Diurnal courses of gross CO 2 uptake and thallus water content of the lichen Ramalina menziesii Tayl. were measured together with the environmental variables temperature, irradiance, and atmospheric humidity at a coastal and an inland site in central California. Twenty-three days, distributed over all seasons and a variety of weather conditions, were recorded at the inland site; 14 were recorded at the coastal site. Conditions in both habitats were similar in that the most frequent reason for photosynthetic inactivity in R. menziesii was insufficient thallus hydration. Irradiance was another factor controlling photosynthetic rates; temperature, on the other hand, had little influence. At the inland site, CO 2 fixation was concentrated in the rainy winter season and very little CO 2 fixation took place during the hot summers. In contrast, fog and high atmospheric humidity that frequently occur during summers at the coast provided sufficient moisture for frequent photosynthetic activity, thus resulting in intermittent activity throughout the year. While extremely high gross photosynthetic rates of up to 15 mg CO 2 g −1 thallus dry weight were measured at the inland site, rates at the coast were always much lower.
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Matthes‐Sears et al. (1986) studied this question.