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

Herbivory results in an immediate increase in the rate of monoterpene emiss ion from conifer tissues to the atmosphere. The current study uses simulate d herbivory and a zero-dimensional photochemistry model with detailed treat ment of monoterpene photooxidation to explore the potential impact of these herbivore-induced monoterpene emissions on local tropospheric chemistry dy namics. Measured monoterpene emission rates from undamaged current-year and year-old needles and wounded current-year needles of ponderosa pine and Do uglas-fir trees were used to calculate whole-canopy fluxes expected from bo th a ponderosa pine and a Douglas-fir forest with 0%, 10%, and 25% damage t o current-year needles. Fluxes from ponderosa pine forests with 10%- and 25 %-damaged foliage are potentially 2- and 3.6-fold higher, respectively, tha n fluxes from forests with no herbivory. Douglas-fir forests experiencing 1 0% and 25% foliar damage can emit 1.6 and 2.5 times higher fluxes, respecti vely, than forests with no damaged foliage. The model simulations suggest t hat the fluxes resulting from even low-level herbivore damage (10% foliar d amage) 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 ra tios of nitrogen oxides (NOx). Ozone production is most sensitive to herbiv ore-induced emissions at NOx concentrations between 0.3 and 7 nmol/mol. The presence of isoprene in coniferous-forest air diminishes the role herbivor y plays in generating local ozone production. The results suggest that defo liation events should be considered to represent an important potential con trol over local oxidative tropospheric chemistry and to play an important r ole in perturbing local ozone dynamics in many rural coniferous forests thr oughout the United States.