G. Zotz et al., IN-SITU STUDIES OF WATER RELATIONS AND CO2 EXCHANGE OF THE TROPICAL MACROLICHEN, STICTA-TOMENTOSA, New phytologist, 139(3), 1998, pp. 525-535
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.