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Diel time courses of microclimate, hydration, and CO2 exchange of Leptogium azureum, L. cyanescens, and L. phyllocarpum (homoiomerous cyanolichens) were measured under quasi-natural conditions at a forest edge of a lower montane, tropical rain-forest (Panama). In addition, responses to experimentally controlled water content (WC), photosynthetic photon flux density (PPFD), and temperature were studied for L. phyllocarpum. Performance of the Leptogium species was compared with two other, but heteromerous, cyanolichens from the same site and treated in earlier publications (Dictyonema glabratum, Sticta tomentosa).

Net photosynthesis (NP) of L. phyllocarpum was adapted to high temperatures with an upper temperature compensation point well above 40 degrees C. The light saturation of NP was highly dependent on WC and occurred at PPFD levels between 100 and 600 mu mol m(-2)s(-1). Light compensation point was about 20 mu mol m(-2)s(-1) and increased with decreasing WC. All three Leptogium species suffered from a strong depression of NP at suprasaturating WC, which reduced CO2 assimilation by 55 to more than 80%, compared to the maximum.

Natural NP was controlled by the interplay of thallus hydration and radiation. In contrast to the heteromerous species, high water holding capacity of the gelatinous lichens, especially of L. phyllocarpum, shortened the periods of inactivity through desiccation, thus essentially extending the daily time span for photosynthetic activity. However, high WC reduced the rates of CO2 fixation. A rough estimate for L. azureum reveals that net photosynthetic carbon gain would be increased by about one third in the absence of suprasaturation depression.

In spite of these limitations, average daily net photosynthetic carbon gain of mature thalli of all three Leptogium species was relatively high [between 6.2 and 9 mgC (gC)(-1)d(-1), as related to thallus carbon content]. However, a very large portion of assimilated carbon - on average 60 to 90% - was lost again through nocturnal respiration which was stimulated by high night temperatures of the continuously moist thalli. The resulting diel carbon balance amounted from 0.6 to 3.6 mgC (gC)(-1)d(-1) Abundance of Leptogium species and of other macrolichens was high in the lower montana forest, it was low in warmer lowland rainforests of the same area. Based upon a literature review for lichens under different climate conditions the existing hypothesis is discussed to what extend this phenomenon might be explained through negative carbon balances due to temperature-induced increases in nocturnal respiration.