Influence of Stand Age And Structure on the Epiphytic Lichen ...

176 THE LICHENOLOGIST Vol.24
older forest, phorophyte specificity being a more important factor for it than
stand age.
Discussion
Although the lichen vegetation <strong>of</strong> the boreal forests has been surveyed quite
extensively in Northern Europe (e.g. Ahlner 1948; Koskinen 1955; Somermaa
1972; Esseen 1981) and North America (e.g. Ahti 1964; Kalgutkar & Bird 1969;
Yarranton 1972; Jesberger & Sheard 1973; Eversman et al. 1987), floristic
research has mostly concerned the southern subzones <strong>of</strong> the boreal forests and
includes little data with which the present results can be compared. Esseen
(1981) found 24 macrolichens on Norway spruce and 18 on Scots pine in
Central Sweden (middle and southern boreal subzones), these figures being
comparable to the numbers <strong>of</strong> macrolichens observed here when the genus
Cladonia is excluded. Moreover, Somermaa (1972) surveyed the epiphytic
lichen vegetation <strong>of</strong> several forest types in Estonia (hemiboreal zone, Ahti et al.
1968), where the number <strong>of</strong> epiphytic lichen species on Scots pines varied from
33 to 50 depending on the forest type. The differences between the total
numbers <strong>of</strong> epiphytic lichen species found by Somermaa (69 on Scots pines and
74 on Norway spruces) and the present figures are surprisingly small considering
the much more favourable macroclimate <strong>of</strong> the hemiboreal zone and the
higher number <strong>of</strong> forest types surveyed by Somermaa.
The differences in lichen vegetation between the end-points <strong>of</strong> the age gradient
are based on differences in species abundance rather than species composition.
The species dominating young stages <strong>of</strong> the pine stands, other than those
mentioned in the results, seemed mostly to have wide ecological amplitudes and
occurred quite uniformly throughout the age series. A direct visual survey <strong>of</strong>
the abundance <strong>of</strong> the species revealed some interactions between age and other
environmental parameters that did not emerge from the species ordinations.
For example, the abundance <strong>of</strong> B. furcellata increased along the age gradient in
those stands classified as ' managed ' but decrease in the ' near natural' stands.
The position <strong>of</strong> B. furcellata in the pine ordination thus indicated indifference
rather than a preference for young stands. In general, the slightly increasing
total cover <strong>of</strong> the epiphytic lichens with age was mostly caused by the increasing
cover <strong>of</strong> P. ambigua, which was second in abundance to H. physodes on both <strong>of</strong>
the phorophytes.
The observed pattern <strong>of</strong> vertical distribution <strong>of</strong> some <strong>of</strong> the epiphytes can
best be explained by the properties <strong>of</strong> the bark. According to Koskinen (1955),
the bark at the base <strong>of</strong> the trunk is moister and less acid because <strong>of</strong> the proximity
<strong>of</strong> the soil, and it may have a greater water-holding capacity (Kalgutkar & Bird
1969). The leprose growth forms <strong>of</strong> Pertusariaceae at least are known to be
adapted to moist conditions (Halonen et al. 1991). Ochrolechia androgyna,
which was more frequent on the lower trunk and <strong>of</strong>ten grows on moss, provides
a good example <strong>of</strong> this feature. The winter snow cover, giving shelter from cold
temperatures, could be a further explanation (Barkman 1958). Wirth (1987)
argues that P. hyperopta, which is a relatively weak competitor on the upper
parts <strong>of</strong> the trunk, is highly abundant on the base because it is well adapted to