Agroforestry for soil and water conservation in the western Himalayan Valley Region of India 2. Crop and tree production

A ten-year-study (1983 to 1992) conducted on nine 15 × 90 m runoff plots at 4% slope compared production efficiency of Leucaena leucocephala and Eucalyptus hybrid based agroforestry as well as monocropping landuse systems in the warm, subhumid climate of the western Himalayan region of India. Treatments for the first sequence were: monocropping systems of leucaena, eucalyptus, Chrysopogon fulvus grass and maize – wheat rotation, and alley cropping systems of grass and crops at 4.5 and 10.5 m alley widths with paired contour tree rows of leucaena and eucalyptus. In the second sequence, alley width increased to 22.5 m in 1989, grass was replaced by turmeric Curcuma longa and paired contour rows of leucaena hedges were introduced in monocropping systems of grain crops and turmeric. Integration of leucaena and eucalyptus trees with crops caused severe reduction of crop yields ranging from 21 to 92% for wheat grain, 59 to 69% for maize grain, 60 to 67% for dry grass and about 50% for turmeric rhizome depending upon the age of trees and alley width. The grain yield of crops stabilized at about 50% reduction with 22.5 m alley width. Total crop biomass (grain + straw) also revealed a similar trend; however, its magnitude of reduction was less severe than for grain. Production of biomass was much lower near the tree rows than in mid alleys. Managing leucaena as contour hedgerows eliminated crop yield reduction in alleys. Performance of grass and turmeric in alleys was not found to be satisfactory. Biomass produced from trees adequately compensated the crop yield reduction. Land equivalent ratios of agroforestry landuses were comparable or even better than monocropping systems indicating suitability of these systems for the western Himalayan valley region. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tree planting pattern effects on forage production in a Douglas-fir agroforest

Resource sharing among agroforestry system components, as expressed by spatial patterns along interfaces between components, is a crucial factor in both understanding present systems and in designing new agroforestry applications. A study of the spatial pattern of forage production surrounding 9–10 year old Douglas-fir trees in a agrosilvopastoral plantation near Corvallis, Oregon, was conducted during 1988 and 1989. Transects of plots were clipped both between trees (tree/tree) and between trees and open pastures (tree/pasture). Best-fit regression models relating forage production to distance from trees (tree/tree R ² = 0.87; and tree/pasture R ² = 0.89) were combined into a single prediction model. Observed forage production increased rapidly with increasing distance from trees during the initial 4 m. Trees had little effect on forage production beyond 4.5 m (approximately 2 canopy diameters) from the nearest tree. Predictions of different combinations of tree density and planting pattern indicated a strong interaction between density and pattern with highly aggregated plantations better able to maintain forage production at high tree densities.     

Soil water competition in a temperate hedgerow agroforestry system in South Africa

An on-farm trial was conducted to determine dry matter production of four fodder tree species and their effect on soil water and maize production. The trees were planted in rows intercropped with maize. The four tree species selected were Acacia karroo Hayne (indigenous fodder tree), Leucaena leucocephala (Lam.) De Wit (nitrogen fixing), Morus alba L. (fodder and fruit), and Gleditsia triacanthos L. (fodder and fuel). Volumetric soil water was measured in the upper 0.3 m of soil in each row of the trial using the time domain reflectometry technique. The neutron probe technique was used for monitoring the water content deeper in the soil. Geostatistical methods were used to analyse treatment differences in the upper 0.3 m of soil. The soil water content did not differ significantly between the maize and tree rows indicating that competition for water in the upper horizon was not the reason for lower maize yields. However, at greater soil depths (75–125 cm) trees in the wide spacing used less water than those in the narrow spacing. Light interception was an important factor in reducing maize yields in the row nearest to the trees. High soil water values recorded during summer indicated that in the current cycle of good rainfall the plants in the agroforestry trial were not stressed. Thus the trees do not compete with the crops for soil moisture in good rainfall seasons. However, this study would need further evaluation for the competition for water for the low rainfall years. Since the trees have access to water at greater depths, they are likely to be more productive into the dry season than shallow rooted crops.     

