Age Dependence of Photosynthesis in the Caribou Lichen Cladina stellaris

The fruticose thallus of the lichen Cladina stellaris (Opiz.) Brodo can be subdivided into individual whorls of branches of known age. Photosynthesis declines steadily with age from a maximum rate of 0.76 milligram CO(2) per gram dry weight per hour in 1-year-old whorls to 0.02 milligram CO(2) per gram dry weight per hour after 15 years. Conversely, the dry biomass of the whorls increases up to age 9 years and then approximately levels off. Photosynthesis in whorls older than 15 years is less than 0.01 milligram per gram per hour. Progressive changes in thallus color with age are associated with the observed photosynthetic decline. Whorls aged 6 years and younger together account for 18% of thallus biomass but 50% of photosynthetic activity. The implications of these results for the idea that the lichen symbiosis results in truly integrated organisms with senescence phenomena akin to those in higher plants is discussed.