Abstract• Existing growth and yield plots of pure and mixed stands of Norway spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica L.) were aggregated in order to unify the somewhat scattered sources of information currently available, as well as to develop a sound working hypothesis about mixing effects. The database contains information from 23 long-term plots, covering an ecological gradient from nutrient poor and dry to nutrient rich and moist sites throughout Central Europe.• An empirically formed interaction model showed, that depending on the site conditions, dry mass growth in mixed stands can range from −46% to +138 % of the growth yielded by a scaled combination of pure stands at equal mixing proportions.• Drawing from the interaction model, overyielding of the mixed stands appears to be triggered by two separate mechanisms. On poor sites, where significant overyielding is commonly found, facilitation by beech offsets nutrient-related growth limitations in spruce. In contrast, overyielding of mixed stands occurs less frequently on rich sites, and appears to be based on an admixture effect, with spruce reducing the severe intra-specific competition common in pure beech stands.• It was concluded that silviculture can accelerate growth of spruce by beech admixtures on poor sites, while growth of beech can be promoted by admixture of spruce, particularly on excellent sites.
Abstract. Atmospheric deposition of nitrogen (N) has exceeded its
demand for plant increment in forest ecosystems in Germany. High N inputs
increased plant growth, the internal N cycling within the ecosystem, the
retention of N in soil and plant compartments, and the N output by seepage
water. But the processes involved are not fully understood, notably the
effect of fructification in European beech (Fagus sylvatica L.) on N fluxes. The frequency
of fructification has increased together with air temperature and N
deposition, but its impact on N fluxes and the sequestration of carbon (C) and N in
soils have been hardly studied. A field experiment using 15N-labeled leaf
litter exchange was carried out over a 5.5-year period at seven long-term
European beech (Fagus sylvatica L.) monitoring sites to study the impact of current mast
frequency on N cycling. Mean annual leaf litterfall contained 35 kg N ha−1, but about one-half of that was recovered in the soil 5.5 years
after the establishment of the leaf litter 15N exchange experiment. In
these forests, fructification occurred commonly at intervals of 5 to 10 years, which has now changed to every 2 years as observed during this
study period. Seed cupules contributed 51 % to the additional litterfall
in mast years, which creates a high nutrient demand during their decomposition
due to the very high ratios of C to N and C to phosphorus (P).
Retention of leaf litter 15N in the soil was more closely related to
the production of total litterfall than to the leaf litterfall, indicating
the role of seed cupules in the amount of leaf N retained in the soil.
Higher mast frequency increased the mass of mean annual litterfall by about
0.5 Mg ha−1 and of litterfall N by 8.7 kg ha−1. Mean net primary
production (NPP) increased by about 4 %. Mean total N retention in soils
calculated by input and output fluxes was unrelated to total litterfall,
indicating that mast events were not the primary factor controlling total N
retention in soils. Despite reduced N deposition since the 1990s, about 5.7 out of 20.7 kg N ha−1 deposited annually between 1994
and 2008 was retained in soils, notably at acid sites with high N/P and C/P
ratios in the organic layers and mineral soils, indicating P limitation for
litter decomposition. Trees retained twice as much N compared to soils by
biomass increment, particularly in less acidic stands where the mineral soils
had low C/N ratios. These results have major implications for our
understanding of the C and N cycling and N retention in forest ecosystems.
In particular the role of mast products in N retention needs more research in
the future.
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