composition of trees in the agroforestry homegardens of Sidama Zone, Southern Ethiopia, and analyses physical and socioeconomic factors influencing diversity and composition of trees in the systems. A total of 144 homegardens were surveyed from 12 sites. In total, 120 species of trees and shrubs were recorded of which, 74.2 % were native to the area. The mean number of tree species per farm was 21. Density of trees varied between sites with mean values ranging from 86 to 1,082, and the overall average was 475 trees ha^?1. Four different crop-based enset (Enset ventricosum (Welw.) Cheesman)-coffee homegarden types were recognized and they differed not only in the composition of major crops but also in the diversity, density and composition of trees. The composition, diversity and density of trees is influenced by physical and socioeconomic factors. The major physical factors were geographical distance between sites and differences in altitude of farms. The most important socioeconomic factors were farm size and access to roads. Tree species richness and density increased with farm size. Increased road access facilitated marketing opportunities to agricultural products including trees, and lead to a decline in the basic components of the system, enset, coffee and trees. In the road-access sites, the native trees have also been largely replaced with fast growing exotic species, mainly eucalypts. The decrease in diversity of trees and perennial components of the system, and its gradual replacement with new cash and annual food crops could jeopardize the integrity and complexity of the system, which has been responsible for its sustenance.     

An evaluation of the Acacia albida-based agroforestry practices in the Hararghe highlands of Eastern Ethiopia

Growing Acacia albida as a permanent tree crop, on farmlands with cereals, vegetables and coffee underneath or in between, is an indigenous agroforestry system in the Hararghe highlands of Eastern Ethiopia. However, there is practically no systematic record or data on the merits and benefits of this practice.The paper presents the results of an investigation into the effects of the presence of A. albida on farmlands on the yield of maize (Zea mays L.) and sorghum (Sorghum bicolor L. Moench). Twenty seven plot pairs each consisting of one plot underneath the A. albida foliage cover and the other in the open, away from the tree-on farmers' fields, in a 40 km radius around the Alemaya College of Agriculture, were sampled and the yield components analyzed. A statistically significant increase in crops yields by 56% on average was found for the crops under the tree canopies compared to those away from the trees. This increase was caused by the improvement in 1000 grain weight and number of grains of plants under the tree, indicating that the trees enhanced the fertility status of the soil and improved its physical conditions in terms of crop growth.Additional benefits from the A. albida trees include supply of fuelwood and fodder. Quantitative estimates of these outputs as well as their monetary values are presented in the paper. However, in order to realize these benefits to a discernible extent, higher stand densities of the tree than at present are required.Based on an enquiry about the farmers attitude towards A. albida, the prospects for an extension of this promising agroforestry technique are discussed against the background of the state and trends of development of agriculture in the area. It is surmised that despite some shortcomings like the relatively slow and highly variable growth of A. albida and a conflict with the spreading cultivation of Ch'at (Catha edulis Forsk.), the prospects of extension of this technique are good. It is recommended that its propagation should be incorporated into the programmes of the extension agencies of the various governmental agencies concerned with land use.     

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.     

Intercropping Acacia albida with maize (Zea mays) and green gram (Phaseolus aureus) at Mtwapa,Coast Province,Kenya

Long-term agroforestry demonstrations/trials using Acacia albida and other nitrogen fixing multipurpose trees/shrubs were initiated in mid-1982 to assess soil and crop productivity at a coastal lowland site characterized by low soil fertility, weed problems and consequent poor crop yields. Growth performance (height and diameter at breast height, dbh) of Acacia albida under eight densities rotationally intercropped with maize (Zea mays) and green gram (Phaseolus aureus), crop grain yields, soil fertility changes and weed control were assessed for a 5-year period (May 1982 to March 1987). A parallel-row systematic spacing field layout was used. Intercropped Acacia albida mean hight and dbh were 140 and 24% respectively higher than tree-only controls by the fifth year. Growth rate was low during the first year but increased in subsequent years to mean height and dbh of 9 m and 10 cm respectively by March 1987. While differences in dbh were significant, those between stand heights were not. Crop yields, especially under higher tree densities, declined considerably due to unexpected shade which also caused significant reductions in weed biomass. Soil fertility levels remained unchanged during the experimental period relative to the initial status, and differences between the intercropped Acacia albida plots and the tree — or crop — only control appeared not to be significant. We conclude that an understanding of the mechanism regulating leaf fall/retention phenomena of Acacia albida is crucial towards determining the intercropping potentials of the species.     

The effects of understory vegetation on P availability in Pinus radiata forest stands: A review

In many second-rotation Pinus radiata forest planta-tions, there has been a steady trend towards wider tree spacing and an increased rate of application of P fertiliser. Under these regimes, the potential for understory growth is expected to in-crease through increased light and greater nutrient resources. Therefore, understory vegetation could become a more signifi-cant component of P cycling in P. radiata forests than under closely-spaced stands. Studies have shown that growth rates and survival of trees is reduced in the presence of understory vegeta-tion due to the competition of understory vegetation with trees. Other studies have suggested that understory vegetation might have beneficial effects on nutrient cycling and conservation within forest stands. This review discusses the significance of understory vegetation in radiata pine forest stands, especially their role in enhancing or reducing P availability to forest trees.     

