Skógræktarritið - 15.05.2001, Side 163

Figure 3
C/N ratios between non-structural car-
bohydrates and total nitrogen content
in stem and root tissue of fertilized
(+N| and non-fertilized (-N) birch
seedlings grown for three months at
Fana (50 m) and Kvamskogen (450 m).
Some plants (open columns) are har-
vested before leaf fall (14.9.87) and
some (hatched columns) after
(25.10.87). Means of five replicates
with ±2 s.e.
stem tissue, with a correspond-
ing decrease in root tissue, indi-
cating preference for root growth
at low temperatures. This is a
useful adaptation, since it would
increase the capacity of roots for
absorbing nutrients from the soil
and restoring the balance
between source and sink (cf.
Thornley 1972).
A substantial part of the nitro-
gen in roots of fertilized plants
was translocated back to the
stems in fall, indicating sec-
ondary growth, while root growth
was preferred in non-fertilized
plants and at high elevations
(low temperatures). The experi-
ment seemed to confirm the
observation made by Chapin
(1980) that plants from high lati-
tudes and altitudes tend to keep
a high nitrogen concentration in
their tissue when being deprived
of nutrients. The highest ATP-
linked growth respiration rates
were found in birch seedlings
from high elevations at southern
latitudes (BH), as a metabolic
compensation to a climate where
light conditions are favorable for
photosynthesis, but where the
growing season is relatively short
(Crawford 1989). The increased
competition in southern vs.
northern habitats, and at higher
temperatures are reflected in the
higher shoot/root ratios in south-
ern ecotypes and the general 1-
5°C higher optimum tempera-
tures in all three populations for
Figure 4
Dark respiration rates (pl g hr
“*) in mature leaves (L), 4 mm stem
segments (S) and roots
(R) of fertilized (+N) and non-fertilized
(-N) birch seedlings, grown fortwo
months at high and low temperatures
(see Table 2b). The respiration rates
are measured with (hatched) orwith-
out cyanide and averaged over SHAM
concentrations, with ±2 s.e. on the
total cyanide-resistant respiration vai-
ues.
stem elongation rates than for
leafgrowth (Skre 1991 b).
ln total, the existence of an
alternative respiration pathway
as a growth-regulating mecha-
nism, maintaining the balance
between nutrient absorption in
roots and carbon fixation in
leaves (Thornley 1972) is strong
support for dark respiration
being a major limiting factor at
low temperatures.
SKÓGRÆKTARRITIÐ 2001 1, tbl
161