| Summary: | The structure of lichen dominated vegetation is described within a series of raised beach ridges found along the strip of coastal tundra in northwestern Ontario. The patterns of distribution of the most abundant species correlated with the topographic influences of individual beach ridges more than with the influences of the general developmental sequence of vegetation from the coast of Hudson Bay, inland. The importance of topography to the structure of this vegetation was examined by comparing ridge top and bottom positions of raised beach ridges in terms of edaphic factors, snow cover and microclimatic factors. A new experimental system was developed to test for adaptation of net photosynthetic rates in the lichens Alectoria ochroleuca ( Hoffm. ) Massal. and Cetraria nivalis (L.) Ach. to the contrasting ridge surfaces from which they were collected. Morphological adaptations to these beach ridge surfaces were also examined. Seasonal patterns of net photosynthesis in both species showed that intra and interspecific differences in seasonal acclimation were important to the observed patterns of distribution in the field. Thallus morphology was found to provide optimal water relations for photosynthetic production. Experiments showed further that the potential for control of evaporative loss is present in lichens. This study shows that the patterns of distribution observed in the field, are consistently related to the topographic relations of individual beach ridges. These patterns are also consistent with both the physiological and morphological requirements exhibited by the plants, and with the defined physical characteristics of the surfaces on which they grow. Doctor of Philosophy (PhD)
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