The Bornh�ved lake district is located in the north of Germany, near
the Baltic Sea; it includes agroecosystems, beech forest, alder bog, grassland,
and a lake. The surface of the catchment is about 5 km2, including 1 km2
of lake surface. The dominating soil types are dystric arenosols on the
hills.
All results are based on a climate data set of 60 years with a maximum
increase of 2.7�C; rainfall was assumed to be unchanged. Potential evapotranspiration
(pET) would increase by about 100 mm, but actual evapotranspiration (aET)
would differ only slightly from observed values because soil water storage
would become a limiting factor. During summer, there may be considerably
more drought stress. Soil moisture repletion in autumn and winter would
take considerably longer: The profile would not be filled until January.
As a result, farmers would have to change their land use from spring cereals
to winter crops or install irrigation systems on the light soils.
The extent of the relatively flat catchment area depends on the amount
and spatial distribution of groundwater recharge. Because of higher evapotranspiration,
groundwater recharge would be reduced by 25% (100 mm/year), and the catchment
area of the lake would decrease. Overall, the inflow into Lake Belau could
decline by about 50%.
The discharge of the rivers could drop by about 30% compared with present
conditions. The frequency of low water levels could increase, causing
additional stress for flora and fauna in the rivers. Water quality in
the eutrophic Lake Belau is determined by the load of the river that flows
through the lake. As a result of decreasing discharge, there would be
higher nutrient concentrations in the groundwater and the river water;
the overall nutrient load will increase substantially.
Under unchanged management, nitrogen concentrations in the soil solution
would rise, but the overall loss would decrease slightly. However, a change
in the crop rotation and a reduction of fertilization from the present
340 kg N/ha to 163 kg N/ha would affect the nitrogen leaching much more.
Vegetation studies were conducted for primary production of alder, vegetation
structure in grassland, soil respiration, changes in the length of the
growth period in beech forest, and production of beech and Carex acutiformis.
Zoological case studies were carried out for molluscs, birds, mice, several
invertebrates, and the food chain of the robin. Particularly interesting
is the decrease in the number of long-distance (trans-Sahara) migratory
birds-a phenomenon that also has been observed in other regions of Germany.
One possible explanation could be that with increasing temperatures, the
nests and territories of these birds already have been occupied by nonmigratory
birds and birds with short passage that can respond more quickly to the
changing conditions of rising temperatures.
Changes in agricultural yield were simulated for wheat, corn, rape, grassland,
and beans. Because of the light soils in the research area, corn yield
will decrease dramatically under scenario conditions, and the risk of
bad harvests will increase. An earlier seed time and the use of other
varieties of corn has almost no effect on yields. Because the agriculture
of the region is dominated by dairy production, food for cattle has the
highest priority. To minimize the risk of bad harvests, farmers should
irrigate or replace corn with grassland, which uses winter rainfall more
efficiently.
Under scenario conditions, rivers and lakes will be more severely affected
than terrestrial ecosystems. The structure of the landscape will be more
pronounced between dry uphill patches and downhill regions with access
to groundwater. The soil water storage capacity will become much more
limiting and will increase the local risk of bad harvests; adaptation
of land-use practices would be necessary. Despite numerous efforts to
predict species composition in terrestrial and aquatic ecosystems, predictions
on a local scale are not possible. Changes should be evaluated by a combination
of models and monitoring programs (predictive monitoring).
Source: H�rmann et al., 1995.
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