30 - 40 % total rainfall

0 - 40 % ground water transpiration

15 - 25 % interception i:ig. 3.5.7. Comparison of the hydrological balance of deciduous and coniferous forests (after Schulze 1982). Coniferous forests are drier than deciduous forests because of the increased evaporation from their wetter surfaces which depends on leaf area index

Nutrient Balance

Beech and spruce differ in their ability to utilise nitrate (Bauer et al. 2000). Coniferous trees have only a low nitrate reductase activity and thus take up ammonium predominantly, leading to nitrate leaching and increasing acidification in coniferous forests. Deciduous trees together with the more vigorous ground vegetation take up nitrate predominantly, so nitrate leaching is very low.

Formation of Humus

Underneath deciduous trees, the formation of raw humus is less than under coniferous trees, as the litter is more alkaline and richer in nutrients than in coniferous forests. The community of humus-decomposing organisms in the soil is thus more active and includes more species. The lower proportion of resins and other secondary metabolites makes decomposition easier, although deciduous trees as well as conifers contain specific materials that slow down decomposition (tannins in oaks, terpenes in conifers). Underneath deciduous forests, depending on cation availability, leaf mull or moder is formed with a larger cation-exchange capacity than in raw humus of conifers.

All these differences might be arguments for changing monocultures of spruce back into deciduous forests. However, at the moment (year

2004), deciduous forests are damaged more than coniferous forests by atmospheric pollutants.

Differences between deciduous and coniferous forests were studied on a transect between northern Sweden and central Italy, where conditions of the field sites were kept as constant as possible, i.e. beech and spruce grew on acid substrates in neighbouring stands (Schulze 2000). There were no conclusive differences in mineralisation and other soil parameters between deciduous and coniferous forests (Fig. 3.5.8; Schulze et al. 2000 a,b). Apparently, some of the differences which we have discussed so far were related to the fact that conifers, either planted or in natural stands, grow on sites which are more acid and nutrient poorer than sites on which deciduous trees grow. If conifers and deciduous trees grow on the same site under the same conditions, differences between tree types are lost.

Data from the European Transect show important features of ecosystems, particularly that production of litter increases with the supply of nutrients from mineralisation and deposition. In contrast, mineralisation compared with the supply of litter is too slow, i.e. the decomposition of litter over a larger area is disturbed. The imbalance increases with N deposition. This means that forest ecosystems in Europe accumulate carbon in the soil and this accumulation increases

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