Arbuscular mycorrhizal symbioses in a cut-and-carry forage production system of legume tree Gliricidia sepium and fodder grass Dichanthium aristatum

Arbuscular mycorrhizal (AM) symbioses may alter the competitive abilities of plant species and facilitate positive interactions such as nutrient transfer between plants. They are therefore particularly interesting components in agroforestry systems. We studied spatial variation of AM colonisation on a cut-and-carry fodder production site (agroforestry plot) of the legume tree Gliricidia sepium and the fodder grass Dichanthium aristatum. Roots of the two plant species were sampled under the tree canopies and on the adjacent grass plot at 1 and 3.5 m from the first tree row where G. sepium roots also occur. Roots of D. aristatum were also sampled on a nearby grass monocrop. Colonisation of arbuscules, vesicles and hyphae in root samples was visually determined, and AM fungal species were identified by DNA sequencing. Colonisation and frequency of types of AM formations varied statistically significantly between the species and sampling points. Arbuscular colonisation in G. sepium roots was higher under the tree canopies than on the adjacent grass plot. Soil nutrient content, particularly P and N, and interspecies competition are the most probable explanations for the observed variation in AM colonisation. Both arbuscular colonisation and arbuscule:vesicle ratio in D. aristatum roots was lower on the D. aristatum monocrop than on the agroforestry plot under or near the tree canopies. Intercropping could stimulate AM symbiosis in D. aristatum. Both plant species formed symbiosis with Rhizophagus intraradices, indicating potential for interplant N transfer via common mycelial networks of AM-forming fungi.     

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.     

Improving adoptability of farm forestry in the Philippine uplands: a linear programming model

In the Philippines, smallholder farmers have become major timber producers. But the systems of timber production practiced have several limitations. In intercropping systems, the practice of severe branch and/or root pruning reduces tree-crop competition and increases annual crop yields, but is detrimental to tree growth and incompatible with commercial timber production. In even-aged woodlots, lack of regular income and poor tree growth, resulting from farmers’ reluctance to thin their plantations, are major constraints to adoption and profitable tree farming. In the municipality of Claveria, Misamis Oriental, the recent practice of planting trees on widely spaced (6–8 m) contour grass strips established for soil conservation suggests ways to improve the adoptability (i.e., profitability, feasibility and acceptability) of timber-based agroforestry systems. Assuming that financial benefits are the main objective of timber tree farmers, we develop a simple linear programming (LP) model for the optimal allocation of land to monocropping and tree intercropping that maximizes the net present value of an infinite number of rotations and satisfies farmers’ resource constraints and regular income requirements. The application of the LP model to an average farmer in Claveria showed that cumulative additions of widely spaced tree hedgerows provides higher returns to land, and reduce the risk of agroforestry adoption by spreading over the years labour and capital investment costs and the economic benefits accruing to farmers from trees. Therefore, incremental planting of widely spaced tree hedgerows can make farm forestry more adoptable and thus benefit a larger number of resource-constrained farmers in their evolution towards more diverse and productive agroforestry systems.     

Microclimate modification and response of wheat planted under trees in a fan design in northern India

The presence of trees in fields may help overcome the physiological stress that happens to late sown wheat (Triticum aestivum L.) in subtropical India. Wheat was planted in an agroforestry system with Eucalyptus tereticornis trees on 7 January 1998 in a fan design that provided different combinations of tree row spacing and orientations. Crop profile microclimatic conditions and the resulting growth responses of the intercropped wheat were studied to explore the potential of agroforestry systems to influence late sown wheat yields under different tree row spacing and orientations. Agroforestry treatments exhibited a potential to optimize the microclimatic conditions for seedling emergence, tillering and earhead emergence at some tree row orientations and distances from the crop. The net radiation distribution at three stages of crop growth indicated that the radiation availability was lower in all the agroforestry treatments than for the sole crop. The tree row orientation and distance influence the growth behavior of the crop but the effect of sun angle (which changes with season) can change their influence over time. The deterioration or amelioration of microclimatic conditions in agroforestry with the passage of time should be expected because of altered interaction patterns between sunrays and tree canopy resulting from changing solar elevation and angle of sunrays. Statistically similar harvest indices in all the treatments despite lower total biological yields in agroforestry treatments revealed that microclimatic conditions under agroforestry were more favorable for wheat growth attributed to reduction in heat load during the post anthesis period. This revised version was published online in June 2006 with corrections to the Cover Date.     

