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The gas exchange of six temperate rain forest lichens was measured in the field in the Urewera National Park, New Zealand. Two shade species, Collema laeve and Sticta Filix, were measured only in the forest; two open ground species, Cladia retipora and CI. aggregata, only outside the forest and two marginal species, Pseudocyphellaria colensoi and P. faveolata, at both locations. The diel measurements for each species were analysed so that a net photosynthesis (Pn) response to both thallus water content (WC) and light (PFD) could be obtained. It proved possible to delineate limits and to determine WC/PFD combinations that would allow maximal Pn at the given habitat conditions. It was found that the lichens were rarely between 3-13% of measurements, at maximal Pn. Lichens were limited mainly by low PFD and high WC in the forest, and by low WC and low PFD outside the forest. At WC which allowed positive Pn, the two open ground Cladia species were unaffected by full sunlight whereas the forest margin lichens were damaged by high PFD within one day. It is suggested that evidence is growing that lichens are rarely able to experience situations where Pn would be maximal. in these forests high production is possible because of the long periods during which the lichens are wet so that Pn continues for long periods although often at much lower than maximal potential.