Jari Oksanen, Toini Holopainen, Tupuna Rovanen, Mikko Kuusinen, Kari Mikkola, Rauni Ohtonen, Jarmo Poikolainen, Aulis Ritari, Marja-Liisa Sutinen, Sari Tarhanen, Henry Väre
Epiphytic lichens growing on the stems and branches of trees use trees
only as their substrate. They are not parasitic and they do not harm the
host trees. Instead, they get their nutrients from rain water and
deposited dust. Epiphytic lichens are very effective in absorbing
nutrients, and this is necessary for their survival and growth. Along with
essential nutrients, lichens absorb many non-essential or harmful
substances as well.
Lichens are the result of symbiosis between algae and fungi. The
thallus of a lichen is actually a minute ecosystem in which the primary
producer, an alga, and a first-degree consumer, a lichen-forming fungus,
live in close interaction. The alga is able to produce carbohydrates by
photosynthesis. The fungus depends on the alga for its supply of
carbohydrates. The symbiotic balance between the alga and the fungus is
fragile and unfavourable conditions can break it down. The more
sensitive partner thus determines the fate of the lichen. Conditions
favourable for one of the two can also lead to the breakdown of the
symbiosis and the death of the lichen. An improvement in the supply of
nutrients, for instance, may enhance the growth of the alga but be
harmful to the fungal partner.
The ability to absorb harmful substances, and the fragility of
symbiosis between the alga and the fungus, make lichens sensitive to air
pollution. Reduction in the number of species (the disappearance of
certain sensitive species), a decrease in the quantities of those surviving,
and the occurrence of pollution-resistant lichens and algae help in
identifying areas exposed to air pollution. Therefore, the lichen
distribution studies (lichen mapping) are among the most widely used
bio-indicator methods.
Pollution injuries can be observed in lichens at the cellular level long
before external damage is apparent. Some conclusions on causes of the
damage can be drawn on the basis of the diagnostic symptoms. This
requires the use of an electron microscope and/or physiological
measurements.
Epiphytic lichens are highly useful bio-indicators. In addition they are the
wintertime fodder of reindeer in northern regions. Consequently, changes in
the abundance or condition of lichens can have direct impacts on reindeer husbandry.
Terricolous lichens are, nevertheless, the more important lichen category in
this respect. The degradation of lichen pastures due to the effect of airborne
pollutants or grazing by reindeer can also influence the soil's temperature
and moisture conditions, microbial activity, nutrient status, and the frost-hardiness
of plant roots (Figs. 36 and 37).
The results of membrane-leakage tests of epiphytic lichens also indicated
that changes had taken place in Inari, Monchegorsk and Salla (Fig.
39). While the natural variability in the physiology of lichens complicated
the interpretation of the results, it was apparent that the emissions of pollutants
from the Kola Peninsula had caused changes in the functioning of cell membranes
in lichens. An increase in the wet and dry deposition of sulphur leads to a
rise in the concentration of sulphur in lichens and to specific changes in lichen
ultrastructure. The characteristic combination of symptoms was observed in epiphytic
lichens throughout the Inari and Salla regions of Finnish Lapland and in the
vicinity of Monchegorsk. The effects of local pollution sources in Finland were
also observed (Figs. 40 and 41).
Furthermore, the ultrastructure of the lichens indicated that nitrogen deposition
south of Salla was greater than elsewhere within the study area. The light staining
of the pyrenoglobules in the chloroplasts and other associated structural changes
in the vicinity of Monchegorsk constitute a hitherto unknown symptom combination.
Experimental research has not yet confirmed the cause of these changes. Presumably,
heavy metals or the combination of heavy metals and sulphur play a role in the
development of these symptoms.
The pine stands on dry and dryish heath forests in the western parts of the
Kola Peninsula clearly differ in their vegetation from corresponding sites in
Finnish Lapland. Intensive grazing by reindeer in Finnish Lapland has changed
the species composition of ground vegetation so much that reindeer lichens (especially
Cladina stellaris) have decreased in abundance and the growth space thus liberated
has been invaded by mosses. The low lichen cover in Finland, compared to the
Kola Peninsula, was found in analyses of satellite imagery and vegetation analyses
of the lichen sample plots and along Line #2 (Figs. 42
and 43). Temperature measurements conducted
in Salla in 1990 and 1991 on the top layer of the soil on grazed and ungrazed
lichen sample plots showed that the lichen layer clearly is an insulating mulch
and that this is important especially during autumn frosts before there is a
cover of snow over the ground (Fig. 44).
Differences in the activity of the soil microbes, and thereby in decomposition
rate in the soil, were found between grazed and ungrazed lichen sample plots.
The metabolic index and biomass of microbes in soils under lichen cover were
at higher level and the time passed from the addition of glucose to the beginning
of microbial growth at lower level compared to soils without lichen cover (Fig.
45a and b). One reason for this may be in the stability of temperature and
moisture conditions due to the protection provided by the lichen cover when
compared to the greater diurnal fluctuations in soils lacking this cover.
Between September 1990 and February 1992 the frost-hardiness of
the roots of pine trees on the Naruska lichen sample plots was at its
lowest
in September (-5 °C) and at its highest in December (-21 °C). The
differences in frost-hardiness among grazed and ungrazed sample plots
were not statistically significant. However, the insulating effect of the
lichen cover becomes important when there is no snow and the ability of
roots to withstand frost may be inadequate.
