Field Measurements of Water Relations and CO2 Exchange of the Tropical,Cyanobacterial Basidiolichen Dictyonema glabratum in a Panamanian Rainforest*

Diel time courses of microclimate, hydration, and CO₂ exchange of the basidiolichen Dictyonema glabratum and its responses to experimentally changed conditions were measured for 14 days in a clearing of a premontane, tropical rainforest (Panama). Net photosynthesis (NP) was adapted to high temperatures and there was no depression of CO₂uptake at the highest thallus hydration. The presence of a CO₂ concentrating mechanism was demonstrated. Decreased NP occurred after desiccation, and apparent photon yield of CO₂ fixation was also sensitive to water loss. Natural NP was controlled by the interplay of thallus hydration and radiation. Regular, daily desiccation and dense cloud cover suppressed production. On average, 72 % of the diurnal photosynthetic gain was lost during the night, and there were days with negative carbon balance. Nevertheless, total carbon gain of the lichen was extremely high. A tentative estimation suggests that annual carbon gain amounts to 228 % of initial thallus carbon content. Because of their strong effect on net production, increased nocturnal temperatures as given at lower elevation, would lead to a negative carbon balance. This might explain the lack of abundance of this and other macrolichens in warmer, lowland rainforests.