Skógræktarritið - 15.05.2001, Side 161

Table 3. Varianœ ratios (F) and significance levels for cyanide-resistant
dark respiration rates in different tissue types of birch seedlings from
three populations, grown at varying temperatures and nutrient levels.
Significance levels are- *P<0.05, **P<0.0l. DF = degrees of freedom.
The SHAM interactions are not included. R2 = square multiple correla-
tion coefficient, SS = sums of squares.
Source DF Leaf discs DF Stem segments DF Root segments
Population 2 7.0+ 2 0.3 2 10.9*
Temperature 1 43.8* 1 4.3° 1 114.7*
Nutrient level 1 17.2* 1 5.0° 1 2.0
Pop x temp 2 1.0 2 0.6 2 17.4*
Pop x nut 2 6.4+ 2 4.0° 2 5.7+
Temp x nut 1 1.6 1 8.7+ 1 Ul bo
Pop x temp x nut 1 2.0 1 0.3 1 0.5
SHAM 3 6.3* 3 2.6 3 7.2*
error ss 68 3.11 51 0.81 70 1.54
total ss 82 8.34 65 1.38 84 6.27
E: ílíli QA2
to the stems prior to leaf abscis-
sion, while at the high-altitude
site and in non-fertilized plants
most of the nitrogen seemed to
be re-translocated back to the
roots (fig. 3). The northern eco-
type (BJ) kept a much higher (2.7-
3.8% vs. 0.9-2.6%) nitrogen con-
tent in its stem and root tissue
than the corresponding plans
from southern ecotypes (Skre
1993), indicating higher metabol-
ic activity as an adaptation to
low temperatures and a short
growing season (cf. Billings
1974).
The C/N ratio in roots of the
coastal BS population increased
in fertilized plants prior to leaf
abscission at the lowland site
while there was a reduction at the
high elevation site (Fig. 3).
Carbon accumulation was
strongest in fertlilized plants.
This indicates that at the low-alti-
tude site, some photosynthesis
was taking place in leaves after
the first sampling in September,
while in unfertilized plants and at
high elevations there was less
photosynthesis after this date.
The results are in accordance
with Ericsson (1979), who found
that increased growth caused by
nitrogen or phosphorus addition,
or high temperature, reduced
starch reserves in Pinus sylvestris
followed by a new increase in car-
bon reserves and C/N ratios after
growth termination.
Non-structural carbohydrates
comprise up to 30% of the root
biomass in arctic plants (Chapin
1979). In the present experiment,
as much as 50% of the root bio-
mass was found to be non-struc-
tural carbohydrates, indicating
that the birch roots have a high
capacity for storage, and that
mountain birch is able to keep a
high root growth rate, even at
very low nutrient levels where the
aboveground growth is restrict-
ed. Root growth therefore seems
to have the priority over shoot
growth at nutrient limitation,
particularly in northern ecotypes.
Dark respiration rates
There was a strong increase in
total and cyanide-sensitive respi-
ration (e.g. growth respiration)
in leaves grown at high tempera-
tures, as a result of nutrient addi-
tion (Fig. 4). The result is in
accordance with Waring et al.
(1986), who found that addition
of nutrients (N, P, K) resulted in
a relatively stronger increase in
respiration rates than in photo-
synthesis.
In nitrogen-deficient plants the
proportion of cyanide-sensitive
respiration relative to total was
higher in roots than in leaves
(Fig. 4).There was almost no
cyanide-sensitive respiration in
leaf tissue grown at high temper-
atures, while there was still some
in roots and stems. Thus, at high
temperatures and low nitrogen
level, growth is directed towards
non-green tissue, especially
roots. This is a useful adaptation
because it would tend to
increase the uptake capacity for
nitrogen and help in restoring
the balance between production
and consumption. When the C/N
ratio is decreased by adding
nitrogen, growth (and growth res-
piration) is again shifted towards
leaf and stem tissue. The signifi-
cant temperature x nutrient
interactions in the cyanide-sensi-
tive respiration rates of leaf and
root tissue (Table 3) and the cor-
responding positive effect of
nutrient level on high tempera-
ture treated roots support this
conclusion (Fig. 4).
Generally, respiration rates
were increased in leaf and stem
tissue by low-temperature treat-
ment, while they were decreased
in roots. The increase was partly
of the cyanide-sensitive type,
indicating shoot growth at the
expense of root growth. Most of
the low-temperature induced
increase in respiration rates,
however, was of the cyanide-
resistant type (e.g. alternative respi-
ration). Lambers (1982) similarly
found that when Plantago was
transplanted into nutrient-defi-
cient solution, the alternative
respiration in leaves increased,
to avoid the production of nutri-
ent-deficient tissue that would
make the plants more suscepti-
ble to water stress.
There was a significant effect of
temperature and population on
alternative respiration rates in
leaf and root tissue and also a
significant population x tempera-
ture interaction in roots, i.e there
was no temperature effect on the
northern (B|) population (table 4).
SKÓGRÆKTARRITIÐ 2001 l.tbl
159