Agroforestry and grass buffer effects on water quality in grazed pastures

Conservation practices including agroforestry and grass buffers are believed to reduce nonpoint source pollution (NPSP) from pastured watersheds. Agroforestry, a land management practice that intersperses agricultural crops with trees, has recently received increased attention in the temperate zone due to its environmental and economic benefits. However, studies are limited that have examined buffer effects on the quality of water from grazed pastures. Six treatment areas, two with agroforestry buffers, two with grass buffers, and two control treatments were used to test the hypothesis that agroforestry and grass buffers can be used to effectively reduce NPSP from pastured watersheds. Vegetation in grass buffer and pasture areas includes red clover (Trifolium pretense L.) and lespedeza (Kummerowia stipulacea Maxim.) planted into fescue (Festuca arundinacea Schreb.). Eastern cottonwood trees (Populus deltoides Bortr. ex Marsh.) were planted into fescue in agroforestry buffers. Soils at the site are mostly Menfro silt loam (fine-silty, mixed, superactive, mesic Typic Hapludalfs). Treatments were instrumented with two-foot H flumes, water samplers, and flow measuring devices in 2001. Composite water samples were analyzed for sediment and total nitrogen after each runoff event to compare treatment differences. Treatments with agroforestry and grass buffers had significantly lower runoff volumes as compared to the control. The loss of sediment and total nitrogen were smaller for the buffered treatments. The results of this study suggest that establishment of agroforestry and grass buffers help reduce NPSP pollution from pastured watersheds. It is anticipated as trees grow and roots occupy more soil volume, the reduction in N in runoff will increase on the agroforestry watershed.     

Modelling growth behaviour in monoculture in subtropical eastern Australia of two eucalypt species that differ in shade tolerance

Growth behaviour of the relatively shade-intolerant species Eucalyptus dunnii and the rather more shade-tolerant Corymbia citriodora was considered during several growth periods over 8–14 years of age in experimental plantation monoculture in subtropical New South Wales. Larger trees dominated growth in any stand; this was consistent with the principle that inter-tree competition in such forests is for light and is asymmetric in that larger trees shade smaller trees but the reverse does not occur. Once competition was established, competition-induced mortality of C. citriodora was less than that of E. dunnii. A model relating tree diameter growth rate to tree diameter, competitive position in the stand and stand basal area was developed. The model showed that smaller, hence partially shaded, trees in stands of C. citriodora were better able to maintain their growth rates than equivalent trees in stands of E. dunnii. It is hypothesised that this reflects the greater shade tolerance of C. citriodora. This pattern of growth led to a more even distribution of tree size classes across the stand in the more shade-tolerant species.     

Dehydrogenase activity and VAM fungi in tree-rhizosphere of agroforestry systems in Indian arid zone

Biological activity in tree rhizosphere and association of VAM fungi with tree-roots could be used as selection criteria for agroforestry trees in arid ecosystems. Dehydrogenase activity and VAM fungi association in rhizosphere of six agroforestry trees (Azadirachta indica, Acacia tortilis, Eucalyptus camaldulensis, Prosopis cineraria and Tecomella undulata) were studied under irrigated and rainfed conditions in the arid zone of India. Higher dehydrogenase activity (9.5 to 16.8∼p kat g soil⁻¹), root colonization (58.3 to 68.5%) and spore density (132.5 to 234.7 spores 100 g soil⁻¹) were observed in tree rhizosphere as compared to that in non-rhizosphere (7.4, 37.7 and 44.4). Irrigation had increased dehydrogenase activity by 22.1% while it reduced root colonization and spore density by 14.2% and 16.2%, respectively. Trees of A. indica had registered maximum growth while E. camaldulensis the least. Findings suggest that selection of A. nilotica, A. indica, and T. undulata for plantations of agroforestry in arid region would reduce the risk and ensure sustainability of the agroforestry system.     

