Mikroklima, Wassergehalt und Photosynthese von Flechten in der küstennahen Nebelzone der Namib-Wüste: Messungen während der herbstlichen Witterungsperiode

Abstract

The coastal fog zone of the Namib Desert (Namibia, Southwest Africa) where there are hardly any higher plants, is known for its rich lichen vegetation. The functional mechanisms that allow existence of these lichens under the special conditions of the “fog desert” are investigated. The study is part of our general efforts to analyse the ecological basis of the photosynthetic production of lichen-dominated habitats in order to explain physiological factors underlying the success of the lichens. During the fall, in April 1988, microclimate, water relations, and photosynthesis of 10 characteristic species were studied in their natural environment in one of the lichen fields north of Swakopmund. Time courses of CO2 exchange and water content were used to estimate the daily primary production and provide information about light dependence of CO2-assimilation of well-hydrated thalli, and about their light (LKP) and moisture compensation point (FKP). Fog is the most important source of water for these lichens. However, dew condensation without fog also results in high degrees of hydration. Maximal water contents of more than 150% of dry weight of the thalli are reached. Soon after sunrise, the hydrated lichens show a steep increase in the rate of net photosynthesis which initially is determined by the increase of photosynthetically active radiation. Subsequently, the lichens lose water, and metabolic activity is then limited by their hydration. Typically, photosynthesis of the drying lichens ceases 2 to 4 hours after sunrise and the water content of the thalli drops below 10%. In addition to hydration through liquid water, water vapor uptake in air of high humidity alone can reactivate the photosynthetic apparatus of the lichens. This may result in a second, smaller peak in CO2 uptake at late afternoon. This general diurnal pattern of photosynthetic activity is modified according to the actual weather conditions. There occurred one day during the measuring period where no CO2 was gained. The characteristics of a species, especially its growth form, determine maximal rate of photosynthesis and the ability of the lichen to make use of the available moisture. The multibranched fruticose species (e.g., Teloschistes capensis, Alectoria spec., Ramalina lacera) reach higher rates of net photosynthesis than the more compact foliose lichens (e.g., Xanthomaculina convoluta, X. hottentotta, Xanthoparmelia walteri), both their maximal apparent quantum use efficiency (initial slope of the light response curve) and their maximal photosynthetic rates at light saturation being greater. Light saturation of the whole thalli in their natural position takes place between 583 (Xanthomaculina hottentotta) and 1856 μE m−2 S−1 (Ramalina lacera) photosynthetically active radiation. LKP varies between 16.3 (Caloplaca elegantissima) and 31.5 μE m−2 S−1 (Xanthomaculina hottentotta), FKP after desiccation in the morning lies between 15.0 (Santessonia hereroensis) and 25.6% (Ramalina lacera) water content relative to dry weight of the thallus. Caloplaca elegantissima, the only crustose species studied which grows on stone pebbles, is characterized by medium rates of photosynthesis when related to dry weight or carbon content of the thallus and by high rates when related to chlorophyll content. Its LKP is very low. In the most productive species Teloschistes capensis, the maximal daily carbon gain during the daylight period amounts to about 0.25% of the thallus carbon content. This is ten times the relative carbon gain of the Xanthomaculina species under the same microc1imatic conditions. The results show that primary production of the cushion forming, fruticose growth form of the lichens is superior in the habitat with heavy fog formation near the coast. This accords with the fact that Teloschistes capensis is the dominant species in these lichen fields. Continuation of the work will be extended over other seasons of the year and will concentrate further on the analysis of the different growth forms as adaptations to the extreme environment. Field investigations will be complemented by measurements of the factor dependency of CO2 exchange under controlled conditions in the laboratory.