Skógræktarritið - 15.05.2001, Side 162

Figure I
Biomass per plant (g DW) of leaf, stem
and root tissue at harvesting
(14.9.1987) in fertilized birch
seedlings, grown fortwo months in
peat at two different altitudes in west-
ern Norway,
Fana (50 m) and Kvamskogen (450 m).
Mean values with 2 s.e. indicated on
total shoot and root biomass.
Treatments are (from left to right): +N,
-N,-P, -NP (the two last treatments
shown only for the BA population
only).
Acomparison between popula-
tions shows that there was rela-
tively more alternative respira-
tion in leaves from the northern
birch population than in leaves
of the southern lowland popula-
tion at low nutrient levels. in
stem and root tissue the propor-
tion of alternative and normal
growth respiration was about the
same level in all three popula-
tions. The suppression of leaf
growth in arctic ecotypes is a
particularly useful adaptation
(Bliss 1971) in an environment
with a long, cold and dark sea-
son.
Beevers (1970) demonstrated
the occurrence of an alternative
dark respiration pathway in age-
ing tissue. The high proportion of
alternative respiration in north-
ern population (B|) leaves at low
nutrient levels may therefore
partly be explained by early age-
ing and partly by the overflow
hypothesis, i.e. that arctic ecotypes
use the alternative pathway to
remove excess assimilates thus
avoiding undesirable growth in
aboveground tissue.
Conclusions
In response to the questions put
forward in the introduction, the
results support the hypothesis
that Betula pubescens has evolved
alternative respiration as an
important growth-regulating
Figure 2
Total nitrogen content of stem (NgT)
and root (NrT) tissue in mg/plant in
fertilized (+N) and non-fertilized (-N)
birch seedlings grown forthree
months at Fana (50 m) and
Kvamskogen (450 m). Some plants
(open columns) are harvested before
leaf fall (14.9.87) and some (hatched
columns) after (25.10.87). Means of five
replicates with ±2 s.e.
mechanism (cf. Lambers 1980).
At low nutrient levels there was a
depression of dark respiration in
leaves and stems, and most of it
was alternative respiration, i.e.
not linked to ATP production.
The tendency was strongest in
the subarctic birch population,
indicating that northern ecotypes
are more adapted to low-growth
strategies than their southern
relatives (cf. Kallio 1984, Chapin
1980). Low temperatures
seemed to increase the alterna-
tive respiration rates in leaf and
160
SKÓGRÆKTARRITiÐ 2001 l.tbl.