FIELD-MEASUREMENTS OF WATER RELATIONS AND CO2 EXCHANGE OF THE TROPICAL, CYANOBACTERIAL BASIDIOLICHEN DICTYONEMA-GLABRATUM IN A PANAMANIAN RAIN-FOREST

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 premo ntane, tropical rainforest (Panama). Net photosynthesis (NP) was adapt ed to high temperatures and there was no depression of CO2 uptake at t he highest thallus hydration. The presence of a CO2 concentrating mech anism was demonstrated. Decreased NP occurred after desiccation, and a pparent photon yield of CO2 fixation was also sensitive to water loss. Natural NP was controlled by the interplay of thallus hydration and r adiation. Regular, daily desiccation and dense cloud cover suppressed production. On average, 72% of the diurnal photosynthetic gain was los t during the night, and there were days with negative carbon balance. Nevertheless, total carbon gain of the lichen was extremely high. A te ntative 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 elevat ion, would lead to a negative carbon balance. This might explain the l ack of abundance of this and other macrolichens in warmer, lowland rai nforests.