CONTROLS OVER MONOTERPENE EMISSIONS FROM BOREAL FOREST CONIFERS

We investigated controls over the emission of monoterpenes from two sp ecies of boreal forest conifers, black spruce (Picea mariana Miller (B .S.P.)) and jack pine (Pinus banksiana Lamb). Monoterpenes are importa nt in plants as carbon-based defensive compounds and in the atmosphere as photochemically reactive compounds that affect ozone and carbon mo noxide concentrations. We examined ecological theories of plant alloca tion to defensive compounds in relation to emission rates of monoterpe nes from the foliage of these two species. Monoterpene emission from p lants is controlled by the vapor pressure of the monoterpenes within p lant 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 emis sion rate from foliage, and demonstrated that emission rate was linear ly related to nitrogen concentration and exponentially related to air temperature, Current theories of plant allocation to carbon-based defe nses predict an inverse relationship between foliar nitrogen and carbo n-based defenses. We found that, under certain circumstances, these th eories were sufficient to predict concentrations and emissions, but un der other circumstances, the theories did not predict monoterpene conc entrations or emissions. These results are discussed in the context of landscape/regional modeling of hydrocarbon emission from vegetation.