Agroforestry systems are believed to provide a number of ecosystem services; however, until recently evidence in the agroforestry literature supporting these perceived benefits has been lacking. This special issue brings together a series of papers from around the globe to address recent findings on the ecosystem services and environmental benefits provided by agroforestry. As prelude to the special issue, this paper examines four major ecosystem services and environmental benefits of agroforestry: (1) carbon sequestration, (2) biodiversity conservation, (3) soil enrichment and (4) air and water quality. Past and present evidence clearly indicates that agroforestry, as part of a multifunctional working landscape, can be a viable land-use option that, in addition to alleviating poverty, offers a number of ecosystem services and environmental benefits. This realization should help promote agroforestry and its role as an integral part of a multifunctional working landscape the world over.

Agroforestry for ecosystem services and environmental benefits: an overview

An Improved Grading System For Measuring Plant Diseases Vol-36

Weeds are known to cause enormous losses due to their interference in agroecosystems. Because of environmental and human health concerns, worldwide efforts are being made to reduce the heavy reliance on synthetic herbicides that are used to control weeds. In this regard the phenomenon of allelopathy, which is expressed through the release of chemicals by a plant, has been suggested to be one of the possible alternatives for achieving sustainable weed management. The use of allelopathy for controlling weeds could be either through directly utilizing natural allelopathic interactions, particularly of crop plants, or by using allelochemicals as natural herbicides. In the former case, a number of crop plants with allelopathic potential can be used as cover, smother, and green manure crops for managing weeds by making desired manipulations in the cultural practices and cropping patterns. These can be suitably rotated or intercropped with main crops to manage the target weeds (including parasitic ones) selectively. Even the crop mulch/residues can also give desirable benefits. Not only the terrestrial weeds, even allelopathy can be suitably manipulated for the management of aquatic weeds. The allelochemicals present in the higher plants as well as in the microbes can be directly used for weed management on the pattern of herbicides. Their bioefficacy can be enhanced by structural changes or the synthesis of chemical analogues based on them. Further, in order to enhance the potential of allelopathic crops, several improvements can be made with the use of biotechnology or genomics and proteomics. In this context either the production of allelochemicals can be enhanced or the transgenics with foreign genes encoding for a particular weed-suppressing allelochemical could be produced. In the former, both conventional breeding and molecular genetical techniques are useful. However, with conventional breeding being slow and difficult, more emphasis is laid on the use of modern techniques such as molecular markers and the selection aided by them. Although the progress in this regard is slow, nevertheless some promising results are coming and more are expected in future. This review attempts to discuss all these aspects of allelopathy for the sustainable management of weeds.

Allelopathic Interactions and Allelochemicals: New Possibilities for Sustainable Weed Management

The ecological principles that define the competitive and complementary interactions among trees, crops, and fauna in agroforestry systems have received considerable research attention during the recent past. These principles have not yet, however, been adequately integrated and synthesized into an operational approach. This paper reviews the ecological and ecophysiological bases for interspecific interactions based on data from site-specific research and demonstration trials from temperate agroforestry systems, primarily from temperate North America. The review shows that information on ecological interactions in several temperate agroforestry systems is inadequate. It is recommended that the future research should focus on exploring new species and systems that have received little attention in the past. Priority research areas should include cultural practices and system designs to minimize interspecific competition and maximize environmental benefits such as improved water quality. Potential for genetic modification of components to increase productivity and reduce competition also needs to be explored. Process-oriented models may be used increasingly to predict resource-allocation patterns and possible benefits for a suite of site and species combinations.

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Interspecific interactions in temperate agroforestry

1 Designs for greenhouse studies of interactions between plants are reviewed and recommendations for their use are provided.

2 Papers published over a 10-year period showed the replacement series design to be the most popular, especially in studying crop–weed interactions. Fifty per cent of the studies involved only two species, although studies testing the interaction between different genotypes of only a few species were also popular.

3 Limitations imposed by the choice of design, the variables measured, and the analysis used on the range of inferences that may be validly drawn from the experiment are frequently not well understood or appropriate for the questions that appear to be addressed. One example is the failure to distinguish the outcome of competition (the long-term outcome of interaction) and the effects of species on each other.

