Per Holm Nygaard

Research Scientist

(+47) 911 38 138

Ås H8

Visiting address
Høgskoleveien 8, 1433 Ås

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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.


Aluminium (Al) is a key element in critical load calculations for forest. Here, we argue for re-evaluating the importance of Al. Effects of two levels of enhanced Al concentrations and lowered Ca:Al ratios in the soil solution in a field manipulation experiment in a mature spruce stand (1996-1999) on tree vitality parameters were tested. In addition, Al solubility controls were tested. Various loads of Al were added to forest plots by means of an irrigation system. Potentially toxic Al concentrations and critical ratios of Ca to inorganic Al were established.The ratio of Ca to total Al was not a suitable indicator for unfavourable conditions for plant growth. No significant effects on crown condition, tree growth and fine root production were observed after three years of treatment. In 1999, foliar Mg content in the highest Al addition treatment had declined significantly. This agreed with the known response to Al stress of seedlings in nutrient solution experiments. No support was found for using the chemical criterion Ca:Al ratio in soil solution, foliar and root tissue as an indicator for forest damage due to acidification. Al solubility was considerably lower than implied by the assumption of equilibrium with gibbsite, particularly in the root zone.The gibbsite equilibrium is commonly used in critical load models. Substitution of the gibbsite equilibrium with an Al-organic matter complexation model to describe Al solubility in soil water may have large consequences for calculation of critical loads. The results indicate that critical load maps for forests should be reconsidered.