Forest Sensitivity to Acid Deposition

In New England and Eastern Canada

Background

Although sulfur emissions have decreased as a result of SO₂ control programs, projected emissions of acidifying sulfur and nitrogen compounds are expected to have continuing negative impacts on forests. These emissions present some of the most serious long-term threats to forest health and productivity in northeastern North America . Excess sulfur and nitrogen deposition may reduce the supply of nutrients available for plant growth. Nutrient depletion leads to increases in the susceptibility of forests to climate, pest and pathogen stress which results in reduced forest health, reduced timber yield, and eventual changes in forest species composition.

Conceived by the Joint Conference of the New England Governors and Eastern Canadian Premiers (NEG/ECP), under the direction of its Committee on the Environment, their 1998 Acid Rain Action Plan called for the formation of a Forest Mapping Working Group to conduct a regional assessment of the sensitivity of northeastern North American forests to current and projected sulfur and nitrogen emissions levels.

The 5 3 Forest Mapping Working Group (FMWG) has been working to identify specific forested areas most sensitive to continued sulfur and nitrogen deposition and to estimate deposition rates required to maintain forest health and productivity.

The assessment process evaluates the capacity of soil chemical processes to replenish plant-available nutrients that are lost due to the enhanced leaching induced by acid deposition.  The nutrient requirement of different forest types, harvesting rates, geology, soil characteristics, and climatic conditions all play a role in how much sulfur and nitrogen deposition can be tolerated without reductions in plant-available nutrients.