Skógræktarritið - 15.05.2001, Síða 175

BROWSING ON OLDER PLANTS
WEEK
Figure 1. The mean number of buds
on mature birch through the summer
of 1998 in simulated grazing and corn
trol treatments. Standard errors of
means are shown.
and 69 mm in July in Heiðmörk in
during the year of study, 1998. In
comparisons to previous years
(1993-1997). May was unusually
wet whereas June and July were
quite dry. The mean daytime max-
imum temperature was 12°C dur-
ing the months of study (May-
July) and maximum daily temper-
atures did not exceed 19°C.
Monthly night time average mini-
mum temperatures were 5°C.
Three measures of seedling
growth (bud number, leaf num-
bers and plant height) showed an
effect of treatment over time. Bud
numbers remained lower through
the growing season on plants
that had received early season
damage (FHuynh.FekJt = 3.690, df =
6, P < 0.01) . Similarly, leaf num-
bers remained lower on plant
that had received early season
damage (FHuynh.Fddt = 4.318, df=6,
P < 0.01). The response was
strong to bud damage, but not
detectable for leaf damage (fig. 3-
4). Seedling height was reduced
by bud damage and to a lesser
extent by leaf damage (FHuynh.Feidt
= 9.516, df = 6, P < 0.001). Dam-
age effects on height were more
pronounced than damage effects
on leaves and buds (fig. 5).
Discussion
Downy birch responses to spring
browsing at an oceanic site in
Figure 2. The mean number of leaves
on mature birch in the summerof 1998
in simulated grazing and control treat-
ments. Standard errors of means are
shown.
BROWSING ON YOUNG PLANTS
Figure 4. The mean number of leaves
on young birch through the summer of
1998 in simulated bud grazing, leaf
grazing and control treatments.
Standard errors of means are shown.
Iceland are similar to what is
reported from continental loca-
tions in northern Fennoscandia
(Hoogesteger and Karlsson 1992,
Karlsson et. al 1996). The indica-
tion is that the birch has a high
ability to recover from early sea-
son browsing. Surprisingly,
younger plants seem to recover
more strongly after damage than
mature plants.
in the present study, early sea-
son bud removal affects the
growth of mature birch. However,
there is no indication of altered
resistance of the downy birch to
herbivory. This is contrary, to the
findings in comparable studies in
continental Fennoscandia (Neu-
vonen et. al. 1988) as well as stud-
ies indicating that resistance to
herbivores may be mediated by
Figure 3. The mean number of buds
on young birch through the summer of
1998 in simulated bud grazing, leaf
grazing and control treatments.
Standard errors of means are shown.
Figure 5. The mean height of young
birch through the summer of 1998 in
simulated bud grazing, leaf grazing
and control treatments. Standard
errors of means are shown.
biotic and abiotic stress to the
plants (Mahdi and Whittaker 1993,
Suomela and Neuvonen 1997).
The climatic data suggest that
bud burst on downy birch in
lceland depends on constant
above freezing temperature as
reported in more southerly
oceanic climates (Billington and
Pelham 1991).
While this study reflects on the
bud and leaf dynamics of downy
birch, more information is need-
ed in the future on the biomass
production and leaf biochemical
content of downy birch in Iceland
in relation to herbivory. The
long-term goal is to make impor-
tant comparisons among areas
where birch grows in a different
soil and climate conditions.
Further analysis of the data set
SKÓGRÆKTARRITIÐ 2001 l.tbl.
173