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would reduce the level of CO 2 concentration in the atmosphere and exclude an
undesirable climate change.
It is supposed that boreal forests are a region for carbon sink. However, their
fires can transform boreal forests into sources of carbon due to direct emissions of
carbon during biomass burning and indirect impacts of
fires on the thermal and
water regimes, as well as the structure and functioning of the ecosystems. The
frequency of
fires in boreal forests during the last several decades has increased and
can increase further under conditions of continuing global climate warming. Thus,
this should lead to a shorter time period of ecosystems
'
recovery in the periods
between the
fires and to enhancing emissions of GHGs to the atmosphere. The CO 2
flux from the ground R s ¼ H 7 þ H 8 þ H 9 þ H 15 (see Fig. 1.30 ) is a second
important
fl
flux of carbon in boreal forests which determines its role in the formation
of the global carbon cycle. The R s value changes due to different processes:
￿
fl
fires lead to a partial removal of vegetation cover and a decrease of soil surface
albedo (the latter determines an increase of surface temperature and the rate of
decomposition of vegetation remains);
a
fire breaks the process of accumulation of organic matter in soil and changes
the balance between the input of detritus and heterotrophic respiration (as a
result of a great input of detritus);
￿
￿
fire brings forth changes of vegetation succession and composition of its
species as well as litter quality.
To better understand the impact of forest
a
fires on the carbon cycle in boreal
forests, Wang et al. (2003) performed measurements and numerical modeling of R s
for black spruce (Picea mariana) with post-fire chronological succession for seven
forest
fires in the north of the Manitoba Province (Canada), under conditions of
good and poor drainage of soils as an example. Studies have been aimed at:
i. a quantitative characteristic of the dependence of R s on soil temperature for
forests of different age;
ii. study of the post-
re succession dynamics of forest stands; and
iii. estimating the annual CO 2 fl
flux from soil surface.
flux depended strongly on conditions of drainage and age of the forest
stand. There was a positive correlation of CO 2 fl
The CO 2 fl
flux with soil temperature (=0.78),
with results of numerical modeling of the
flux differing strongly depending on a
combination of the level of drainage and age of the forest stand. In the period of the
vegetation development season the CO 2 fl
fl
flux from a well drained soil was much
greater than from a poorly drained one. The annual mean values of the CO 2 fl
ux
from the soil in different years constituted 244 (the year 1870), 274 (1930), 350
(1964), 413 (1981), 357 (1989), 412 (1995), and 226 gC m 2 year 1 (1998). Under
conditions of well drained soils the numbers above become 264, 233, 256, 303,
300, 380, respectively, while 146 gC m 2 year 1 in the case of poorly drained soils.
In the winter (from 1 November to 30 April) the values of CO 2 fl
flux varied within
5
-
19 % with respect to the annual
fl
flux. Apparently, a decrease of the
fl
ux at the
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