Because seedlings and mature trees do not necessarily respond similarly to O(3) stress, it is critically important that exposure systems be developed that allow exposure of seedlings through to mature trees. Here we describe three different O(3) Free-Air Exposure Systems that have been used successfully for exposure at all growth stages. These systems of spatially uniform O(3) release have been shown to provide reliable O(3) exposure with minimal, if any, impact on the microclimate. This methodology offers a welcome alternative to chamber studies which had severe space constraints precluding stand or community-level studies and substantial chamber effects on the microclimate and, hence physiological tree performance.