Relationships to Forest Health

The model-based nutrient deficiency metric appears to be a good predictor of independent, on-the-ground indicators of current forest health and productivity. For oak and pine forests in Massachusetts, tree height and canopy transparency were significantly related to foliar Ca levels and to the modeled rate of base cation depletion. Canopies were more transparent and grew to lower heights where base cation depletion rates were higher, and foliar Ca status was lower. At these same sites, root-zone soil base saturation, pH, and Ca/Al ratio were correlated with the modeled base cation depletion rates. A separate evaluation also showed promising results: a comparison of model results with multiple-year aerial surveys of forest damage in Vermont indicated that both the frequency of damage and size of damaged areas were related to modeled base cation depletion. Forests in Vermont also showed greater canopy transparency and higher percentages of trees with chronic dieback where modeled base cation depletion rates were higher.

The critical loads framework can also be used to assess the recovery of ecosystems due to changes in pollution levels and management strategies. Miller (2005) estimated that the cation depletion likely to be previously occurring in 126,000 ha of forest in New Hampshire and 133,775 ha in Vermont was probably reversed by the 36 percent reduction in S deposition that occurred in the region from the late 1990s to the early 2000s. These forest areas should be experiencing recovery of base cation pools.