Millet production under pruned tree crowns in a parkland system in Burkina Faso

As a tree management tool, three treatments of crown pruning (total-pruning, half-pruning and no-pruning) were applied to Vitellaria paradoxa (karité) and Parkia biglobosa (néré) in agroforestry parkland systems in Burkina Faso. The area under each tree was divided into four concentric tree influence zones (Zones A: up to 2 m from the tree trunk, B: up to half of the radius of the tree crown, C: up to the edge of the tree crown and D: up to 2 m away from the edge of the tree crown). Millet production under these zones and outside was assessed during two cropping seasons over the study period of three years and the results showed that tree crown pruning had significant effect on millet production and the highest millet grain yield and total dry matter were produced under total-pruned trees (507 ± 49 and 2033 ± 236 kg ha⁻¹ year⁻¹, respectively). Light transmission, transpiration and soil nutrient status under the trees were also analysed in relation to millet production. The results of the analysis showed that total-pruned trees gave the highest millet production due to the reduction by crown pruning of the effects of large tree crowns on PAR transmission below crowns and rates of transpiration by trees. Soil was more fertile closer to the tree trunks than outside tree crowns. This may also be one of the reasons why millet overall performed better under Zone B than outside tree crowns. The higher production of millet under Zone B than under Zone A, the zone closer to the tree trunk, may be due to lower light intensity and more intense competition for water between trees and crops under Zone A. It was concluded that at least in the short term millet production could be improved by crown pruning of both karité and néré, but long term effects may depend on the ability of the trees to maintain the amelioration of soil fertility and on how quickly the trees recover from pruning. This revised version was published online in June 2006 with corrections to the Cover Date.     

The vertical distribution of fine roots of five tree species and maize in Morogoro,Tanzania

In order to assess the possibility of root competition in agroforestry, the vertical distribution of fine roots (< 2 mm in diameter) of five tree species in pure two-year-old stands was compared to that of mature maize.Cassia siamea, Eucalyptus tereticornis, Leucaena leucocephala andProsopis chilensis had a rooting pattern similar to that of maize, i.e. a slow decline in fine root mass from 0–100 cm soil depth.Eucalyptus camaldulensis had its roots evenly distributed down to 100 cm. On an average, the fine root biomass of the tree species was roughly twice as that of the maize. We conclude that the studied tree species are likely to compete with maize and other crops with a similar rooting pattern for nutrients and water.     

The role of complex agroforestry systems in the conservation of forest tree diversity and structure in southeastern Ghana

The role of different agroforestry systems in the conservation of plant diversity and forest structure has not been directly compared in many agricultural dominated landscapes. In this study, we investigated tree diversity and forest structure in a complex agroforestry landscape traditionally grown for cocoa and mixed food crops and compared these to the natural forest in southeastern Ghana. The study was carried out using 36 25 m × 25 m plots. There was significant difference [95% Confidence Interval (95% CI)] in the native forest/non-crop tree species richness between the natural forest and the agroforest farmlands but species richness was similar between the cocoa and mixed food crops agroforests. The density of native forest/non-crop trees was significantly higher (P < 0.05) in the natural forest but similar between cocoa and mixed food crops agroforest. Similarly, the basal area of native forest/non-crop trees was significantly higher (P < 0.05) in the natural forest but comparable between cocoa and food crops agroforest. Of the 20 most abundant native forest/non-crop trees recorded, 12 of them showed significant responses (P < 0.05) to land use change with nine of the species significantly abundant in the natural forest relative to the agroforest systems. Eighteen native forest/non-crop trees species in the agroforestry systems were commonly recorded as being used; 100% of them being used as fuel wood with 83.3 and 77.8%, respectively, used as medicines and materials. The findings of this study suggests that although complex agroforestry systems are a poor substitute for the natural forest the heterogeneous mosaic landscape in which complex agroforestry forms part can be strategically managed to maximize the benefits of both sustainable agriculture production and conservation of plant diversity by acting as buffer between protected areas and intensively managed areas.     

