Sensitivity of tree seedlings to aluminum. II. Red oak, sugar maple, and European beech

A series of solution culture and greenhouse studies were conducted as part of the ALBIOS project to evaluate the response of northern red oak (Quercus rubra L.), sugar maple (Acer saccharum Marsh.), and European beech (Fagus sylvatica L.) seedlings to Al. Several soil and plant variables were evaluated as possible indicators of plant response to increased Al. Of the soil variables evaluated, 0.01 M SrCl2 extractable Al was found to be well correlated (r = 0.70) with plant response. Changes in root branching frequency were found to be an even more sensitive indicator of potential impact, although routine applications in the field was deemed impractical. Of the three species evaluated, northern red oak was the most sensitive to increasing exhibiting root growth reductions at Al concentrations ranging from 0.12 to 0.28 millimole in two separate experiments. The Ca/Al ratio was found to be particularly important in establishing toxicity thresholds for northern red oak with no Al impacts observed when Ca/Al ratio was greater than 4. In low ionic strength solution culture experiments, shoot growth of European beech was reduced by 40% at an Al concentration of 0.5 millimole. Foliage tissue Al concentrations tended to be less reliable than root concentrations as predictors of plant response. Comparison of established critical tissue levels (CTL) of Al to tissue data from various field sites indicated that only northern red oak (CTL = approximately 6600 microgram g-1) may be near impact levels, whereas soil and soil solution data indicate rhizosphere levels generally well below those producing an impact. All results and thresholds must be extrapolated with caution because there are many interacting factors producing a variety of responses in the face of seemingly similar experimental treatments.