IN-SITU STUDIES OF WATER RELATIONS AND CO2 EXCHANGE OF THE TROPICAL MACROLICHEN, STICTA-TOMENTOSA

Diel (24-h) time courses of CO2 exchange, water relations, and microcl imate of the foliose lichen, Sticta tomentosa (Swartz) Ach., and respo nses to experimentally manipulated conditions were measured at a fores t edge in a lower montane rainforest in Panama. Similar to earlier obs ervations on two other rain forest lichens, daily desiccation suppress ed net photosynthesis (NP) during the period when irradiation was high est. Not surprisingly, the light response curves of NP showed saturati on at rather low light levels. Rehydration was associated with an init ial resaturation burst of short duration, which could be demonstrated both under natural conditions and experimentally. This additional loss of CO2 seems too low to be ecologically relevant. Moreover, high thal lus hydration was also detrimental to NP: at maximum water content net CO2 uptake was depressed by > 50%. Although NP was well adapted to th e prevailing high temperatures, the latter also stimulated dark respir ation. On average, almost 60 % of the diurnal carbon gain was lost dur ing the night. In spite of these limitations, the integrated 24-h C ga in was quite high, on average 0.5 % of the thallus C content. Whilst t hese figures were determined for horizontally exposed samples, we also assessed the role of different exposures on photosynthetic performanc e. Diel C gain was highest under conditions of semi-shade (westerly ex posure), which allowed long periods of activity, whilst much higher ir radiance at other exposures could not be utilized for photosynthetic p roduction: easterly exposed thalli dried out even faster than horizont ally exposed samples.