Growth and nut production of black walnut in relation to site, tree type and stand conditions in south-central United States

Black walnut (Juglans nigra) is a prime tree species for agroforestry practices in the United States providing highly prized wood and nuts for human consumption and wildlife. In 54 black walnut stands in south central United States, the site index (i.e., mean dominant height [DH], at age 25 years) ranged between 5.2 and 21.4 m, and was independent of stand density. There were no differences in height and stem diameter (DBH) growth rates between stands with improved varieties and native stock. Most stands were in a “free growth” stage because of either early age or wide spacing. Mean annual increments in DBH and height were positively related both for improved varieties and native stock. Understory competition had a substantial detrimental effect on DH. In a 26 year-old stand, trees growing within Kentucky blue grass (Poa pratensis) had a site index 5 m greater than trees growing within tall fescue (Festuca arundinacea). Predicted nut yields in 2002 varied between 0 and 1370 kg of hulled nuts per ha. Improved varieties had, in general, higher nut yields than predicted from a nut yield-DBH equation developed for individual trees. Nut yields were highly variable both within and among stands, and were related to DBH in native stock but not in improved varieties.     

Assessing farmers’ interest in agroforestry in two contrasting agro-ecological zones of Rwanda

Uptake and management of agroforestry technologies differs among farms in Rwanda and needs to be documented as a basis for shaping future research and development programs. The objective of this study was to investigate current agroforestry practices, farmers’ preferences, tree management and perspectives for agroforestry technologies. The study consisted of a combination of a formal survey, a participatory tree testing, farmer evaluation and focus group discussions in the Central Plateau (moderate altitude) and the Buberuka (high altitude) agro-ecological zones. A survey and a tree testing exercise with a range of species: (timber species—Eucalyptus urophyla, Grevillea robusta; legume shrubs - Calliandra calothyrsus, Tephrosia vogelii; and fruit species—Persea americana and Citrus sinensis) were carried out in Simbi (Central Plateau) and Kageyo (Buberuka) with farmers from different wealth status who received tree seedlings for planting, managing, and evaluating. Simbi had more tree species farm^?1 (4.5) than Kageyo (2.9). Fruit trees occurred most frequently in Simbi. Grevillea robusta, Calliandra calothyrsus and Tephrosia vogelii were mostly established along contours, fruit trees in homefields and Eucalyptus urophyla trees in woodlots. Survival was better on contours for Grevillea robusta (58–100 %) and Calliandra calothyrsus (50–72 %). Tree growth was strongly correlated with the total tree lop biomass in Eucalyptus urophyla (R ² = 0.69). Grevillea robusta was most preferred in Simbi and Eucalyptus urophyla and Calliandra calothyrsus in Kageyo. The study provided information useful for revising the national agroforestry research and extension agenda and has important implications for other countries in the highlands of Africa.     

The vertical distribution of fine roots of five tree species and maize in Morogoro,Tanzania

In order to assess the possibility of root competition in agroforestry, the vertical distribution of fine roots (< 2 mm in diameter) of five tree species in pure two-year-old stands was compared to that of mature maize.Cassia siamea, Eucalyptus tereticornis, Leucaena leucocephala andProsopis chilensis had a rooting pattern similar to that of maize, i.e. a slow decline in fine root mass from 0–100 cm soil depth.Eucalyptus camaldulensis had its roots evenly distributed down to 100 cm. On an average, the fine root biomass of the tree species was roughly twice as that of the maize. We conclude that the studied tree species are likely to compete with maize and other crops with a similar rooting pattern for nutrients and water.     

Agroforestry on alkali soils: Effect on some management practices on initial growth,biomass accumulation and chemical composition of selected tree species