4 Studies in which only final yield is measured are severely limited as to the inferences which may be drawn. Effects due to interspecific interaction during the course of the experiment cannot then be separated from pre-existing differences, and interpretation may be biased towards species whose individuals were initially larger. In addition, measurements at several times are necessary to understand the changing dynamics of species interaction.

5 Simple pair-wise mixtures can assess the effect of treatment factors on the outcome of competition. Replacement series and related diallel designs generally produce results that may be size-biased even when initial interspecific differences are known. Additive designs (including target–neighbour designs), despite confounding density with species proportions, offer considerable scope for addressing mechanistic questions about interspecific interactions. Designs that allow response surface analysis can avoid many of the problems inherent in the other methods, but all need to be adjusted for initial interspecific differences. Designs for multiple species experiments are still largely untested, although several designs have been used. At the level of the individual plant, hexagonal fan designs permit study of the effects of varying the spatial pattern, and the densities and the relative proportions of interacting species, but suffer from lack of independence and lack of randomization.

Designs for greenhouse studies of interactions between plants

Safety-net role of tree roots: evidence from 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.

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

Growth response of longleaf pine (Pinus palustris Mill.) seedlings to fertilization and herbaceous weed control in an old field in southern USA

Whole bean (var. "Eastern Butterwax") plants and isolated cells were used to investigate possible mechanisms of action of glyphosate [N-(phosphonomethyl)glycine]. Results showed that glyphosate was quickly absorbed by the whole plant but not by individual cells and that it caused a rapid reduction in leaf dry matter accumulation, leaf expansion, leaf angle, and stomatal aperture without affecting the water status of the plant. Glyphosate also caused a rapid reduction in cellular uptake of (86)Rb and (32)P which preceded its detrimental effects on photosynthesis, RNA and protein synthesis, and respiration of isolated cells. This reduction in ion absorption was not due to a loss of membrane integrity, decrease in energy supply or chelation of ions. It was concluded that glyphosate was directly inhibiting the ion absorption process of bean leaf cells.

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Effect of Glyphosate on Intact Bean Plants (Phaseolus vulgaris L.) and Isolated Cells.

The degree of tree-crop competition for nitrogen (N) and its effect on fertilizer-use efficiency and N movement were examined in a pecan (Carya illinoensis K. Koch)-cotton (Gossypium hirsutum L.) alley cropping system. Assessment of competition was accomplished via the installation of a belowground polyethylene root barrier in half the number of plots in order to provide two treatments–barrier and non-barrier. The percentage of N derived from fertilizer (NDF) and fertilizer-use efficiency (UFN) were determined using 15N-enriched ammonium sulfate (5% atom enrichment) applied at 89.6 kg N ha−1. In cotton, the barrier treatment resulted in higher leaf (38%), stem (66%), seed cotton (55%) and total (58%) biomass compared to the non-barrier treatment. Total N content in leaf, stem and seed cotton was 67% higher in barrier compared to non-barrier treatment. Percentage of NDF in cotton leaf and stem was significantly lower in barrier (15.8% and 17.3%, respectively) compared to non-barrier treatment (20.4% and 21.2%, respectively). For UFN, this trend was reversed, with plants in barrier treatment having a higher percentage of UFN. Root trenching did not affect pecan foliar N concentration, canopy N content, NDF or UFN. In soil, N recovery at 90–120 cm depth was lower in non-barrier treatment, indicating tree root uptake of fertilizer N. Although tree roots in non-barrier treatment had access to fertilizer N, competition was mainly for N already in the soil, since fertilizer was applied after major seasonal nutrient demands of the trees had been met. Overall, the alley cropping system in this study exhibits potential for efficient N cycling, given the apparent ability of pecan trees to intercept and uptake N fertilizer from deeper soil layers and return to surface soil via litterfall.

Competition for 15N-labeled fertilizer in a pecan (Carya illinoensis K. koch)-cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

Barry J. Brecke

Papers

Safety-net role of tree roots: evidence from a pecan (Carya illinoensis K. Koch)-cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

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

Growth response of longleaf pine (Pinus palustris Mill.) seedlings to fertilization and herbaceous weed control in an old field in southern USA

Effect of Glyphosate on Intact Bean Plants (Phaseolus vulgaris L.) and Isolated Cells.

Competition for 15N-labeled fertilizer in a pecan (Carya illinoensis K. koch)-cotton (Gossypium hirsutum L.) alley cropping system in the southern United States