The ecology of Rhizocarpon superficielle. I. The rock surface boundary-layer microclimate

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Abstract
The thermal environment of the crustaceous lichen R. superficiale (Schaer.) Vain. is documented for spring and summer conditions using selected data characteristic of a range of thallus hydration conditions. During extended rainfall periods hydrated thallus temperatures largely parallel air temperature, usually remaining below 10.degree. C. However, quite often periods of shower activity alternate rapidly with intervals of full solar isolation, particularly during summer thundershower activity. This can result in hydrated thallus temperatures, fluctuating from 5.degree. to 20.degree. C within minutes. An equally wide range of hydrated thallus temperatures is also evident under spring snowmelt conditions. Under the high levels of convective cooling typical of the exposed habitat of R. superficiale, thallus temperatures under maximum solar radiation conditions in midsummer rarely rise above 32.degree. C. Under still conditions thallus temperatures rapidly reach a potentially stressful level of 45.degree. C and it is probable that strong convective cooling in summer plays an important role in the ecology of R. superficiale.