Skógræktarritið - 15.05.2001, Síða 183

Fig. 1
the quality of seeds in the seed
rain affects the success of the par-
ticular year's seed rain in seedling
recruitment. Seeds should also
drop on suitable microsites for
seed germination and seedling
recruitment, which does not nec-
essarily happen (Schupp 1995).
A soil seed bank is the product
of a seed rain of many years,
even decades. The seeds accu-
mulating in the seed bank have
several alternative fates (Simp-
son et al. 1989). Some of them
may germinate, or some may
lose their viability because of
physiological death or patho-
gens. Some of them can be eaten
by predators or become buried in
deep layers of the soil. Rabino-
wich and Rapp (1980) and Schott
and Hamburg (1997) recorded a
reduction in the seed number
from the seed rain to the seed
bank of prairie and old field veg-
etation. This pattern reflects an
effective eradication of the seed
bank via seed germination and
death. In an alpine undisturbed
heath the seed bank (germinated
in a green house) and the seed
rain are dense. However, en-
vironmental conditions almost
completely prevent seedling
emergence from the seed bank in
the field (Welling and Laine a, in
prep.). On the other hand, in an
alpine meadow, seedling emer-
gence from the seed bank in the
field is effective.
Drought and low temperatures
limit seed germination, and
therefore affect seedling recruit-
ment from the seed bank. On the
other hand, needle ice activity
and soil drought are important
constraints on seedling survival.
Disturbed, open patches facili-
tate seed germination and seed-
ling survival. This is because dis-
turbances decrease competition
by destroying vegetation and
providing better light, tempera-
ture and nutrient conditions for
the seedlings (Chambers 1995).
Surprisingly, comparable seed-
ling emergence was found in
gaps and closed vegetation of an
alpine meadow and heath
(Welling and Laine a, in prep.). In
the meadow, a shift in the domi-
nant seedling flora from gaps to
closed vegetation seems to be a
reason for the comparable seed-
ling emergence in these patches.
In the heath, probably drought'
prevents seedling emergence in
gaps as effectively as the thick
moss cover in closed vegetation.
Several plant traits, for in-
stance diaspore morphology,
seed size, state of clonality and
growth form, have a role in the
success of plants during the
regeneration pathway. The con-
sequences of seed size - small or
large - are apparent in regenera-
tion. Small seeds dominate the
seed rain (Chambers 1993,
Welling and Laine b, in prep.)
and the seed bank (Chambers
1993, Thompson et al. 1998). On
the other hand, a large seed is an
advantage in colonization, espe-
cially that of closed vegetation
(Kiviniemi 1999). Appendaged
diaspores have an ability to dis-
perse longer distances than non-
appendaged diaspores, but ap-
pendages may limit burial in the
soil (Rabinowich 1981). The dias-
pores which are not buried are
exposed to predation, heat and
drought. Thus, appendiges may
limit seedling emergence and
recruitment.
Many herbs and sedges are
known to demand light for seed
germination, but grasses are tol-
erant to shade (Grime et al. 1987,
Chambers 1987, Schutzand Rave
1999). Herbs and sedges should
therefore demand gaps for seed
germination. On the other hand,
grasses should germinate in
closed vegetation effectively.
However, narrow leaves may
limit seedling recruitment of
grasses in closed vegetation. In
clonal plants seedlings are com-
monly rare (Eriksson 1992).
Clonal plants are abundant in
standing vegetation, however,
because of their large size and
effective vegetative spreading.
In all, particular plant traits go
SKÓGRÆKTARRITIÐ 2001 l.tbl.
181