This project will test and improve computer models designed to predict forest responses to deposition of ammonium (NH4) and nitrate (NO3) due to atmospheric pollution. The investigators will determine how increased nitrogen (N) inputs influence forest growth, the uptake and release of atmospheric CO2 by forests, and N retention by forests at decadal time scales. This will be done by resampling forests that were labeled with 15N (at non-radioactive isotope) tracers in the early 1990s and testing models by comparing predicted redistribution of 15N on experimental plots in central Massachusetts with measurements of tracers. The revised models will then be to predict 15N tracer movements on a forest in Maine. After a second round of testing, the improved models and GIS (Geographic information systems) software will be used to predict how N deposition is influencing forest growth and forest-atmoshphere C exchanges in the northeastern U.S.
