Trees depend on our atmosphere to obtain the necessary carbon dioxide (CO2) and moisture for their growth. An acid rain testing site is located at the eastern end of the Windy Ridge Trail. This site began collecting data in the early 1970's as part of a nationwide project established by the National Atmospheric Deposition Program to monitor the intensity and location of acid rain centers. Trees are damaged by acid rain when it falls on their foliage. Acid rain may also damage trees by leaching nutrients out of the soil, which are vital to tree growth.

Between 1987 and 1991, the effects of ozone (O3) on Douglas-fir growth was studied. (Ozone is created when car emissions, containing nitrous oxide (NO), undergo an oxidizing reaction in the presence of ultraviolet light). Because we are fairly isolated from large cities at Pack Forest, we could expose individual trees to ozone and then compare them to non-exposed trees. Plastic chambers were fitted around research trees that were then fumigated with ozone. This ozone was found to interfere with trees' biological processes, such as photosynthesis and respiration. New research by the University of Washington Field Station for Protected Area Research has also shown that ozone levels are higher in the areas surrounding Mt. Rainier National Park than in urban areas because prevailing winds carry ozone emissions from Seattle and Tacoma to the area. Unique atmospheric conditions at higher elevations also contribute to these higher levels because they do not allow the ozone to dissipate, as it normally does each evening in lower elevations.

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