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ŠUMARSKI LIST 13/2005 str. 40     <-- 40 -->        PDF

P. Kantor: POSSIBILITIES OF MOUNTAIN FORESTS IN REDUCING HIGH WATERS AND FLOODS Šumarski list SUPLEMENT (2005) -39
After the fall on the forest soil precipitation water
largely infiltrates into it. In addition to the saturation of
soil pores and subsequent seepage the infiltrated water
gets through the system of water paths to a parent rock
(ducts in the forest soil created by roots of trees, animals
etc.), runs down and accumulates in the parent
rock hollows and depressions. If the parent rock stratum
is impermeable the infiltrated water changes into
the subsurface runoff. If the parent rock stratum is of
the fissure type the water permeates to groundwater
participating in underground runoff.


And just here, there is a substantial difference in the
water-management effectiveness of forests as compared
with agricultural soils the infiltration capacity of which
being usually markedly lower. Also during rainstorms
inducing floods in summer 1997, 1998 and in August
2002, surface runoff in spruce and beech was not decisive
(Sach, Kantor, Černohous 2000). And once
again, it is necessary to mention that in the given case it
refers to ordinarily managed forest stands. Increased
surface runoff (although generally not dangerous) is
usually noticed only during spring snowmelt in unmixed
beech stands where the compact layer of leaves
shows lower infiltration capacity than spruce litter. Moreover,
the intensity of thawing in leafless beech stands
under sunny days is even 30 % higher than in closed
spruce stands.


Under the usual regime of precipitation, water in
soil is drawn by roots of trees to ensure their physiological
processes (transpiration). Intensity of transpiration
in broadleaves is usually 2 to 5 times higher than
in coniferous stands. With respect to markedly higher
values of the biomass of spruce stand needles as
against the foliage biomass in beech stands differences
between transpiration of conifers and broadleaves are
usually not marked. This very important finding was
corroborated particularly by German studies already


30 to 40 years ago (L a d e f o g e d 1963, M i t s c h e r lieh
1971, Schmaltz 1969). Similarly, mature
stands of spruce and beech in the Orlicke hory Mts.
consumed for this form of evaporation virtually the same
amount of precipitation, viz on average 180 to 200
mm per year.


Finally, the certain part of soil moisture is evaporated
right from the soil surface - in our case it referred
to about 80 mm per year in both types of stands.


Data on the water regime of spruce and beech
stands on balance plots in the Orlicke hory Mts. are given
in Table 1.


Thus, it is evident that forest ecosystems are the
considerable consumer of precipitation water. Not only
findings from the Orlicke hory Mts. but also all available
published sources have corroborated substantially
higher consumption of water by spruce stands as compared
with beech stands - see Table 2. Markedly higher
interception in a spruce stand is unambiguously the
main cause of these differences.


From the viewpoint of reducing high waters and
floods the maximum water capacity of forest soils is of
exceptional importance. It is the highest amount of water
which can be retained by soil. In our actual case in
the Orlicke hory Mts. (light-textured sandy-loam up to
loamy-sandy Cambisol with a 50 % admixture of soil
skeleton), its value amounted to about 270 mm at the
soil depth of 70 cm. In the course of summer months,
the actual water content in the soil of both stands ranged
between 170 and 190 mm not decreasing below
150 mm in the period without precipitation. In the given
case, the forest soil of mountain spruce and beech
stands was able to retain and accumulate 40 to 60 mm
precipitation water, maximally 100 mm. In the moment
of achieving full water capacity, the soil can be compared
to a sponge fully saturated by water which is not
able to retain a millimeter of other precipitation.


CONCLUSION


Based on the long-time series of experimental studies
not only in the Orlickć hory Mts. but also in the
Beskids and through the analysis of a number of foreign
studies it is possible the following basic findings to
be regarded as proved:


1.
Surface runoff and subsequent soil erosion are quite
negligible in forest stands. This fact refers not only
to natural forests but also commercial forests. Even
on clear-felled areas, erosion processes (with the
exception of boulder localities) are not the result of
mere felling the trees but they are always a manifestation
of the poorly organised use and movement of
heavy means of mechanization and other activities
of man. The absence of surface runoff in the forest
(the runoff being also eliminated there by the system
of water paths created by roots of trees, animals
etc. in the forest soil) as against its often occurrence
on agricultural land is thus the first and very important
condition for the reduction of floods in the
landscape. As a textbook example, it is possible to
mention a local flood of 15 July 2002 in the Hodoninka
river watershed (district of Blansko). In virtually
forest-free parts of the cadaster of Crhov and
Olešnice, in the course of 2 hours some 100 to 170
mm precipitation fell which immediately flew out
on the surface of agricultural land into watercourses
causing many-million damage to property and unfortunately
even the loss of two human beings.