Lichen carbon gain under tropical conditions: water relations and CO2 exchange of three Leptogium species of a lower montane rainforest in Panama

Diel time courses of microclimate, hydration, and CO2 exchange of Leptogium azureum, L. cyanescens, and L. phyllocarpum (homoiomerous cyanolichens) we re measured under quasi-natural conditions at a forest edge of a lower mont ane, tropical rain-forest (Panama). In addition, responses to experimentall y controlled water content (WC), photosynthetic photon flux density (PPFD), and temperature were studied for L. phyllocarpum. Performance of the Lepto gium species was compared with two other, but heteromerous, cyanolichens fr om 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 leve ls between 100 and 600 mu mol m(-2)s(-1). Light compensation point was abou t 20 mu mol m(-2)s(-1) and increased with decreasing WC. All three Leptogiu m species suffered from a strong depression of NP at suprasaturating WC, wh ich reduced CO2 assimilation by 55 to more than 80%, compared to the maximu m. Natural NP was controlled by the interplay of thallus hydration and radiati on. In contrast to the heteromerous species, high water holding capacity of the gelatinous lichens, especially of L. phyllocarpum, shortened the perio ds of inactivity through desiccation, thus essentially extending the daily time span for photosynthetic activity. However, high WC reduced the rates o f CO2 fixation. A rough estimate for L. azureum reveals that net photosynth etic carbon gain would be increased by about one third in the absence of su prasaturation depression. In spite of these limitations, average daily net photosynthetic carbon gain of mature thalli of all three Leptogium species was relatively high [betwe en 6.2 and 9 mgC (gC)(-1)d(-1), as related to thallus carbon content]. Howe ver, a very large portion of assimilated carbon - on average 60 to 90% - wa s lost again through nocturnal respiration which was stimulated by high nig ht 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 litera ture review for lichens under different climate conditions the existing hyp othesis is discussed to what extend this phenomenon might be explained thro ugh negative carbon balances due to temperature-induced increases in noctur nal respiration.