Testing the aluminium toxicity hypothesis: A field manipulation experimentin mature spruce forest in Norway

Aluminium (Al) has been considered to be a central element for risk evaluat ion of forest damage due to acidification. It has been hypothesized that Al reduces root growth, nutrient uptake and forest vitality. However, forest monitoring studies fail to show correlations between soil acidification and forest health. In general, no direct relation between Al concentration and forest health has been established. Here, Al concentrations in soil soluti on were manipulated by weekly additions of dilute AlCl3 to levels that are believed to be unfavorable for plant growth. Four treatments (in triplicate ), including a reference and three Al addition levels, were established. Ef fects of enhanced Al concentrations on fine root growth, nutrient uptake an d crown condition in a mature Norway spruce forest in Norway were tested (1 996-1999). After three years of manipulation, crown condition, tree growth and fine root growth were not affected by potentially toxic Al concentratio ns. However, the Mg content in current year's needles decreased at the high est Al addition treatment. The Mg/Al ratio of fine roots of the same treatm ent had declined too, which suggests that Al blocked Mg uptake at the root surface. The manipulation will be continued for two more years.