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ŠUMARSKI LIST 11-12/1997 str. 17     <-- 17 -->        PDF

SUMMARY: A part ofeastern Slavonia where the mute ofthe Danube-Sava
canal is envisaged, stretches over the basins oftwo rivers: the Danube and the
Sava. With the exception ofthe Nuštar elevation, it is aflat terrain in which the
dijferences in the micro-relief amount to several meters only. The lowland forests
ofthe region are characterized by numerous forest communities whose diversity
is conditioned by the micro-relief and the water supply.

The life of these forests is crucially dependent on groundwater. During the
growing period, abundant supplies ofvvater are ensured by the roots of trees
reaching down to groundwater levels. It is a knownfact that tree species growing
in lowlandforests need larger guantities ofwaterfor transpiration than they
receivefrom rainfalling in the vegetationperiod. Hygrophytic trees, such as the
pedunculate oak, narrow-leaved ash, black alder, and white willow transpire
between 400 and 700 mm water in the growing period. In the Spačva forest basin
the annual precipitation is about 600 to 700 mm, ofwhich one halffalls in
the vegetation period. Without the additional groundwater, lowlands forests
would not be able to grow.

These forests contain three different groups of forest ecosystems. These are:

1. Forests of pedunculate oak and common hornbeam grovving on the elevated
parts ofthe micro-relief,
2. Slavonian forests of pedunculate dtik in lower micro-relief positions.
Before the river Sava was regulated, these forests were regularly flooded, and
3. Forests ofnarrow-leaved ash and forests of black alder in the lowest positions
in which water is retained over the largest part oftheyear. During July,
A ugust and September the water recedes, thus makingpossible the growth of forest
In ali three groups ofsites and their forest associations, water is the dominant
ecological factor. Forest ecosystems čame into being when plant and animal
species merged and adapted to certain site conditions, the most important
being water in various forms: precipitation, flood, or groundwater. Over the
centuries, water and other site factors have had a constant and regular influence
on, and have determined the development of a particular forest ecosystem.
Every change in water conditions causes a disturbance in the forest ecosystem.

Hydrological conditions in the lowland forests ofeastern Slavonia depend
on the network ofwaterways. In the Danube river basin, it is the river Vuka with
its tributaries, and in the Sava one, it is the Bosut with its tributaries.

The Spačva forest basin and other lowlandforests are situated in the lowest
parts ofthe region. Thus, they are constantly supplied with water, either in the
form ofsurface or groundwater, which enables the pedunculate oak and other
lowland tree species to transpire abundantly. The importance ofwaterflows far
the supply oflowland forests with water is very high. With their floods, but also
with "keeping" the groundwater in the region, thev play a crucial role in the
sense ofa water gradient.

These most valuable lowland forests ofCroatia are directly dependent on the
micro-relief floods and groundwater. A large number oftree species and relatively
numerous forest communities have allfound their place within a narrow
range ofheight differences. Their complexity makes them highly susceptible to
mutual and unpredictable synergetic action ofthe existing factors.

These forests have reached their full potential precisely under specific ecological
conditions, and can only štand very small digressionsfrom the usual site
conditions. Ifsite conditions change, these forests become unstable.

The hydrological conditions in these forests are suited to the growth of pedunculate
oak and other loivland forest species. Each disturbance causing
groundwater levels to drop or rise more significantly, or surface water to stagnate
in the forest area, would have an unfavourable impact on forest trees.

In order to preserve the lowland forests in the Spačva basin and the forests
directly affected by the envisaged Danube-Sava canal runningfrom Vukovar to
samac, it is necessary to maintain the present regime ofsurface and groundwater
in the region. As available data on the existing state of groundwater are not

Šumarski list br. 11-12, CXXI (1997), 579-592