Indicators of chemical recovery from acidic deposition

Forest Soil base saturation of 20% or higher (i.e., the percent of available cations in the soil that are bases) Calcium to aluminum molar ratio in the soil solution of 1.0 or greater 

Aquatic Stream and lake pH of 6.0 or higher (except where pH is lower under background conditions) Stream or lake ANC of 50 peq/L or higher Stream or lake concentrations of dissolved inorganic aluminum less than 2 jjmol/L 

Terrestrial recovery is even more difficult to project than aquatic recovery. Given the life span of trees and the delay in the response of soil to decreases in acidic deposition, it is reasonable to suggest that decades will be required for affected trees on sensitive sites to recover once chemical conditions in the soil are restored. Overall, the timing and extent of chemi cal and biological recovery depend on how soon and how significantly emissions that cause acidic deposition are reduced. Moreover, human influences (e.g., land disturbance, introduction of exotic or invasive species), in addition to acidic deposition, can delay biological recovery after chemical recovery has occurred. 

While there is considerable information about the response of surface waters to decreases in acidic deposition under chronic conditions, much less is known about how episodic acidification responds to these changes. Laudon and Hemond (2002) reported decreases in episodic acidification following decreases in atmospheric sulfur deposition in northern Sweden. Unfortunately, comparable data sets have not been developed for other regions. 

Recovery of Acid-Sensitive Ecosystems with Future Decreases in Emissions 

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Table 3.3. Indicators of chemical recovery from acidic deposition...

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