Influence of umbrella pine (<Emphasis Type="Italic">Pinus</Emphasis><Emphasis Type="Italic">pinea</Emphasis> L.) stand type and tree characteristics on cone production

Most umbrella pine (Pinus pinea L.) stands are managed as agroforestry systems, whose main production is fruit, due to the edible and highly nutritious kernels, and are frequently associated to natural or seeded pastures and grazing. The stands have low density, in order to enhance crown growth and fruit production. Nevertheless, cone production, both with regard to number and weight, varies greatly between stands, trees and years. In this study were selected three agroforestry systems, representative of umbrella pine stands whose main production is fruit, and one stand representative of the timber production system, where fruit is the secondary production. It was evaluated the variability in cone production as a function of the tree’s diameter at breast height and crown diameter and the individual tree’s competition status. The results indicate that stands managed in agroforestry systems with lower competition and individuals with larger diameter at breast height and crown diameter tend to produce more and heavier cones per tree. The first two principal components of the principal component analysis explain 84 % of the variance in cone production, trees’ dimensions and competition index. Tree competition status has a negative impact on production per tree.     

Nitrogen-Fixing Trees in Small-Scale Agriculture of Mountainous Southeast Guatemala

Abstract

Intensive, subsistence agriculture on hillsides of southeast Guatemala has caused extensive soil degradation. This retrospective study evaluated the success of an agroforestry treatment in improving the sustainability of mountainous agricultural systems in terms of soil nutrient status and erosion control. Three years after maize (Zea mays) and pasture fields were alley-cropped with N-fixing Gliricidia sepium trees, soil nutrient levels were examined and compared to nonalley-cropped controls. Agroforestry treatment showed significantly higher soil organic matter (SOM) over paired plots with no alley-cropping (mean 4.3% vs. 3.2% C, p < 0.05). Mean total N was also higher, with 0.12% in the nonagro-forestry control plots compared to 0.16% underthe agroforestry treatment (p <0.05). Some improvement in soil water-holding capacity(WHC) was also observed in the agroforestry systems. Differences in soil levels of available P, ranging from 8.0 to 64.4 μg/g, were not significant and correlated more with site than with agroforestry treatment versus control. Specific soil and site properties such as texture and slope as well as land use and ground cover management largely influenced improvements in soil nutrient status under the agroforestry treatment at each site. Despite increases in soil nutrients, maize plant productivity showed no response to the agroforestry treatment in terms of leaf chlorophyll index or maize plant height. Three years after the initiation of agroforestry, soil erosion rates showed no difference from paired controls; rates were correlated with ground cover and soil characteristics such as texture and surface rockiness rather than with agroforestry treatment. While soil nutrient status had not reached levels optimum for maize growth after three years, the positive trajectory of change in soil nutrients suggests the potential for using agroforestry systems in mountainous regions of Guatemala to increase the sustainability of agricultural production.     

Growth and yield of eight agroforestry tree species in line plantings in Western Kenya and their effect on maize yields and soil properties

Between October 1988 and August 1992, field experiments were carried out in West Kenya to evaluate the suitability of Leucaena leucocephala, L. collinsii, Gliricidia sepium, Calliandra calothyrsus, Sesbania sesban, S. grandiflora, Senna siamea and S. spectabilis to provide a range of agroforestry products and services. The initial objective was to establish the growth rates and wood and leaf yields of these tree species, when planted in single rows. After the initial evaluation, it was evident that valuable additional information could be collected if the trees were converted to hedges and their effect on intercropped maize and soils was studied. At 21 months after planting, different species and provenances ranged in height between 3.5 and 6 m and varied considerably in phenotypic appearance. Wood production (1988–1990) varied from 3 to 33.8 t ha⁻¹ and leaf production varied from 0.62 to 10.1 t ha⁻¹. During intercropping (1990–1992), leaf production varied from 0 to 10.9 t ha⁻¹. Maize yields were higher in association with Leucaena and Gliricidia than with Calliandra, Sesbania and Senna. Cumulative maize grain and stover yields over four seasons were positively correlated with the total amount of tree leaves applied (r² range, 0.70–0.95). The effect of tree leaf mulch on crop yields decreased over time for all species. Leaves with high nutrient contents, which decompose fast (Leucaena, Gliricidia, Sesbania) are likely to have been more effective in sustaining crop yields than leaves with lower nutrient contents (Senna) or more complex decomposition patterns (Calliandra). Simple “leaf input-crop output” budgets to calculate the reserves for N, P and K in different systems explained crop yield differences in some cases. Compared to the fertility status of “zero-mulch” control plots, the status of soil C, N, P, K, Ca, Mg and S was to varying degrees improved under Leucaena, Gliricidia and Sesbania, much less under Calliandra but not under Senna. First season grain yields were related to the soil fertility status at the end of the tree fallow. The results of these experiments suggest that under subhumid tropical conditions with soils of relatively poor nutrient status, where light and water are not likely to be the major limiting factors to crop production, the application of sufficient quantities of high quality tree mulch may positively influence maize yields. When agroforestry tree species with contrasting decomposition and nutrient release patterns are evaluated jointly, it is more difficult to demonstrate a general relationship between quantities of mulch applied and improvements in crop yields and soil fertility levels. Therefore, further chemical, physiological and phenotypic characterization of free species with potential for fallow and intercropping systems is required.