Overstory #84 - Patchiness: Living in the Real World
Patchiness: Living in the Real World
Agroforestry systems are characteristically diverse, containing a mosaic plants and animals within a broad range of natural land features, climates, and soils. Editions #72 and #74 of The Overstory by Robert Chambers explored how observing and exploiting microclimates (small-scale environments) can be extremely useful in developing sustainable land use systems.
In this edition, special guest author Peter Huxley discusses how spatial differences, or patches, are an important aspect of understanding and designing agroforestry systems.
What Is 'Patchiness'?
We all live in a very 'patchy' environment, some more than others, and tropical agroforesters more than most! Patchiness is a form of variability in space. The term expresses diversity, a difference in the nature of something in degree or quality. Spatial differences can be gradual (a 'cline'), or they can be more abrupt as in patchiness, where some area is distinguishable in some way from those around it. Patchiness often refers to the unlike nature of many adjacent small areas. Sometimes patchiness is used in a wider context to refer to any kind of dimensional variability, for example, changes throughout a three-dimensional soil profile. Change can occur over time, too, so that any particular pattern of patchiness can evolve into another.
Disadvantages and advantages
There are two ways of looking at patchiness. It can be seen is being a nuisance that we want to eliminate or avoid, or as something potentially useful that needs to be understood, explored and, perhaps, exploited. Locational variability encountered in the field when setting down an experiment falls into the first category. Researchers need to be aware of soil fertility changes across an experiment (without necessarily understanding them), so that they can design experiments to take account of, and effectively 'remove' them from, the comparisons and contrasts between treatment effects. Farmers in a seasonally arid area may watch with horror as a rainstorm sweeps across their valley, wetting and resuscitating neighbours' droughted crops, but missing their own plots. Farmers who grow tea from seedlings will observe patches of higher and lower yielding bushes, an expression, in part, of their underlying genetic variability, in part of soil patchiness. These are a few examples of different ways in which environmental or biological patchiness is seen as non-beneficial or adverse.
There are many other examples, however, where patchiness is welcomed. An excellent example is the way home gardens have evolved. Here, through skill and shared knowledge and experience (trial-and-error, at first) farmers have optimised the numbers and kinds of plants grown, and have planted them in the best arrangements and in the most suitable environmental conditions so that they can all flourish together (Torquebiau, 1992). Such pragmatic designs also explore and exploit the vertical environment. Almost certainly many of them will have maximised environmental resource capture and, probably also, environmental resource use efficiency. They will also have optimised labour inputs and the flow of rewards in the form of various products and cash incomes.
Even farmers using relatively simple cropping practices still tend to manipulate them spatially so as to take advantage of particular areas that they know will suit different kinds of crops. Agroforestry, too, will be best served by such a practice, but more needs to be known about the specific environmental requirements of the trees. There is also competition to consider.
Of course, we remain quite unaware of a great deal of patchiness. However well-graded and carefully cultivated a plot may be, it is seldom that much is known about the different layers below ground and what effects it has on root systems. One of the most useful tools for agroforestry field experimenters is still a soil auger.
Agroforestry increases patchiness
Growing crops alone as monocultures will tend to diminish at least surface patchiness; the frequent cultivation will mix up the topsoil and make it more homogeneous. However, what can be expected from agroforestry? Mixtures of woody and non-woody plants will tend to increase existing patchiness over time. This will occur above ground as a consequence of the environmental changes brought about by the taller, woody species at the tree-crop interface (shade and temperature and humidity changes, shelter, rainfall redistribution, and so on). Below ground there will be soil changes brought about as a result of litterfall, mulching, fine root turnover, macropore formation, and so on. These 'microsite' changes are an important, potentially beneficial aspect of trees on farmers' fields or in the wider landscape, but they will tend to be localised and form patches where trees do not form such a complete cover as they do in forests or commercial tree plantations. Indeed, tree-crop interfaces are the observed and measurable outcome of created patchiness, in part due to these immediate (short-term) effects of the plant components on one another and, as time goes by, as a consequence of changes (long-term) in the soil at this interface.
Questions of scale
The question of scale is all-important in exposing and defining patches. In dealing with environmental variability, for example, we expect things that are close together to be alike; but how close is 'close'? For any level of change it often becomes possible to discriminate more and more patches as smaller and smaller areas are defined. Thus, choosing a scale level that will show the kinds of limits we want to observe is essential; too small a scale just increase the work without reward.
