Epiphytische Flechten im Bereich einer chilenischen „Nebeloase“ (Fray Jorge) II. Ökophysiologische Charakterisierung von CO2-Gaswechsel und Wasserhaushalt

Abstract

In a previous communication (Part I) the epiphytic lichen vegetation in the region of the fog oasis Fray Jorge was reported. For 2 lichen communities, the habitat conditions have been characterized through microclimatological measurements. The present paper considers CO 2 gas exchange and water relations of these lichens. Under controlled conditions, photosynthesis and respiration are reported as influenced by water content of the thalli, light intensity, and temperature, for selected species ( Everniopsis trulla and Usnea lacerata from the Oropogonetum loxensis and Heterodermia spinulosa and Ramalina cactacearum from the Ramalinetum cactacearum). The maximal rates of net photosynthesis and dark respiration of the 4 species were very similar. They exhibited medium range of CO 2 exchange capacity when compared with maximum rates of species from other climatic regions. Moistened with liquid water, photosynthetic activity began at a thallus water content of 26 to 29 % (on a dry weight basis). Subsequently, net photosynthesis increased almost linearly with increasing water content. Very high water content resulted in a substantial depression of CO 2 uptake for the species of the Ramalinetum. This apparently is due to increased diffusion resistances within the fully-moistened thalli. Characteristically, such a depression is much smaller or nonexistent for the species of the Oropogonetum. Water vapor uptake in the absence of condensation by the dry lichen thalli can also enable photosynthetic activity. For example, Ramalina cactacearum already exhibited CO 2 uptake in equilibrium with a relative air humidity of 82 %, which corresponds to a water potential of —251 bar (at 10°C). In equilibrium with nearly saturated air, the lichens reached rates of photosynthesis which are similar to those which are attained at optimal water contents after moistening with liquid water. However with water vapor uptake, the same rates of net photosynthesis are possible at substantially lower thalli water contents than would be the case if the thalli were moistened with liquid water. Differences in activation of mitochondrial respiration and in the diffusive conductance of the thalli for CO 2 may be the reasons for thie phenomenon. This has important implications for production ecology. Based upon the CO 2 exchange measurements in the laboratory, the dependency of net photosynthesis of the experimental lichens on water status, light and temperature is presented in interpolation diagrams. These allow estimation of rates of CO 2 uptake for a given combination of environmental factors. With the weather data obtained from the field measurements it is thus possible at least approximately to simulate gas exchange performance of the lichens under their natural conditions. Fog condensation results in water contents of the lichen thalli which — according to the simulations — allow maximal rates of net photosynthesis. With typical fog weather conditions, a positive net CO 2 uptake is possible for the species of the Oropogonetum from briefly after sunrise until about 13 h when the moisture compensation point is finally reached. The daily photosynthetic CO 2 gain under such circumstances amounts to about 5.47 mg CO 2 per g dry weight. Conditions for production are much less favourable for the species of the Ramalinetum. However, the simulations show that in spite of the lack of moistening by fog in these habitats, water vapor uptake from the ambient air alone enables carbon gain in the morning. The duration and rate of net photosynthesis is very dependent on the humidity conditions during the night and on the rate of drying by solar radiation in the morning hours. Under conditions of high humidity at night, Heterodermia spinulosa attains a daily gain of 2.39 mg CO 2 per g. The capability of the thalli to use air humidity as their main water source and their ability to conduct positive photosynthesis at extremely low thallus water potentials, are necessary characteristics for the existence of these species in the Ramalinetum.