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

Diel (24-h) time courses of microclimate, water relations, and CO2 exchange were measured under quasi-natural conditions at a forest edge in a lower montane, tropical rainforest in Panama for six Lobariaceae (Lobaria crenulata, L. dissecta, Pseudocyphellaria aurata, P. intricata, Sticta sublimbata, S. weigelii). Responses to experimentally controlled water content (WC), photosynthetic photon fluence rate (PPFR), and temperature were studied in most detail with P. aurata. Photosynthesis was well adapted to high temperatures, and all species exhibited 'shade plant' characteristics with low light compensation points and low light saturation. Lobatia and Pseudocyphellaria species suffered from a strong depression of net photosynthesis (NP) at suprasaturating WC; suprasaturation depression was less in cyphellate Sticta species. Photosynthetic capacity correlated with thallus nitrogen concentration, and maximal NP rates of the cyanobacterial Sticta species was 4 to 5 times higher than that of the green algal Lobatia species. However, high rates of NP were uncommon and brief events under natural conditions; the different environmental factors were rarely optimal simultaneously. Similar to earlier observations with other rainforest lichens, NP ceased during the period of highest irradiation on most days due to desiccation. During moist periods low light often limited carbon fixation, and high thallus hydration was often detrimental to NP. In spite of these limitations the maximal daily integrated net photosynthetic carbon income (SigmaNP) was quite high especially for the Sticta species [17(.)3 and 24(.)1 mgC (gC)(-1) day(-1) for S. sublimbata and S. weigelii, respectively]. High nocturnal carbon loss, due to high night temperatures and continuous hydration, resulted in frequent negative diel carbon balances (SigmaC) in all species. The average nocturnal carbon loss amounted to 83 and 70% SigmaNP for P. aurata and P. intricata, respectively and to 64 and 59 % of SigmaNP for S. sublimbata and S. weigelii, respectively. Their average diel SigmaC was as high as 3(.)7 and 5(.)3 mgC (gC)(-1) day(-1). In contrast, SigmaC was much lower for the other species, it amounted to only 0(.)18 mgC (gC)(-1) day(-1) for L. crenulata. Thus, the Sticta species stood out amongst the species studied for their most successful adaptation of photosynthetic productivity to the habitat conditions in the lower montane rainforest.