Nutrient cycling in pure spruce and mixed species stands

Ever since the 19th century, when it became popular to plant Norway spruce (Picea abies) outside its climatic range to reforest devastated forest land in Central Europe, spruce and beech (Fagus sylvatica) stands have been contrasted in their effects on the forest soil. It is nowadays considered prudent, close-to-nature forest practice, to convert secondary spruce stand into mixed spruce-beech stands, even though such mixtures have often not been the natural vegetation at most of the sites in question. Critical reviews on tree-soil interactions concede acidification by spruce but partly question its negative consequences on stand growth. Effects of admixture of beech to secondary pure spruce stands are investigated within this study with special emphasis on nutrient cycling in these forest ecosystems. We hypothesize that a) input of carbon and macro nutrients by litter and roots is a function of forest vegetation type; b) this carbon input as well as the chemical quality of the associated input of nutrients has a marked effect on element stores and turnover; and c) silvicultural methods, e.g., admixture of beech versus spruce, can be used to manipulate nutrient stores of forest ecosystems and to manage nitrogen retention and release. This issue is important because ground water levels are already critical from the viewpoint of human health. The methodological part will focus on four lines: i) Improvement of the theoretical basis for understanding and describing the impact of forest management on nutrient cycling using regression analysis of soil and nutritional parameters. ii) Modeling and predicting effects of forest management on soil processes and nutrient fluxes within forest ecosystems on the local scale. iii) Distinguishing pathways of nutrient turnover in pure spruce and mixed spruce-beech stands by means of natural abundance measurements of stable isotopes (C, N, O, Sr), since it has been proposed that the C-13, N-15, O-18-nitrate and Sr-87 signals of vegetation and soil may provide useful tools in evaluating nutrient dynamics of forest ecosystems. iv) Linking microbial nutrient turnover in pure spruce and mixed spruce-beech stands with isotopic signatures of C and N, because during microbial transformations of C and N, microbes discriminate against the heavier isotopes.

Ever since the 19th century, when it became popular to plant Norway spruce (Picea abies) outside its climatic range to reforest devastated forest land in Central Europe, spruce and beech (Fagus sylvatica) stands have been contrasted in their effects on the forest soil. It is nowadays considered prudent, close-to-nature forest practice, to convert secondary spruce stand into mixed spruce-beech stands, even though such mixtures have often not been the natural vegetation at most of the sites in question. Critical reviews on tree-soil interactions concede acidification by spruce but partly question its negative consequences on stand growth. Hence, effects of admixture of beech to secondary pure spruce stands were investi-gated within this study with special emphasis on nutrient cycling in these forest ecosystems. Nutrient fluxes via throughfall, stemflow, litterfall and soil solution were monitored for selected study sites. Due to a higher filtering capacity of atmospheric pollutants for spruce than for beech canopies throughfall fluxes of H+ , SO 4 -- and NO 3 - were clearly higher under spruce than under adjacent mixed spruce-beech stands. Soil solution concentrations of all analysed anions and cations were about twice as high under pure spruce than under the mixed stand with low fluctuation during the sampling period. Element losses under spruce, calculated by multiplying soil solution concentrations with modelled water outputs from the stands (surface flow, interflow and seepage to groundwater), are suggested to be a manifold of those from mixed stands. This issue is important because ground water levels are already critical from the viewpoint of human health. Preliminary results, using 87Sr/86Sr-ratios, indicate that beech is a real Ca pump (uptake from deeper soil horizons) but this Ca is mainly of atmospheric source and not, as expected, sup-plied by weathering. We conclude that even in mixed spruce-beech stands Ca is leached in high amounts together with nitrate, sulfate and organic anions through the shallow top soil, which is rooted by spruce. Uptake of Ca in deep soil layers by beech minimizes substantial loss of Ca and other base cations, which would be inevitable under pure spruce stands. CO 2 and N2 O emissions were more pronounced in the mixed forest than in the pure spruce stand. Tree species do influence the emission rates as they regulate the nutrient supply, the N deposition rate and the soil water regime of a forest ecosystem. Both gases are well known greenhouse gases. In comparison to N fertilized agricultural soil estimated N2 O loss from the studied forests is low (1-3 kg N2 O-N ha -1 a -1 ). However, since forests cover ap- proximately 50% of Austria, managing N2 O emission via tree species composition could be a contribution to the national greenhouse balance. The use of stable N isotopes enabled the detection of the main processes of N 2 O production: denitrification in the mixed stand and nitrification in the pure spruce stand.