The ecology of Rhizocarpon superficial. II. The seasonal response of net photosynthesis and respiration to temperature, moisture, and light

The seasonal gas-exchange response matrix for the crustaceous lichen Rhizocarpon superficiale (Schaer.) Vain, is presented. Maximum rates for net photosynthesis of ca. 1 mg CO2 g−1 h_1 are generated at 900 μE m−2 s−1 illumination between 50 and 60% relative thallus moisture content and at thallus temperatures of 14 °C down to 1 °C, during all seasons of the year. This broad response to temperature as well as the apparently high resistance to heat stress in Rhizocarpon is discussed in relation to the special characteristics of its boundary-layer habitat. The absence of any seasonal photosynthetic capacity change is discussed in terms of the constraint imposed by the random fluctuations in the thermal operating environment of the lichen. We postulate that the predictability of the operating environment largely determines the extent and the level of any photosynthetic adaptation.