An investigation was made of the degree of physiological differentiation in response to high temperatures, in the lichen Ramalina menziesii Tayl. The range of this species extends from cool coastal rainforest in British Columbia, Canada to hot and dry coastal desert in Baja California del Sur, Mexico. Thalli were collected from four extreme positions within the range and exposed to temperature pretreatments ranging from 20 to 70 °C in 10 °C increments for 7-5 h. Rates of net photosynthesis, dark respiration and calculated gross photosynthesis were then determined. There was physiological differentiation among the four populations at a narrow range of pretreatment temperatures so that Thalli derived from the site with the least heat stress show the least resistance, while Thalli derived from the site with the most heat stress show the most heat resistance. This suggests that physiological as well as morphological plasticity is important to the ecology of this lichen. It also suggests that high temperatures alone do not act to restrict the distribution of this species.
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