This field study started in July 1982 on a typical alkali soil (Aquic Natrustalf, ESP 99.7) examined the growth responses to some management practices in a unified system planned to establish agroforestry. The planting of 3 salt tolerant tree species with two methods: on flat natural surface (FSPB) without rainwater conservation and on ridges (0.6 m high, 1.5 m at top and 2.5 m at base) having parallel trenches of the same section to store 300 mm of monsoon rainwater; constituted the main plot treatments. The tree planting with and without forage grassDiplachne fusca linn. in the inter-row space and planting in shallow (15 × 60 cm) and deep (15 × 180 cm to cross hard pan) augerholes filled with amendment treated soil (2 kg gypsum, 8 kg FYM, 50 g N, 10 g zinc sulphate and original soil) formed the sub and sub-plot treatments replicated 4 times in a split-split plot design.The mean plant height ofEucalyptus tereticornis smith;Acacia nilotica L; andParkinsonia aculeata L. in 2 years period was 273 and 328, 240 and 240 and 211 and 199 cm respectively with and without rainwater conservation. The corresponding height of the 3 tree species was 314 and 287, 250 and 231 and 207 and 203 cm with and without grass in the inter-row space. Similarly the plant height was 247 and 354, 182 and 298, 172 and 238 in shallow and deep augerholes respectively. The tree height and basal diameter differences with and without rainwater conservation and grass growth remained non-significant but deep augerhole planting was markedly superior to the shallow augerhole planting. The 2 year biomass accumulation also followed the same trend. The grass competed with trees for moisture during hot dry summer months and increased plant mortality particularly in the shallow augerholes and more so on ridges. The plant roots, essentially, remained confined to the amended soil of the augerholes in FSPB but proliferated in the loose soil of ridges or grass and submergence ameliorated surface soil of trenches.Acacia nilotica accumulated low sodium and had the lowest Na:Ca and Na:K ratio. It was found more promising than eucalyptus and parkinsonia as it experienced low mortality and had better chemical constitution to tolerate adverse alkali soil environment. The rainwater conservation system needed further evaluation before drawing final conclusions.     

Growth of widely spaced trees. A case study from young agroforestry plantations in France

Pessimistic forecasts often suggest that widely spaced trees enjoying free growth (no competition with other trees) will fail to provide high quality timber. This challenges the temperate agroforestry practice of planting widely spaced trees to produce high quality timber. We analyse tree growth data from recent temperate agroforestry plantations aged three to eight years, featuring low tree plantation densities (50 to 400 stems ha−1), the association of trees with intercrops (silvoarable systems) or animals (silvopastoral systems), and the use of plastic shelter tubes to protect trees (1.2 to 2.5 m high) and avoid damage by cattle or sheep in pastures or facilitate maintenance in silvoarable systems. The plantations are located in climates ranging from Mediterranean, dry central temperate plains, cold and wet central uplands to mild oceanic areas. Some plantations included a forestry control (high density of plantation, no tree shelter, no intercropping nor grazing). Trees were evaluated for height and diameter growth and stem form (straightness and absence of knots). Trees in most agroforestry plots grew satisfactorily, often faster than in forestry control plots. In some experimental plots, average annual height increments as high as 1 m and diameter increments as high as 1.5 cm were observed. Few agroforestry plantations were unsuccessful, and the reasons for the failures are discussed: animal damage in silvopastoral plots, but also a wrong choice of tree species unsuitable for local soil and climate characteristics. From these early results we can formulate some guidelines for designing future agroforestry plantations in temperate climates, concerning tree density, use of tree shelters and care required for widely spaced trees. This revised version was published online in June 2006 with corrections to the Cover Date.     

Competition for water in a pecan (Carya illinoensis K. Koch) – cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

Understanding the belowground interactions between trees and crops is critical to successful management of agroforestry systems. In a study of competition for water in an alley cropping system consisting of pecan (Carya illinoensis) and cotton (Gossypium hirsutum) in a sandy loam soil (Rhodic Paleudult) in Jay, Florida, root systems of the two species were separated by trenching to 120 cm depth. A polyethylene barrier was installed in half of the plots. Spatial and temporal variations in soil water content, root distribution and water uptake by both species, and leaf area development and height of cotton were measured. Interspecific competition for water was greater in the non-barrier treatment near tree rows than at the alley center. Competition became evident 3 to 4 weeks after emergence of cotton and increased during the following 7 to 8 weeks. Compared with the non-barrier treatment, the barrier treatment had higher soil water content and better growth of cotton (height, leaf area, and fine root biomass). Cotton lint yield in the barrier treatment (677 kg ha^–1) was similar to that in a sole-crop stand, but higher than in the non-barrier (502 kg ha^–1) treatment. Lint production efficiency of plants was higher in the interior rows in the non-barrier treatment (0.197 kg lint per square meter of leaf area, compared to 0.117 kg in the barrier treatment). The results suggest that trenching or even deep disking parallel to the tree row may reduce competition for water, but the impact on tree growth cannot be established from this study. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.