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Abstract: Global temperature rise is suggested to be greater and more rapid in polar regions. There has been a clear temperature rise of 0.056°Cy−1 in the Antarctic Peninsula and this has led to changes in higher plant extent and range. In the more extreme environments of the main continent the vegetation is scattered and composed of lichens and mosses. There is interest in the possible effects of global climate change on these communities acting through changes in temperature and precipitation. Lichens have been extensively used to date the substrates on which they are growing using the techniques of lichenometry. The slow growth and longevity of lichens particularly suites them for this use. We present evidence that there appears to be a substantial (two orders of magnitude) cline in lichen growth rate from the warmer, wetter and more productive Peninsula to the cold Dry Valleys at 77°S latitude. The differences in growth rate reflect the precipitation and temperature regimes at the different sites. The large range in growth rates coupled with the simplicity of measuring lichen growth using modern techniques suggests that this could be an excellent tool for the detection of climate change in continental Antarctica. [Copyright &y& Elsevier]

Abstract: Although there is only negligible rainfall, frequent nocturnal fog, dew and high air humidity support a luxurious lichen vegetation in the coastal zone of the central Namib Desert (Namibia). In earlier publications, we have studied ecophysiological performance of a series of epilithic and terrestrial lichens. Here, we have extended this work to three epiphytic species (Heterodermia namaquana, Ramalina lacera, and Xanthoria turbinata) that inhabit the sparse perennial shrubs growing in this area. Our intention, monitoring lichen CO2 exchange, their water relations and microclimate conditions, was to determine the functional mechanisms that allow these epiphytes to exist under the special conditions of a fog desert. Measurements were conducted mainly during the spring season. The epiphytic lichens showed response patterns very similar to the epilithic and epigaeic species at the same site. Their metabolism was activated through moistening by dew and/or fog during the night and, in the very early morning, they exhibited the typical brief peak of net photosynthesis (NP) between sunrise and desiccation. The thalli were almost completely dry for the remainder of the day. Average duration of the positive NP during the morning peak was about 3h. Dew condensation, alone, resulted in activation that provided 58–63% of integrated carbon income (ΣNP) as compared to fog (plus dew). In the late afternoon, there was a tendency for hydration to increase again, due to water vapour uptake at higher air humidity, and this allowed on some days a brief additional period of very low rates of photosynthesis shortly before sunset. Light response of photosynthesis showed “sun-plant” characteristics with saturation around 1000μmolm−2 s−1 photosynthetically active photon flux density (PPFD). Light compensation point (LCP) of CO2 exchange after sunrise was highly dependent on actual water content (WC) for X. turbinata: at low hydration it was ca. 10μmolm−2 s−1 PPFD whilst, at high WC, it was almost 80μmolm−2 s−1 PPFD. In contrast, LCP of R. lacera was almost independent of WC. This phenomenon was probably due to differences in thallus structure. Maximal attained NP and daily ΣNP both showed a saturation-type response to previous maximal nocturnal WC. Neither parameter was increased substantially when higher maximal thallus WCs were produced by experimental moistening in the night. All three species, despite their different morphologies, performed optimally at the highest nocturnal moistening achieved by natural fog and were not able to make use of higher hydration. The three studied epiphytes were similar in their chlorophyll-related rates of NP. Due to lower chlorophyll content, dry weight and carbon-related NP of X. turbinata was only about one-third of that of the other two species. The average carbon income on days with fog and/or dew hydration during the spring season amounted to 2.4 and 2.1mgC(gC)−1 day−1 (related to thallus carbon content) for H. namaquana and R. lacera, respectively. This primary production was of similar magnitude to those found for the terrestrial species at the same site. [Copyright &y& Elsevier]

