Preliminary studies of the physiological ecology of Umbilicaria lichens

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Abstract
Lichens which grow as clones or large anastomosing mats are often hard to handle experimentally because of the difficulty in defining an individual plant. Lichens in the Umbilicariaceae, however, are normally monophyllous and thus may be more convenient experimental material for the study of physiological ecology. The response of 5 Umbilicaria spp. to a variety of experimental treatments was examined to determine the suitability of using such plants as an experimental system in physiological ecology, when using the discrete sampling method of IR gas analysis. The treatments used included the time-course response of net CO2 exchange following resaturation, the effect of ventilation, the response to the concentration of CO2, the response to irradiance and the response to a low temperature pretreatment. The species could be considered suitable if steady-state rates of gas exchange extend over several hours, if rates of CO2 exchange were not influenced by ventilation, if rates of CO2 exchange were independent of CO2 concentration over at least a 100 ppm range, if the plants could be stored for periods of time up to 1 mo. without significant alteration of their response to environmental conditions, and if the equipment used enabled light-saturated rates of net CO2 exchange to be obtained. Steady-state rates of gas exchange exist from 1-10 h in U. vellea and U. papulosa and from 1-20 h in U. mammulata, U. muhlenbergii and U. deusta. None of the species has ventilation-dependent rates of fixation and all species show no limitation of CO2 exchange at CO2 concentration between 200 and 350 ppm. Only U. vellea shows altered CO2 exchange after 1 mo. of dry- or wet-frozen storage at -25.degree. C and U. deusta is not light saturated using the growth and incubation chamber facilities that have been constructed. Umbilicaria spp. thus appear to be suitable as experimental material for further use in physiological ecology.