Seasonal Change in the Pattern of Net Co2 Exchange in Umbilicaria Lichens

SUMMARY The response of net photosynthesis and respiration in five species of Umbilicaria to thallus moisture content at three temperatures (0, 14, 28 °C) and four light intensities (0, 100, 330, 700 μmol m‐2 s‐1) was studied in collections made in summer and winter. The pattern of response to these variables suggests that each species is uniquely adapted to the cliff environments in which they grow. A simple model suggests further that each species can maintain a positive energy balance provided that its distribution does not extend to sites with temperatures in excess of certain values. None of the species show photosynthetic acclimation to temperature, yet they all exhibit large seasonal changes in their response structures. The significance of such patterns is discussed.