Abstract: Although the coastal zone of the Central Namib Desert (Namibia) has negligible rainfall, frequent fog, dew and high air humidity support a luxurious lichen flora. Large areas of soil crust communities are dominated by the multibranched, fruticose Teloschistes capensis interspersed by a (still indeterminable) Ramalina species. In earlier communications, based on field measurements in autumn, we began the analysis of functional mechanisms that allow these lichens to exist under the special conditions of a fog desert. We have extended this work by monitoring lichen CO2 exchange and water relations in spring and by experiments under controlled conditions. In both seasons, nocturnal hydration, by fog and/or dew, activated dark respiration of the lichens which was followed, after sunrise, by a short period of positive net photosynthesis (NP) that continued until metabolic inactivation occurred from desiccation. Dry thalli of T. capensis were able to reactivate NP through water vapour uptake alone, beginning at an air relative humidity of 82%, i.e. at a water potential of −26.3MPa; the moisture compensation point during desiccation was at 13% thallus water content (WC, dry weight related). Optimal WC for photosynthesis was around 100%, and both species showed a large and extended suprasaturation depression of CO2 assimilation. Light response showed “sun-plant” characteristics with saturation >1000μmolm−2 s−1 photosynthetically active photon flux density (PPFD). However, due to rapid desiccation, the combination of light saturation with optimal WC very rarely occurred under field conditions. Light compensation point after sunrise was highly dependent on actual WC: at low hydration, it amounted to only ca. 10μmolm−2 s−1 PPFD so that even the smallest levels of hydration could be used for carbon gain before desiccation took place again. This phenomenon was probably due to a hydration gradient in the thallus branches during transient moistening so that the outer photobiont layer was favoured in contrast to the internal mycobiont which remained dry longer and did not contribute respiratory CO2 loss. Fully hydrated thalli had light compensation points around 50μmolm−2 s−1 PPFD. Extended desiccation of 1–3 days had no impact on the magnitude and recovery of photosynthesis but, imposed desiccation of 10 days reduced NP in lab and field experiments and caused an extended period of recovery. “Resaturation respiration” was not detected in the field data, although it was present after experimental moistening of dry thalli. In spring, the higher fog frequency and intensity increased maximal nocturnal WC, maximal attained NP as well as integrated daily carbon income (ΣNP) compared to the autumn measurements. NPmax and ΣNP depended on maximal nocturnal WC with a saturation-type response. In terms of carbon gain both species seem to be optimally adapted to nocturnal moistening up to 160% WC and were not able to make use of higher degrees of hydration, a feature that might well influence their habitat selection. Maximal daily carbon-related ΣNP for T. capensis was 4.6mgC (gC)−1 day−1. A rough estimate of the annual (projected) area-related carbon balance (photosynthetic income minus respiratory losses) based on published fog and dew frequencies and personal observations was 15–34mgC m−2 yr−1. [Copyright &y& Elsevier]

Abstract: CO2 exchange and water relations of selected lichen species were investigated in the field and also in the laboratory, at a height of 3106m above sea level in the Austrian Alps, during the short snowless summer period from middle of July to the end of August. In the course of the field investigations, clear summer days were quite rare. Altogether 14 diurnal courses of CO2 exchange were measured spanning a time of 255h of measurements. The air temperatures measured close to the ground ranged between −0.7 and 17.1°C and their daily fluctuation was lower than 10.7°C. Fog was present for more than one-third of the measuring period and relative humidity (RH) exceeded 90% in almost half of the time. Temperature optimum of net photosynthesis (NP) of Xanthoria elegans and Brodoa atrofusca determined in the laboratory increased with increasing photosynthetic photon flux density (PPFD) from 1.5 to 11.3°C and the maximal CO2 uptake was found to be at 10°C. In the field the lichens were metabolically active at air temperatures between −0.7 and 12.8°C. The light compensation points (LCP) of both lichen species ranged in the laboratory between 50 and 200μmolm−2 s−1 PPFD (0–20°C) and in the field between 22 and 56μmolm−2 s−1 PPFD (3–8°C). At 30°C the NP of X. elegans surpassed the LCP, whereas B. atrofusca remained below the LCP. NP in X. elegans did not reach light saturation at 1500μmolm−2 s−1 PPFD. NP in B. atrofusca reached light saturation at low temperatures (−5 to +5°C). At higher temperatures light saturation was almost detectable. On sunny days the lichens in the field were metabolically active only for 3h during the early morning. In this time they reached the maximal values or values close to their maximal CO2 uptake in situ. Under dry weather conditions the lichens dried out to a minimal water content (WC) of 5–12% which is below the moisture compensation point (MCP) of 34–25%. The optimal WC was between 90% and 120% dry weight (DW) in B. atrofusca and Umbilicaria cylindrica, in X. elegans between 140% and 180% DW. Species specific differences in water-holding capacity, desiccation intensity and in the compensation points of temperature, light and moisture are responsible for differences in metabolic activity. The lichens were active during less than half of the observation time. Total time of NP of X. elegans was 24% of the measuring period, for U. cylindrica 22% and for B. atrofusca 16%. [Copyright &y& Elsevier]