Mixing ratios and emission rates of monoterpenes were measured above a ponderosa pine plantation in the Sierra Nevada mountains before, during and after a pre-commercial thinning in spring 2000. The thinning removed and left onsite approximately one half of the plantations biomass. Monoterpene fluxes increased tenfold during the thinning and pinene mixing ratios in excess of 3 ppb were observed, possibly altering regional atmospheric chemistry. The increase was mostly because of higher basal emission rates, but small changes in the temperature-dependence were also found. Using an emission-model based on these responses, the additional monoterpene emissions due to the thinning were estimated to increase emissions by a factor of forty, and yearly emissions by a factor of five. Using US tables of absolute timber removal and on site residue volumes from logging and thinning activities, we calculate that current US monoterpene emissions may be underestimated by several percent.