Tag: microclimate
Productivity, microclimate and water use in Grevillea robusta-based agroforestry systems on hillslopes in semi-arid Kenya
This paper describes a multi-disciplinary project to examine the changing interactions between trees and crops as the trees in semi-arid agroforestry systems establish and mature; the project is one of the most detailed and highly instrumented long-term studies of tree and crop growth, system performance, resource capture, hydrology and microclimate ever carried out within an agroforestry context. Its primary objective was to compile a comprehensive experimental database to improve the mechanistic understanding of tree/crop interactions and support the development and validation of process-based simulation models describing resource capture and tree and crop growth in semi-arid agroforestry systems.Grevillea robusta A. Cunn. (grevillea) trees were grown as mono-cultures or in mixtures with cowpea (Vigna unguiculata L.) or maize (Zea mays L.) over a 68-month period. Allometric approaches were used to determine seasonal and annual growth increments for leaf area and leaf, branch and trunk biomass in grevillea. Crop performance was examined during each growing season, while the spatial distribution of tree and crop roots was established during the latter stages of the experiment using coring and mini-rhizotron approaches. Detailed hydrological studies examined effects on the soil water balance and its components (precipitation, interception, runoff and soil moisture status); equivalent measurements of spatial and temporal variation in microclimatic conditions allowed the mechanistic basis for beneficial and detrimental effects on understorey crops and the influence of proximity to trees on crop performance to be examined. Transpiration by grevillea and water movement through lateral and tap roots were measured using sap flow methodology, and light interception by the tree and crop canopies was routinely determined.This multi-disciplinary study has provided a detailed understanding of the changing patterns of resource capture by trees and crops as agroforestry systems mature. This paper provides an overview of the underlying rationale, experimental design and core measurements, outlines key results and conclusions, and draws the attention of readers to further papers providing more detailed consideration of specific aspects of the study.
Understorey microclimate and crop performance in a Grevillea robusta-based agroforestry system in semi-arid Kenya
The influence of dispersed trees on microclimatic conditions, gas exchange and productivity of maize (Zea mays L.) in a Grevillea robusta-based agroforestry system in semi-arid Kenya was examined to test the hypothesis that the benefits of shade seen in savannah ecosystems may be outweighed by competition for below-ground resources. Meristem temperature, cumulative thermal time, intercepted radiation, spatial distribution of shade and gas exchange were determined for maize grown as sole crops, in an agroforestry system, or under shade nets providing 25 or 50% reductions in incident radiation to discriminate between effects of shade and below-ground competition. The major benefit of shade was to reduce exposure to the supra-optimal temperatures experienced in many tropical regions, and which are predicted to become increasingly common by climate change models. However, although trees decreased photosynthetic photon flux density (PPFD) incident on understorey maize by ca. 30%, the yield reduction was much greater than in the 25% shade net treatment in four seasons providing contrasting rainfall. Maize yield was unaffected by 50% artificial shade in the driest season (168 mm) but decreased with increasing shade when rainfall was high (628 mm). Shade reduced meristem temperature and delayed flowering by 5-24 days depending on treatment and seasonal rainfall. Thermal time to flowering in the agroforestry system doubled from 600 to 1200 C day as rainfall decreased. Photosynthetic and transpiration rates for understorey maize were similar to the 25 and 50% shade treatments when rainfall was high, but were ca. 10% of those for unshaded sole maize in dry seasons. PPFD-saturated photosynthetic rate was initially similar in all treatments but fell sharply in the agroforestry system as the season progressed. Radiation conversion coefficients did not differ between unshaded sole and understorey maize. The results suggest that the ameliorative influence of tree shade was greater in low rainfall seasons, as in savannah systems, but that potential benefits were outweighed by below-ground competition. This may be managed by root-pruning trees.
Can the ecosystem mimic hypotheses be applied to farms in African savannahs?
The first ecosystem mimic hypothesis suggests clear advantages if man-made land use systems do not deviate greatly in their resource use patterns from natural ecosystems typical of a given climatic zone. The second hypothesis claims that additional advantages will accrue if agroecosystems also maintain a substantial part of the diversity of natural systems. We test these hypotheses for the savannah zone of sub-Saharan Africa, with its low soil fertility and variable rainfall. Where annual food crops replace the natural grass understorey of savannah systems, water use will decrease and stream and groundwater flow change, unless tree density increases relative to the natural situation. Increasing tree density, however, will decrease crop yields, unless the trees meet specific criteria. Food crop production in the parkland systems may benefit from lower temperatures under tree canopies, but water use by trees providing this shade will prevent crops from benefiting. In old parkland trees that farmers have traditionally retained when opening fields for crops, water use per unit shade is less than in most fast growing trees introduced for agroforestry trials. Strong competition between plants adapted to years with different rainfall patterns may stabilise total system productivity — but this will be appreciated by a farmer only if the components are of comparable value. The best precondition for farmers to maintain diversity in their agroecosystem hinges on the availability of a broad basket of choices, without clear winners or ‘best bets’.
A framework for quantifying the various effects of tree – crop interractions
Early assessments of the potential benefits of agroforestry at the farm level were based largely on the assumption that it is possible to extrapolate from existing information on forestry and agriculture (Huxley, 1983; Nair, 1993), and partly on observations of trad-itional agroforestry systems that showed increased growth of understorey vegetation (Ong and Leakey 1999; Kho et al., 2001). Various negative effects have also been rec-ognized, such as competition for moisture, excessive shading and allelopathy, although these have attracted much less attention from scientists. Most of the evidence of benefits and drawbacks of agroforestry continues to be qualitative or indirect, i.e. extrapolated from a wide range of systems, creating often un-realistic expectations of the benefits of agro-forestry technologies (Garcia-Barrios and Ong, 2004).
Tree-crop interactions: a physiological approach
Agroforestry is rapidly being transformed from an empirical, largely anecdotal collection of beliefs and practices into an emerging science in the field of natural resource management. This book is a major contribution towards this goal, and is aimed at students, research workers and practitioners in agroforestry and applied plant physiology. The authors have applied principles of plant ecology and crop physiology to develop more precise approaches that quantify biological (tree-crop) interactions in agroforestry systems. The various models developed, particularly the tree-crop interactions equation, provide practical but rigorous approaches for both above- and below-ground processes. The book focuses on two basic resources: water and light. Tree-crop interactions for nutrients are not treated in depth as there are very few data on this subject. There are 10 chapters by various authors: (1) A framework for quantifying the various effects of tree-crop interactions (Ong, C. K.); (2) Mixed cropping of annuals and woody perennials: an analytical approach to productivity and management (Ranganathan, R.; Wit, C. T. de); (3) Mulch and shade model for optimum alley-cropping design depending on soil fertility (Noordwijk, M. van); (4) Principles of resource capture and utilization of light and water (Ong, C. K.; Black, C. R.; Marshall, F. M.; Corlett, J. E.); (5) Microclimatic modifications in agroforestry (Brenner, A. J.); (6) The water balance of mixed tree-crop systems (Wallace, J. S.); (7) Biological factors affecting form and function in woody-non-woody plant mixtures (Huxley, P.); (8) Tree-soil-crop interactions on slopes (Garrity, D. P.); (9) Root distribution of trees and crops: competition and/or complementarity (Noordwijk, M. van; Lawson, G.; Soumaré, A.; Groot, J. J. R.; Hairiah, K.); and (10) Woody-non-woody plant mixtures: some afterthoughts.