The Thermal Operating Environment of Corticolous and Pendulous Tree Lichens

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Abstract
The thallus temperatures of Hypogymnia physodes, Letharia vulpina and Usnea fulvoreagens during full solar radiation in contrasting canopy positions are compared with corresponding air temperature data. The thallus temperature of Hypogymnia growing in the boundary layer of the bark surface is always well above air temperature despite strong convectional cooling by wind in the canopy. Hypogymnia at ground level achieves even higher thallus temperatures in the absence of any marked degree of convective cooling. Usnea with its very fine branches and resultant high surface area to weight ratio radiates energy very efficiently and remains very close to air temperature. Letharia with its bulkier thallus and caespitose habit occupies and intermediate position with thallus temperatures rising to .apprx. 8.degree. C under full radiation conditions. The data are discussed in relation to the thermal limits and net photosynthetic temperature optima found in lichens, and the importance of defining the thermal operating environment of lichens rather than the overall latitudinal climate is emphasized.