Photosynthetic productivity of the epilithic lichen Lecanora muralis: long-term field monitoring of CO2 exchange and its physiological interpretationIII. Diel, seasonal, and annual carbon budgets

Abstract

Summary Carbon dioxide exchange of the epilithic, green algal lichen Lecanora muralis and the weather conditions in its natural habitat in the Botanical Garden, Wurzburg (Bavaria, Germany), were monitored by means of an automatic cuvette. The continuity of the data over a total of 15 months of measurements enabled integrals to be calculated of diel, monthly, seasonal, and annual sums of photosynthetic carbon income (σNP), respiratory carbon loss (σDR) and carbon budget (net primary production, σC = σNP – σDR). Annual σNP for the period July 1995 through June 1996 was 50021 mg m −2 year −1 but respiratory processes lost 57% of this so that σC amounted to 21494 mg m −2 year −1 (20.7% of thallus carbon content). If summer 1996, which had a much smaller productivity than summer 1995, is included, annual σC was only 15.8% of carbon content for the period October 1995 through September 1996. The lichen showed extremely high variability in its daily CO 2 exchange, and performance depended almost completely on the momentary hydration conditions. Diel σNP and σC were not greatly influenced by thallus temperature, and manner of moistening was more important than high PPFD. Days with favourable primary production occurred in all seasons. Negative diel σC was frequent through the year and carbon loss occurred on 25%; of the 274 days of the year on which the lichen was metabolically active. The various weather response types, because of their characteristic influence on the mode and extend of thallus hydration, differed considerably in their contribution to the annual carbon balance of the lichen. 74 days of heavy rain during the year produced only 4.2%; of the annual σC, because, under such conditions, σDR was high and σNP was limited by depression of thallus gas exchange due to suprasaturation. In contrast, days with dew (40.0% of annual σC in 105 days) or those with nocturnal rain and subsequent drying (28.5% in 57 days) were the best contributors to annual σC. Differences in the frequency of occurrence of the different weather condition types through the year meant that lichen productivity apparently followed a seasonal pattern, each season resulting in a positive carbon balance. Autumn and summer 1995 were the most productive seasons, and the lowest was spring with no more than 18% of the autumn production. In summer, the season which was covered twice during the measurements, σC was less than one fourth in 1996 of that in 1995, underlining the high variability in the photosynthetic production of the lichen. Thus, firm generalisations are still not possible about seasonality of production for this species. There was no correlation between monthly σC and monthly rainfall. Due to the complex interdependencies of CO 2 exchange of L. muralis simple predictions of its carbon budget from meteorological standard data are likely to be very difficult or even impossible.