Forest die-back and impaired tree vitality have frequently been ascribed to Al-toxicity and Al-induced nutritional disorders due to increased acidification of forest soils. Therefore, in this experiment effects of A1 were studied on growth and nutrient uptake with seedlings of five different forest tree species. During growth in culture solutions with and without AI all five species proved to be very Al-tolerant, despite high accumulation of AI in roots. In the coniferous evergreens Douglas-fir and Scots pine shoot as well as root A1 concentrations were significantly higher than in the deciduous broad-leaved species oak and birch. Larch showed intermediate AI levels.In none of the five species did AI reduce nutrient concentrations or the Ca/A1 ratio to values below the critical level. Besides differences in AI accumulation, coniferous and broad-leaved species also differed with respect to uptake and assimilation of nitrogen. Due to extra NH~ uptake, oak and birch showed a much higher N uptake and higher NH~ preference than the coniferous species. Especially with oak this high NH4 + preference in combination with a low specific root surface area resulted in a high root proton efflux density. In comparison to both broad-leaved trees and Scots pine the NO3 reduction capacity of larch and Douglas-fir was extremely low. This may have important consequences for both species if grown in NO;--rich soils.

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