The calcium/aluminum (Ca/Al) molar ratio of the soil solution provides a valuable measurement endpoint or ecological indicator for identification of approximate thresholds beyond which the risk of forest damage from Al stress and nutrient imbalances increases. The Ca/Al ratio can also be used as an indicator to assess forest ecosystem changes over time in response to acidic deposition, forest harvesting, or other processes contributing to acid soil infertility. Based on a critical review of literature on Al stress, we estimate that there is a 50:50 risk of adverse impacts on tree growth or nutrition when the soil solution Ca/Al ratio is as low as 1.0, a 75% risk when the soil solution ratio is as low as 0.5, and nearly a 100% risk when the soil solution Ca/Al molar ratio is as low as 0.2. The Ca/Al ratio of the soil solution can be corroborated with other complementary indices. Our analysis found that threshold conditions for potential forest impacts from Al stress are indicated by four successive measurement endpoints: (i) soil base saturation less than 15% of effective CEC; (ii) soil solution Ca/Al molar ratio less than or equal to 1.0 (for 50% risk); (iii) fine root tissue Ca/Al molar ratio less than or equal to 0.2 (for 50% risk); and (iv) a foliar tissue Ca/Al molar ratio less than or equal to 12.5 (for 50% risk). With appropriate precautions and caveats, these sequential indices based on the Ca/Al ratio provide a means of distinguishing site conditions where Al stress is likely to affect tree growth adversely.
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