INTRODUCTION
The lichen cover protects roots againts sudden and extreme changes in temperature
and soil water status. This is meaningfull especially during the fall and early
winter, when the air temperature is below zero and the protecting snow cover is
still missing. The effect of lichen cover on the seasonal changes in soil
temperature and frost hardiness of adult Scots pine (Pinus sylvestris L.)
trees was studied between September 1990 and February 1992 in the dry heath pine
forest.
MATERIALS AND METHODS
The study area locates in Finnish Lapland (67 oN, 29 oE)
and it was divided into two sample plots: the lichen (Cladonia arbuscula
and Cladonia uncialis) covered plot (ungrazed sample plot) was fence off
the area completely grazed by reindeers (grazed sample plot). Air (2 m above
ground) and soil (5 cm and 20 cm depth) temperatures were measured continuously
every second hour. The frost hardiness of the roots from the humus layer and
from the mineral soil (at the depth of 10 cm) was measured by electrolyte
leakage method. The survival of the roots was revealed by TTC method.
RESULTS
The lichen cover insulated very effectively the first freezing temperatures
before the first snowfall (first week of October) during the fall. Eventhough the
air temperature was below -30o C, the soil temperature at 5 cm stayed
above 0o C on the ungrazed sample plot and it was -4o C on
the grazed sample plot. Between September 1990 and February 1992 the frost
hardiness of the roots of pine trees on the lichen sample plot was at its lowest
in September (-5o C) and at its highest in December (-21o
C). The differences in frost-hardiness among grazed and ungrazed sample plots
were not statistically significant. However, the frost hardiness of the roots was
all the time higher in the humus layer when compared to roots from the mineral
soil.
DISCUSSION
The insulating role of the lichen cover becomes important in the situations,
where the snowcover is exceptionally late. Also the grazing by reindeers can
weaken the insulating effect of lichen cover and can cause injuries to the roots
during the extremely cold mid-winter days of subarctic. The ecophysiological
significance of lichen cover will be discussed.
Marja-Liisa Sutinen The Finnish Forest Research Institute Rovaniemi Research Station P.O. Box 16 FIN-96301 Rovaniemi, Finland E-mail: Marja-liisa.Sutinen@metla.fi