The complex structure of the Damar agro forest illustrates the intense competition for light that goes on in their environment, in which other climatic factors are not limiting at most time of the year. To assess the importance of canopy openness which determines not only light distribution amongst individual trees but also other local microclimatic factors such as temperature and relative air humidity, a three-dimensional model was developed. This individual-based model simulates forest dynamics on the basis of establishment, growth and death of individual trees. Modeling is indeed a valuable tool to test the consistency of functioning hypothesis one can hardly test in the field given the lack of controlled conditions and given the long-term response one must expect in dealing with trees
Tag: light
Utilisation of light and water in tropical agriculture
The resource capture approach developed by John Monteith has been applied in studies of a wide variety of plant species and cropping systems in the tropics over the past 18 years. The purpose of this review is to highlight the progress made and the new challenges which lie ahead. The foundation for this approach was the establishment of ‘response surfaces’ for the development and growth of tropical crops using controlled-environment facilities. The concepts of light interception and thermal time developed were then used to investigate the mechanisms responsible for overyielding in intercropping systems and genotypic differences in the drought adaptation of crops in the semi-arid tropics. The most significant achievements were in the understanding of temporal and spatial complementarity in intercropping and agroforestry systems and the development of plant growth models. More recently, the same concepts have been extended to the capture of below-ground resources in agroforestry systems and rain forests. The most serious remaining challenge is to extend this approach to studies of complex multispecies systems in the humid tropics.
A simple model of light interception by trees in multi-species, multi-strata forests
A simple model of light interception was designed to represent light resource variability in multi-strata multi-species agroforests. Dealing with many different species makes it unpractical to develop a detailed model of light interception taking into account structural characteristics of each species (foliage lay-out, distribution of leaf orientation, radiation characteristics of leaves, etc.) as it has been done in other cases (Cescatti 1997; Bartelink 1998). The hypothesis to be tested was that a simple 3D description of trees inside a plot with average crown “density” estimated by photographs would allow a reasonably precise description of light received at any point. The model was tested against hemispherical photographs taken at ground level
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.
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.