Critical Loads and Target Loads: Tools for Assessing, Evaluating and Protecting Natural Resources Ellen Porter Deborah Potter, Ph.D. National Park Service U.S.D.A. Forest Service WESTAR Council November 15-16, 2005 Air Resource Management Programs

Why do we need them? What are critical loads/target loads? How are they developed? How are they used? Information needs Overview

Why do we need them? Atmospheric Deposition Effects –Effects from nitrogen deposition Chemical changes in soils and trees Nitrogen saturation in high-elevation soils; runoff into lakes Altered terrestrial and aquatic plant communities –Effects from sulfur deposition acidification of lakes and streams altered soil chemistry/nutrient cycling mobilization of aluminum in soil altered growth of spruce-fir forests Need for evaluation and assessment –How much deposition is too much?

“You’ve got to be careful if you don’t know where you’re going because you might not get there.” Yogi Berra

History of Critical/Target loads Europe: –multi-national, coordinated approach (International Cooperative Programmes) for critical loads research and implementation; critical/target loads used to set emissions reductions goals. Canada: –critical/target loads used to set emission reductions goals. U.S. Federal Land Managers (National Park Service, Forest Service, Fish and Wildlife Service): –use screening thresholds, limits of acceptable change, and deposition analysis thresholds to assess deposition. U.S. EPA: –currently does not incorporate critical loads in air pollution increments or standards.

Critical load/Target load Critical load: “The quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.” (Nilsson and Grennfelt 1988) Critical loads can be developed for any pollutants.

Effect to specific resource Load (kg/ha/yr) harmful effect critical load no harmful effect

Target loads –Target Load: The level of exposure to one or more pollutants that results in an acceptable level of resource protection; may be based on political, economic, or temporal considerations. “Protective” target loads vs “Interim” target loads

Interim Target Load Current deposition Deposition reduction Glide path Deposition (kg/ha/yr) Time Protective Target Load is set below critical load to ensure that critical load/harmful effect is not reached. An Interim Target Load is used if current deposition is above the critical load, to establish a glide path towards the critical load, and eventually, the protective target load. Re-evaluate; adjust Critical Load Protective Target Load Current deposition

Load (kg/ ha /yr) Changes in soil chemistry Change in plant communities Episodic acidification Forest health decline Critical loads are defined for specific indicators and effects. Chronic acidification

Scientists conduct empirical studies to identify resources sensitive to deposition Scientists derive critical loads from empirical studies and modeling analyses. Federal manager is guided by agency policy in selecting sensitive resources and indicators of change; defines ”harmful” changes to sensitive resources based on policy goals. Decisions about interim or sustainable levels of N and S deposition on federal lands are made by federal manager, with consultation with air regulators and others if target loads will be used for emissions control strategies. CRITICAL LOAD DEVELOPMENT SCIENCE FEDERAL MANAGER

Lawrence and Huntington, USGS publication WRIR Ecosystem Approach

Critical Load Calculations Simple, Steady-state, Mass-Balance Model Empirical critical loads Dynamic models Input +/- Retention = Output Observation, experiment PnET, MAGIC, CENTURY Model Examples

Critical Load Calculations Simple, steady-state, Mass-Balance Model Empirical critical loads Dynamic models total deposition rate (S, N) soil properties soil solution chemistry vegetation (nutrient uptake, storage) bedrock composition water chemistry (BCC, ANC, N, S) Model Example Data Inputs

Empirical Critical Loads Based on observed (actual) ecosystem response at known deposition rates, e.g., impacts to soil, water, plant and animal communities may consider results of field studies and mesocosm experiments to link cause and effect managers need a conservative loading rate that protects the most sensitive ecosystem components

Using the Critical Load Concept  Land Management Planning  Resource Planning Act  National Environmental Policy Act  State Implementation Plans  Regional Planning Organizations  New Source Review  Assess the success of air pollution regulatory programs (e.g., cap-and-trade programs)

Using Critical and Target Loads To better evaluate and communicate how pollution is affecting natural resources and what is needed to protect and restore them… To our own decision makers To the regulatory community that sets the rules for controlling air pollution To the public whose support is needed for positive change to occur

Challenges Synergistic effects Ecosystem complexity Lag time for ecosystem response Long-term ecosystem recovery / state Cost of complex models (data gathering)

Increased communication and collaboration between land managers and scientists on resource management needs to meet resource protection goals. Inventory and monitor sensitive resources; Identify / refine models for estimating critical loads in both aquatic and terrestrial ecosystems; Collaborate to use critical loads in air regulatory planning processes at the national, state, and local level. Future Direction

Summary Use loading rate models to … identify and attain loading rates (e.g., kg N or S per ha per year) that will protect sensitive ecosystems and allow degraded ecosystems to recover