The Physiological Response of sub-Arctic Lichens to Their Abiotic Environment

Abstract

The effect of environmental factors on metabolic processes of sample thalli, of four sub-Arctic lichen species from Abisko, Sweden (68.37˚N, 18.69˚E). Nephroma arcticum, Cladina mitis, C. stellaris and C. rangiferina, which are found in contrasting niche habitats within the same ecosystem, have been studied. An oxygen electrode apparatus has been used to study the effects of air temperature, thallus irradiance intensity and climatic regime during thallus storage, on the rate of net photosynthesis and dark respiration of sample thalli, in a series of studies on the four lichen species named above.
Environmental temperature and light intensity both significantly affected the rate of thallus net photosynthesis (NP) in samples of all four species. The mean rate of thallus NP was significantly depressed at 17⁰C in all four species due to an exponential increase in respiratory rate with rising temperature. Maximal NP was achieved at 5⁰C in N. arcticum, 10⁰C in C. rangiferina, and NP at 5 & 10⁰C were not significantly different in C. mitis and C. stellaris. Optimum light intensity for photosynthesis was 340 µmol m-2 s-1 in all four species; however, temperature significantly increased the light compensation point of photosynthesis. Furthermore, mean light compensation points were greater in the more shaded adapted species N. arcticum and C. mitis; suggesting, as found in vascular plants, lichens exhibit photosynthetic characteristics adapted to their abiotic environment. C. mitis thalli did not provide any evidence of plastic adaptation to temperature over a six day storage period; whereas, N. arcticum samples exhibited increased NP after six days at 10 & 15⁰C compared to non-acclimatized samples. Prolonged storage at 10⁰C appeared to be detrimental to samples of C. stellaris. Responses to temperature and light intensity were similar to those found, in similar species, in photosynthetic rate investigations utilizing IRGA apparatus; validating the use of O2 electrode experiments using small samples of thallus tissue, as used in this investigation.
The response of species, within this study, to environmental factors was slightly different to those obtained in similar studies for North American populations, which suggests that lichens may exhibit some within-species genetic variation between lichen populations from different continents. Analysis of moisture availability, from past climate-data of the region from which the lichen populations are found, suggests that the growth season of sub-Arctic lichens is very different to that of vascular plants that are within the same ecosystem. Future warming may cause longer, wetter Arctic Autumns and Springs, which would benefit lichen biomass, but environmental warming would suppress lichen growth and promote vascular plant productivity.