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Are the Xeromorphic Trees of Tropical Upper Montane Rain Forests Drought- Resistant?

R. C. Buckley, R. T. Corlett and P. J. Grubb
Biotropica
Vol. 12, No. 2 (Jun., 1980), pp. 124-136
DOI: 10.2307/2387728
Stable URL: http://www.jstor.org/stable/2387728
Page Count: 13
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Are the Xeromorphic Trees of Tropical Upper Montane Rain Forests Drought- Resistant?
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Abstract

Most tropical upper montane rain forests are in cloud for prolonged periods and in most years are subject to only a moderate drought-stress or none at all. However, the leaves of their constituent trees and shrubs have several features which are characteristic of plants that grow in areas which are regularly subject to strong drought-stress. Experiments made on cut shoots and detached leaves in Malaya and Jamaica show that, in general, the xeromorphic montane plants are not characterized by an ability to restrict water loss more than trees of lowland rain forest; that they and the lowland trees usually appear to shut their stomata at small water deficits; and that relatively small water deficits are lethal for both these groups. The examined trees and shrubs of the tropical upper montane rain forests differ from the undoubtedly drought-resistant sclerophylls of mediterranean climates, which are able to survive much larger water deficits and mostly need large water deficits to induce appreciable stomatal closure. Among the tropical montane plants studied, two minority groups do restrict water loss very effectively: the conifers and the facultative epiphytes. The stomata of tested species from upper montane rain forest in Malaya failed to close when leaves were dried at 34-35⚬C. These species therefore differ from semidesert xeromorphs which shut their stomata when subjected to simultaneous heat-stress and drought-stress. It is suggested that the upper montane rain forest flora in Malaya has been recruited through an environmental sieve which has demanded an ability to live on strongly acidic soils which were poor in mineral nutrients; some of the species passing the sieve are strongly tolerant of water-stress, but most are not. The evolutionary rationale of the xeromorphism seen in the majority of montane trees and shrubs remains enigmatic.

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