Stereocaulon virgatum Ach. has colonized lava flows deposited on the west flank of the volcano La Soufrière. The mean annual rate of acetylene reduction was 43·4 nmol C2H4 gdw−1 h−1 with maximum rates during the prevalent cloud/shroud meteorology of 101 and minimum rates during rare high insolation events of 0·63 nmol gdw−1 h−1. Percentage thallus moisture was the major variable controlling nitrogenase activity. During cloud/shroud conditions the upper 90% of the lichen canopy reduced 85% of the acetylene. Canopy shading reduced intra-canopy temperatures allowing the basal 10% of the canopy to fix nitrogen during insolation shocks. Basal portions of pseudopodetia exhibited reduced rates of ethylene production when exposed to canopy surface light intensities during cloud/shroud conditions. The recovery pattern of nitrogenase following desiccation during an insolation shock is presented. Rates of photosynthesis during cloud/shroud conditions were high, reaching 50% of those attained during saturating light intensities.

Thalli of five species of epiphytic lichens were collected from one moribund spruce-fir stand in the Algonquin Highlands in Ontario, Canada. Extracts of all were subjected to isoelectric focusing and stained for activity of 16 enzyme systems. Gels were scored for the presence or absence of individual bands in each enzyme system, and the degree of polymorphism of all detectable enzymes was evaluated using a variability measure developed for use with presence/absence data. Esterase and alkaline phosphatase showed the greatest amount of polymorphism and superoxide dismutase was the least variable enzyme system. The degree of enzyme variability in each of the five species was probably a reflection of its past evolutionary history and bore little relationship to apparent potential for sexual reproduction.

The response of net photosynthesis, dark respiration and acetylene reduction to temperature, moisture and light intensity were examined for Stereocaulon virgatum growing in the cloud/shroud zone on the tropical volcano La Soufrière, Guadeloupe, French West Indies. Rates for both acetylene reduction and net photosynthesis were maximal at saturating water contents, a pattern attributed to the finely branched nature of the phyllocladoid branchlets and the exposed position of spherical cephalodia, both of which minimize the formation of surface and interhyphal water films. Under conditions typical of those during cloud/shroud periods (13–16°C), thalli of S. virgatum exhibit many characteristics seen in other shade-tolerant lichen species. Net photosynthesis was light saturated at 300 μmol m−2 s−1 PAR, while the photocompensation point was less than 25 µmol m−2 s−1 PAR. Net photosynthetic uptake of carbon dioxide was optimal at 27–34°C, at which point light saturation was near 700 µmol m−2 s−1 PAR and the photocompensation point between 50 and 100 µmol m−2 s−1 PAR. Thalli of S. virgatin exhibited temperature-dependent sensitivity to high insolation. Only at 20°C were thalli able to tolerate high light exposure without reduction of apparent quantum yield. Exposure to high light intensity at 40°C inhibited the apparent quantum yield by almost 40% and acetylene reduction by 95%. This suggests brief periods of insolation shock may exert an influence disproportionately higher than either their frequency or duration. Thalli are normally exposed to cloud/shroud conditions but net photosynthetic uptake was maximal only during periods of elevated thallus temperature experienced at the onset of an insolation shock. However, with prolonged high insolation exposure and further elevation of thallus temperatures and thallus desiccation, severe impairment of subsequent photosynthetic activity ensues. S. virgatum may be characterized as a shade-tolerant species but its physiology is more adapted in some respects to conditions experienced during rare periods of full insolation.

The value of 19 epiphytic lichen species as air pollution indicators was tested in the Khamar-Daban mountain range in east Siberia. Frequency of occurrence of a species was used as the indicator characteristic. Altitude and distance from the pollution source were considered as environmental factors. The methodology of fitting log-linear models to contingency tables with presence/absence (frequency) scale as an additional dimension is presented. The significance of changes in species frequency is studied by examining the significance of the model parameters, which account for interactions of the environmental factors with the ‘frequency factor’. Only two species, Parmelia caperata and P. olivacea, were found to be good pollution indicators in that they showed a uniform monotonic (unidirectional) response to the factor of distance at all altitudes. The frequency of 12 species changed significantly and monotonically with distance in one or two altitudinal zones.