The Physiology of Survival at the Limits of Farming in the Dry Tropics

  • G. R. Squire
Conference paper
Part of the NATO ASI Series book series (volume 16)


The dominant form of agriculture changes systematically along any spatial gradient of increasing dryness in the tropics. Between humid and arid regions, dense stands of perennial trees give way to annual or biennial cereals, legumes and root crops, and these in turn to mixed arable and animal husbandry, and eventually to extensive rangeland. Such transitions reflect changes in two main attributes of the commonest plants. One is the ‘return’ of resources intercepted by dry matter invested in the resource- capturing structures (τ), a value that decreases as the resources become scarcer. For example, if expressed as total radiation intercepted per unit of vegetative dry matter in shoots, τ is typically 5 MJ g-1 for perennial palms and 1 MJ g-1 for short-season dryland cereals. The other major attribute is the minimum dry mass (Wv) that plants must accumulate before any economic yield is produced. Wv ranges from 105 g plant-1 for some tree crops to 10–20 g plant-1 for large annuals, 1–4 g plant -1 for small cereals and legumes, and a small fraction of a gram for rangeland grasses. In contrast to these two attributes, the conversion ratios change much less systematically in relation to aridity. Transitions — whether from maize to millet, or arable to rangeland — increase the chance that the returns of dry matter will exceed Wv, and therefore that surplus dry matter will be available for economically important structures. For example, the transition from arable cropping to rangeland seems to occur when the total dry matter per plant is similar to Wv for the most adaptable dryland cereals, τ and Wv are inherently determined by physiological attributes of the plant. Particularly important are the longevity of leaves and roots, their expansion rates and area/mass or length/mass relations, the determinacy of growth habit, and the sensitivity of reproduction to drought and high temperature. Generally, longevity is the most important attribute and the one that can perhaps be most influenced by husbandry, water-harvesting practices or by reducing stand density. This paper examines the way these physiological attributes discriminate both between species and among stands of the same species, using examples from different tropical environments.


Pearl Millet Coconut Palm Bambara Groundnut Investment Efficiency Field Crop Research 
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Copyright information

© Springer-Verlag Berlin Heidelberg 1993

Authors and Affiliations

  • G. R. Squire
    • 1
  1. 1.Scientific Adviser’s UnitScottish Office Agriculture and Fisheries DepartmentEdinburghUK

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