Herbivore-induced monoterpene emissions from coniferous forests: Potential impact on local tropospheric chemistry

Herbivory results in an immediate increase in the rate of monoterpene emission from conifer tissues to the atmosphere. The current study uses simulated herbivory and a zero-dimensional photochemistry model with detailed treatment of monoterpene photooxidation to explore the potential impact of these herbivore-induced monoterpene emissions on local tropospheric chemistry dynamics. Measured monoterpene emission rates from undamaged current-year and year-old needles and wounded current-year needles of ponderosa pine and Douglas-fir trees were used to calculate whole-canopy fluxes expected from both a ponderosa pine and a Douglas-fir forest with 0%, 10%, and 25% damage to current-year needles. Fluxes from ponderosa pine forests with 10%- and 25%-damaged foliage are potentially 2- and 3.6-fold higher, respectively, than fluxes from forests with no herbivory. Douglas-fir forests experiencing 10% and 25% foliar damage can emit 1.6 and 2.5 times higher fluxes, respectively, than forests with no damaged foliage. The model simulations suggest that the fluxes resulting from even low-level herbivore damage (10% foliar damage) are large enough to increase local tropospheric production of ozone and organic nitrates and to suppress hydroxyl radical (OH) concentrations. In both Douglas-fir and ponderosa pine forests, the predicted magnitude of the perturbations to each of these chemical species increases linearly with the extent of foliar damage and is critically dependent on local mixing ratios of nitrogen oxides (NO(x)). Ozone production is most sensitive to herbivore-induced emissions at NOx concentrations between 0.3 and 7 nmol/mol. The presence of isoprene in coniferous-forest air diminishes the role herbivory plays in generating local ozone production. The results suggest that defoliation events should be considered to represent an important potential control over local oxidative tropospheric chemistry and to play an important role in perturbing local ozone dynamics in many rural coniferous forests throughout the United States.