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Harvard University (1982)

POIKILOHYDRY AND DESICCATION TOLERANCE IN SOME XEROPHYTIC MOSSES

ALPERT, PETER

Titre : POIKILOHYDRY AND DESICCATION TOLERANCE IN SOME XEROPHYTIC MOSSES

Auteur : ALPERT, PETER

Université de soutenance  : Harvard University

Grade : Doctor of Philosophy (PhD) 1982

Résumé
The high tolerance of drought and extreme temperature which characterizes many desiccation tolerant, poikilohydric plants would seem to enable them particularly to colonize arid regions. However, such plants are relatively scarce in xeric habitats. It was hypothesized that the rapidity and frequency of desiccation of poikilohydric plants in xeric habitats induced starvation for essential metabolites, both by limiting the time available for CO(,2) assimilation and by favoring respiration over photosynthesis. The ecological role of poikilohydry and desiccation tolerance was examined in some xerophytic mosses growing on granite boulders at a site in the inland chaparral of San Diego County, California. The five species most intensively studied were Grimmia apocarpa Hedw., G. laevigata (Brid.) Brid., Hedwigia ciliata (Hedw.) P.-Beauv., Orthotrichum rupestre Schleich. ex Schwaegr., and Tortula ruralis (Hedw.) Gaertn, Meyer & Scherb. var. crinata De Not. Infrared gas analysis of net CO(,2) exchange and investigation of water relations, recovery from drought, internal movement of assimilated 14-CO(,2), microdistribution, microclimate, and macroclimate show that these imposed by their chaparral habitat, and can utilize even brief periods of favorable climate at any time during the year. Among these mosses, the species with higher water loss resistance and drought tolerance occupy more xeric microsites. However, the comparative physiology of the mosses fails to account fully for differences in their microdistributions. It may be that complex interactions between plant responses and environmental factors in this system can only be apprehended by modelling potential net carbon balance as a function of species, microtopography, and climate. The data collected here should suffice for this purpose. On the other hand, transplant experiments suggest that dispersal and establishment also limit microdistributions of the mosses. It appears that poikilohydry and desiccation tolerance enable these mosses to colonize a stressful and otherwise largely unoccupied habitat, but neither to survive in its most xeric portions nor to grow in habitats of greater disturbance or competition. Functional links between poikilohydry and desiccation tolerance may explain why desiccation tolerant species are not more numerous and more abundant.

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