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Photosynthetic photon flux density (PPFD), air relative humidity and thallus temperature (TT) were measured in Usnea antarctica over a period of 12 months at a coastal rock on King George Island, South Shetland Islands. TT had an annual amplitude of c. 54 K with the maximum of +27.4°C recorded in December and minimum of −27.3°C in July. Daily maximum TT exceeded 0°C every month except in June and July when they were below −0.4°C and −1.6°C, respectively. Daily minimum temperatures were always below zero. Diel courses of PPFD showed pronounced seasonal differences between summer (more than 2000 μmol m−2s−1) and winter (less than 50 μmol m−2s−1 in July). Daily sum of PPFD was highest (more than 30 mol m−2d−1) in December and lowest (0.1 mol m−2d−1) in July. A photosynthesis model was used to estimate the potential annual primary production from habitat PPFD and TT. The estimated potential annual carbon balance of U. antarctica was 323 mg CO2 g−1 dry weight y−1 assuming that the lichen was always at optimal moisture. The potential carbon balance indicates that primary production is severely limited by low PPFD and subzero TT in the austral winter. Nevertheless, the PPFD and TT would allow metabolic activity during winter but with a negative carbon balance during May–August. Spring and autumnal months are revealed as probably the most important periods for lichen primary production because PPFD exceeds photosynthetic light compensation point during the daytime and ambient moisture conditions frequently favour a positive carbon balance.

The lack of a statistical record compels us to base many of our conclusions in matters of ancient history on inference from casual notices. These notices, often obscure, are liable to misinterpretation. Even when rightly interpreted, their authority and their bearing on each other are often doubtful. In particular, there is an ever-present difficulty arising from the loss of other evidence, probably larger in bulk and not inferior in value. The discovery of new evidence sometimes upsets conclusions that at an earlier stage of inquiry were reasonable enough. Re-examination of old evidence often modifies them. But an early conclusion sometimes for lack of sufficient challenge glides into an assumption: if it falls in with the general views of historians, it imperceptibly affects their attitude as inquirers and gains authority by lapse of time.

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