Acid deposition biological responses

The first step in the application of the concept was to determine the critical load values for the different regions of eastern Canada. This was done using historical measurements of lake acidity in concert with the Integrated Assessment Model (IAM) which links atmospheric transport and deposition models with water chemistry and empirical biological response models. Details of the method are given in Jeffries and Lam (1993). 

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. 

Scientists have developed computer models that depict the physical, chemical and biological processes within forest watersheds. Watershed acidification models can be used as research and management tools to investigate factors responsible for the historical acidification of soil and water as well as the ecosystem response to anticipated future changes in acidic deposition. In order to effectively predict the pH, ANC and aluminum concentrations in streams, all major chemicals must be accurately simulated (e.g., sulfate, nitrate, calcium, magnesium). The acidification model PnET-BGC was used for this assessment because it has been rigorously tested at Hubbard Brook and other sites in the northeastern United States, and it allows the user of the model to consider the ecosystem response to multiple chemicals simultaneously. Other frequently used acidification models include MAGIC (Cosby et al. 2001), and NuCM (Lui et al. 1992). 

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