Patchiness can be observed at very small scales. Above ground the flutter of leaves and the creation of sunflecks through a canopy is one such example, reflecting changes in both space and time. This process is of some importance for shade-loving lower-storey plants. Categorising the different within-canopy arrangements of cohorts of leaves or the ways fine roots of woody plants can be clustered are examples of relatively small-scale patchiness in agroforestry systems that could well be functionally important.
Very little land is absolutely flat. Soil surface topography changes over quite short distances, with effects that often are overlooked. For example, the puddles that form in hollows can bring about temporary waterlogging after heavy rainstorms; this can often be seen in cotton and groundnut crops. A reverse effect may occur in dry regions on sandy soils, such as in the Sahel, where millet will often germinate more rapidly in such spots. Microcatchments for water are a beneficial form of artificially created patchiness.
Below ground, a good example at the 'micro' scale is the way fine roots explore the soil. Such roots branch and grow towards fertile patches of soil. They also die in places where soil conditions contain life-threatening elements: e.g.. toxic levels of aluminum, extreme pH, and so on. Thus, at this scale, the root system is dynamically exploring the best of the soil it finds itself in. No wonder that soil analysis, where samples are commonly combined and variability is averaged out. may not always present conditions quite as the plant might see them. In agroforestry, there are additional implications for competition and complementarity if fine root clusters can occupy different microspaces, and so share the environmental resources to a certain degree, or if the trees and crops explore different macrospaces (i.e., if the trees root more deeply).
At this somewhat larger scale, the exploitation of nutrients and water in soil profiles by the root system as a whole certainly also encourages patchiness. Phosphorous uptake, for example, unlike that of soluble nutrients, is highly dependent on the level of fine root growth to soil peds where it is available. Again, in agroforestry the exploitation of soil water by a mixture of woody and non-woody plants can leave the soil water status in the profile extremely patchy, something that is less commonly found under agricultural crops.
Above ground, single trees offer an extraordinarily complex set of microclimatic patterns. Studying the single tree is not simple. Not only is distance from the trunk important, but also orientation. Shade patterns, and hence soil surface temperatures and ambient humidities above it, are dominated by the east-west passage of the sun. The rainfall shadow and leaf litter dispersal will be affected by prevailing winds, whatever their direction might be. Canopy throughfall will depend on the density of the canopy, and also on leaf size, which affects the size of droplets falling to the soil. It will also be supplemented from time to time by stemflow which, itself, projects another pattern around the trunk. In terms of biomass additions and nutrient recycling, working from what happens around an individual tree to the landscape level requires an understanding of the resource flows in the larger system, and the extent to which the individual trees contribute to this. The patchiness within such areas and the overall effect of scattered trees at a higher scale level have so far hardly been investigated (van Noordwijk and Ong, 1996).
Differences in tree behaviour (phenology) can present another form of observable patchiness. Natural vegetation or managed tree mixtures often contain species that at any one time are behaving differently. Indeed, herein lies a key to more effective environmental resource capture in agroforestry.
Patchiness is part of the world around us. Whether dealing with the 'micro' level, the plot, farm or higher levels of scale, the spatial relationships involved need to be exposed and their interactions understood. Nowhere do we find forms of land use where this is more essential than in agroforestry.
Van Noordwijk, M. and C.K. Ong. 1996. Lateral resource flow and capture - the key to scaling up agroforestry results. In: Agroforestry Forum 7:29-31.
Torquebiau, E. 1992. Are tropical home gardens sustainable? In: Agriculture, Ecosystems and Environment, 41:189-209.
This article is excerpted with the kind permission of the author and publisher from:
Huxley, P. 1999. Tropical Agroforestry. Blackwell Science, Oxford, UK, pp. 241-6.
Special offer by the publisher on Tropical Agroforestry for Overstory subscribers: The regular price of the book is £72 (£69.50 + p&p). The special price is a reduction of 20%, giving a price of £58.00 inclusive of p&p.
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About the Author
Peter Huxley is an Associate of the Oxford Forestry Institute. He is former Director of Research Development at the International Centre for Research in Agroforestry (ICRAF), agroforestry expert, and author of Tropical Agroforestry. Since leaving ICRAF in 1992, Peter Huxley has been involved mainly in running short courses around the world on subjects covered in Tropical Agroforestry, and in writing about agroforestry. As an Associate of the Oxford Forestry Institute he is currently helping to put together a regional East African Universities Programme to improve the sustainability of rural productivity and enhance human welfare in the Lake Victoria Basin under conditions of community change and continuing climatic variability.