Net photosynthetic recovery in subarctic lichens with contrasting water relations

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Abstract
The rates of net photosynthetic recovery after wetting for six subarctic lichens were related to their drying rates under laboratory conditions. Net photosynthetic recovery was described by the three parameters of the Von-Bertalanffy equation: T o and K, related to the rates of resaturation respiration and gross photosynthetic recovery, and P max, the photosynthetic maximum attained at full recovery. The time to full photosynthetic recovery ranged from 143 to 510 min and was positively correlated with the drying rate of the thallus. In order from most to least rapid recovery, the species are Coelocaulon divergens, Cetraria cucullata, Alectoria ochroleuca, Cladina stellaris, Nephroma arcticum, and Cladonia sulphurina. In nature the high evaporative resistance or low waterholding capacity characterizing fast-drying species will result in short, frequent cycles of wetting and drying which induce carbon losses. In such situations a rapid photosynthetic recovery should be adaptive since it increases photosynthetic carbon gain during a period of metabolic activity. We hypothesize that fast-drying species achieve their rapid photosynthetic recovery by an increased desiccation-tolerance which has a metabolic cost associated with it. In slowdrying species a rapid recovery is not favored by natural selection since these species can take advantage of longer photosynthetic activity periods and are exposed less frequently to deleterious wetting and drying cycles. Future studies of lichen distribution and productivity should take into account the recovery phenomena.