The survival potential of lichens in a given habitat is determined by the response of CO exchange to photosynthetically active radiation (PhAR), thallus temperature, and thallus relative water content (RWC). Therefore morphologically similar lichens from contrasting climatic environments 1) should differ in their CO exchange responses, and 2) these differences should reflect adaptations to their climatic regimes. The CO exchange responses of a subarctic (55ยฐN, 67ยฐW) Cladina stellaris (Opiz) Brodo population and a temperate (29ยฐN, 82ยฐW) Cladina evansii (Abb.) Hale and W. Culb, population were used to test these two related hypotheses.Infrared gas analysis with lichens collected in September-October 1975 established that the two populations differed in their responses to incident PhAR, thallus temperature, and thallus RWC. Net photosynthesis in C. stellaris had an optimum at a lower temperature and a greater relative photosynthetic capacity at low temperatures than did C. evansii. Cladina evansii maintained net photosynthesis above 35ยฐC thallus temperature; C. stellaris did not. In both species the optimum temperature for net photosynthesis increased with increasing irradiance. The C. stellaris light saturation point was consistently lower than that of C. evansii. Both species had maximal rates of net photosynthesis at 70-80% relative water content. In C. evansii the CO exchange rates, expressed as percentages of the maximum rate, declined more rapidly under suboptimal conditions. The absolute CO exchange rates of C. evansii were greater than those of C. stellaris. At 20ยฐC and 90-95% RWC, resaturation respiration occurred in both species and continued until 6-7 h after wetting.Contrasts in the temporal patterns of thallus condition at each collection site suggest that not all differences in the two response surfaces reflect climatic adaptation. The two populations appear well adapted to incident PhAR and thallus temperature regimes but the 70-80% RWC optimum for net photosynthesis common to both species is puzzling since their water regimes differ markedly. The overall adaptedness of the CO exchange responses in the two species cannot be judged without a comprehensive quantitative analysis of carbon balance under differing climatic regimes.