We investigated controls over the emission of monoterpenes from two species of boreal forest conifers, black spruce (Picea mariana Miller (B.S.P.)) and jack pine (Pinus banksiana Lamb). Monoterpenes are important in plants as carbon-based defensive compounds and in the atmosphere as photochemically reactive compounds that affect ozone and carbon monoxide concentrations. We examined ecological theories of plant allocation to defensive compounds in relation to emission rates of monoterpenes from the foliage of these two species. Monoterpene emission from plants is controlled by the vapor pressure of the monoterpenes within plant tissues, and vapor pressure is controlled by two parameters, air temperature and terpene concentration within the tissues. We measured the concentration of terpenes and nitrogen within foliage and the emission rate from foliage, and demonstrated that emission rate was linearly related to nitrogen concentration and exponentially related to air temperature. Current theories of plant allocation to carbon-based defenses predict an inverse relationship between foliar nitrogen and carbon-based defenses. We found that, under certain circumstances, these theories were sufficient to predict concentrations and emissions, but under other circumstances, the theories did not predict monoterpene concentrations or emissions. These results are discussed in the context of landscape/regional modeling of hydrocarbon emission from vegetation.