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Stand dynamics and the gap initiation prior to gap formation are not well- understood because of its long- term nature and the scarcity of late- successional stands. Reconstruction of such disturbance is normally based on historical records and den-droecological methods. We investigated gap initiation and formation at the fine- scale stand level in the old- growth reserve of Karlshaugen in Norway. Given its long- term conservation history, and thorough mapping in permanent marked plots with spatially referenced trees, it provides an opportunity to present stand development before, during, and after gap formation. Late- successional decline in biomass was recorded after more than 50 years of close to steady state. Gaps in the canopy were mainly cre-ated by large old trees that had been killed by spruce bark beetles. Snapping by wind was the main reason for treefall. Long- term dominance of Norway spruce excluded downy birch and Scots pine from the stand. Comparisons of the forest floor soil prop-erties between the gap and nongap area showed significantly higher concentrations of plant available Ca within the gap area. Plant root simulator (PRS™) probes showed significantly higher supply rates for Ca and Mg, but significantly lower K for the gap compared to the nongap area. Soil water from the gap area had significantly higher C:N ratios compared to the nongap area. Fine- scale variation with increasing distance to logs indicated that CWD is important for leaking of DOC and Ca. Our long- term study from Karlshaugen documents gap dynamics after more than 50 years of steady state and a multiscale disturbance regime in an old- growth forest. The observed dis-turbance dynamic caused higher aboveground and belowground heterogeneity in plots, coarse woody debris, and nutrients. Our study of the nutrient levels of the forest floor suggest that natural gaps of old- growth forest provide a long- lasting biogeo-chemical feedback system particularly with respect to Ca and probably also N. Norway spruce trees near the gap edge responded with high plasticity to reduced competition, showing the importance of the edge zone as hot spots for establishing heterogeneity, but also the potential for carbon sequestration in old- growth forest.



This thesis deals with effects of acidification, fertilisation and addition of Al on boreal vegetation as studied in different field- and laboratory experiments. The results are discussed in relation to the natural vegetation dynamics and critical loads of S and N for forest soils. Field experiments with artificial acid rain caused damage to bryophytes like Pleurozium schreberi and Dicranum polysetum at pH 2.5 and pH 3.0. The presence of Melampyrum pratense was reduced when treated with pH 2.5 and pH 3.0. For the pH 2.5 treatment, a decreased leaf production and decreased internal Mg concentration of Vaccinium myrtillus leaves were recorded. Treatment with pH 2.5 and 3.0 resulted in decreased base saturation in the O and E horizons, in particular Mg2, which may explain the effects on V. myrtillus. Repeated N-fertiliser additions caused vegetation changes, particularly at higher doses (1500 kg N/ha).An increase in cover of the species Deschampsia flexuosa, Molinia caerulea, Agrostis capillaris, Carex canescens, Rubus idaeus, Epilobum angustifolium and Dryopteris assimilis were found 8 years after the last N-application. Chemical analysis of leaves of V. myrtillus demonstrated an increase in N and a decrease in P concentrations on the fertilised plots compared to controls.The concentration of exchangeable Ca2, Mg2and K in the humus layer were reduced in fertilised plots compared to control plots. No differences in pH(H2O) or exchangeable acidity in the humus layer were found between fertilised plots and control plots. Laboratory experiments with Mg-limited Norway spruce seedlings showed that 80 M Al3 and a constant molar Ca/Al ratio of 0.2 decreased the uptake of Ca2 and Mg2, and reduced root length growth. However, no indication of an ameliorating effect of K on Al were seen, therefore these experiments give no support for including K into the critical load criterion. In the critical load calculation it seems that the molar Ca/Al ratio has been emphasised too much, and in particular that the scientific evidence of the critical chemical value (Ca/Al=1) is not well documented. A lot of the processes going on in the forest ecosystem are oversimplified or even left out in the present calculations. Long-term monitoring of forest vegetation at Karlshaugen, under moderate deposition regime of S and N, showed that other processes than deposition of S and N determine the vegetation dynamics, in particular the development of the dominant canopy species and field layer species seems to be much more important because of their influence on light and nutrients. Reduced frequency and persistence were shown for many field layer species and cryptogames during a 60 years period. A comparison of pH in soilwater from O-layer between 1961 and 1991 showed an increase in median pH of 0.1 pH unit for the area