Skógræktarritið - 15.05.2001, Side 174

growth, reproduction and insect
resistance of birch in Iceland.
The most important vertebrate
seed feeder in Icelandic birch
forests is the redpoll (Carduelis
flammea), while other birds feed
on birch seeds to a iesser extent,
The natural history of the in-
sect species feeding on birch in
Iceland is quite well recorded
(Ottósson 1982), including
information on moth species
(Ólafsson and Björnsson 1995)
that feed as caterpillars on the
birch. Among the caterpillars
that may cause the greatest dam-
age are two species of tortricid
moths, Epinotia solandriana and
Acleris notana. Two species of
geometrids are also common,
Operophlera brumata and Rheuma-
ptera hastata, while the third geo-
metrid, Erannis defoliaria, is com-
mon only in southeast lceland.
The noctuid caterpillar, Diarsia
mendica, feeds opportunistically
on birch, but more commonly
feeds on other dwarf shrubs,
including the related dwarf birch
(Betula nana). Outbreaks of in-
sects on downy birch in Iceland
are reported by biologist in this
and past centuries. Caterpillars
may cause widespread damage
to both buds and leaves of birch
in Iceland, but the nature and
scale of insect outbreaks is likely
different from the situation in the
neighbouring countries.
The experiments described
here explore the regrowth of
young and mature plants after
simulated early season grazing.
In addition, the field study ad-
dresses the importance of early
season grazing for the resistance
of birch to insects.
Materials and methods
A field study was conducted in
1998 in Heiðmörk (64°N, 21°W),
a study site protected from sheep
grazing, where downy birch forms
scattered shrublands. Most of
the birches at the site are mature
plants, seedlings were rare dur-
ing the study. During the sum-
mer caterpillar infestation of the
plants was high, as shown by a
high number of folded leaves
called caterpillar tents. Epinotia
solandriana caterpillars were the
most common, but Operophtera
brumala also occurred. The
ground vegetation consisted of
dwarf shrubs, forbs, graminoids
and mosses.
A group of forty mature birches
were assigned to the study. The
following factors were examined:
(1) The effect of simulated grazing
on birch growth, and (2) the
effects of simulated grazing on
insect resistance, as estimated
from the occurrence of caterpillar
tents. On mature birch, apical
buds were removed before bud
burst, while control plants were
untreated. The bud removal simu-
lated early season browsing by
sheep on mature plants. Indivi-
dual birches on each plant were
randomly assigned to the study.
All apical buds were removed on
the assigned branch and adjacent
branches, removing a total of forty
apical buds on each plant. Regular
recordings were made every other
week on branch segments origi-
nally consisting of approximately
twenty buds, counting the num-
bers of buds, leaves, shoots and
caterpillartents.
Few seedlings grew at the
study site, but experiments on
seedlings were made in outdoor
plots at the nearby Icelandic
Forest Research Station, Mógilsá.
This allowed an indirect compari-
son of the herbivory tolerance of
young as compared to mature
birches. Cutting treatments sim-
ulated grazing effects on leaves
and buds in the early season.
Sixty birch seedlings were orig-
inally raised from seeds in indi-
vidual plant pots in unheated
greenhouses and subsequently
transferred to open plots in cli-
mate conditions similar to the
nearby Heiðmörk area. Five trays,
with an equal number of plants
in each treatment, were placed in
the open plots and the trays were
regularly rotated to ensure uni-
form conditions. There were two
cutting treatments and an uncut
control. The plants were desig-
nated to different treatments:
partial foliar damage (1/4 of every
other leaf), bud damage (every
other bud removed), and control.
Regular recordings were made
every other week of seedling
growth in terms of addition of
leaves and buds, extension of the
main shoot and individual
branches. The experiment was
run during the summer.
The analysis of data was made
using a repeated measures
analysis for multifactor experi-
ments (Potwin et al. 1990, Winer
et al. 1991) and the statistical
package SPSS 8.0 for Windows.
Results
Apical bud removal on mature
birch in the early season reduced
bud production as the season
progressed (Pillai's trace F429 =
5.126, P < 0.01, fig. 1). The treat-
ment also reduced leaf produc-
tion (F, 32 = 9.102, P < 0.01), but
no change in the effect on leaf
production was detected through
time (fig. 2). Budbreak occurred
during a dry period when the air
temperature had exceeded 0°C
for three weeks. An increasing
number of caterpillar tents were
recorded as the number of ex-
panded leaves increased. Simul-
ated browsing on the plants did
not affect how many caterpillar
tents were added through the
season (P > 0.05). There was not a
strong relationship between
increased leaf expansion and the
maximum daily temperatures. The
mean monthly precipitation was
190 mm in May, 25 mm in June,
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SKÓGRÆKTARRITIÐ .2001 l.tbl.