Biogenically produced and emitted hydrocarbons, such as monoterpenes and isoprene, are important trace gas species that affect the oxidative capacity of the atmosphere. Hydrocarbon emissions are species specific and are regulated by many environmental variables; the most important variables are thought to be temperature and light. During the production of hydrocarbons, plants use a measurable amount of the total carbon assimilated. This paper will report on the emission of monoterpenes from a Douglas fir / Western Hemlock old growth forest and the emission of isoprene from a managed poplar plantation, with special attention on the carbon balance. The old growth forest site is located at the Wind River Canopy Crane Research Facility in southwestern Washington. The site consists of a 75-meter freestanding construction crane that provides the unique opportunity to access the entire canopy within a 2.3-hectare area. The second site is a Boise-Cascade managed poplar plantation in eastern Oregon with a walk-up scaffolding tower. Branch and leaf level hydrocarbon emissions and physiological activities were measured using a dynamic enclosure at both sites during the summer of 1997, with sampling continuing during the summer of 1998. Daily and seasonal emission trends will be compared with the photosynthetic active radiation (PAR), leaf temperature, and carbon assimilation. This comparison will quantify the role of biogenic hydrocarbons in carbon exchange for two very different terrestrial forest ecosystems. The study of the surface-atmosphere exchanges as a function of atmospheric variables also contributes to the topic of effects of climate on forest ecosystems.