Background
Earth's vegetation is responsible for the production of vast amounts of ozone (O3; Chameides et al., 1988; Harley et al., 1999), but it is also responsible for destroying a lot of O3.  With respect to the latter phenomenon, the authors of this intriguing paper mention three major routes by which O3 exits the air near the earth's surface: leaf stomatal uptake, surface deposition, and within-canopy gas-phase chemical reactions with biogenically-emitted volatile organic compounds (BVOCs).

The first of these exit routes, according to Goldstein et al., accounts for 30-90% of total ecosystem O3 uptake from the atmosphere (= O3 destruction), while the remainder has typically been attributed to deposition on non-stomatal surfaces.  However, they note that "Kurpius and Goldstein (2003) recently showed that the non-stomatal flux [from the atmosphere to oblivion] increased exponentially as a function of temperature at a coniferous forest site," and that "the exponential increase with temperature was consistent with the temperature dependence of monoterpene emissions from the same ecosystem, suggesting O3 was lost via gas phase reactions with biogenically emitted terpenes before they could escape the forest canopy."

In a study designed to take the next step towards turning the implications of this observation into something stronger than a mere suggestion, Schade and Goldstein (2003) demonstrated that forest thinning dramatically enhances monoterpene emissions.  In the current study, Goldstein et al. take another important step towards resolving the issue by measuring the effect of forest thinning on O3 destruction, in an attempt to see if it is dramatically enhanced in parallel fashion to the thinning-induced increase in monoterpene emissions.

What was done
In a ponderosa pine (Pinus ponderosa L.) plantation in the Sierra Nevada Mountains of California, USA, a management procedure to improve forest health and optimize tree growth was initiated on 11 May and continued through 15 June 2000.  This procedure involved the use of a masticator to mechanically "chew up" smaller unwanted trees and leave their debris on site, which operation reduced plantation green leaf biomass by just over half.  Simultaneously, monoterpene mixing ratios and fluxes were measured hourly within the plantation canopy, while total ecosystem O3 destruction was "partitioned to differentiate loss due to gas-phase chemistry from stomatal uptake and deposition."

What was learned
Goldstein et al. report that both the destruction of ozone due to gas-phase chemistry and emissions of monoterpenes increased dramatically with the onset of thinning, and that these phenomena continued in phase with each other thereafter.  Hence, they "infer that the massive increase of O3 flux [from the atmosphere to oblivion] during and following mastication is driven by loss of O3 through chemical reactions with unmeasured terpenes or closely related BVOCs whose emissions were enhanced due to wounding [by the masticator]."  Indeed, they say that "considered together, these observations provide a conclusive picture that the chemical loss of O3 is due to reactions with BVOCs emitted in a similar manner as terpenes," and that "we can conceive no other possible explanation for this behavior other than chemical O3 destruction in and above the forest canopy by reactions with biogenically emitted VOCs."

What it means
The authors say their results "suggest that total reactive terpene emissions might be roughly a factor of 10 higher than the typically measured and modeled monoterpene emissions, making them larger than isoprene emissions on a global scale."  If this proves to truly be the case, it will be a most important finding, for it would mean that vegetative emissions of terpenes, which lead to the destruction of ozone, are significantly greater than vegetative emissions of isoprene, which lead to the creation of ozone (Poisson et al., 2000).  In addition, there is substantial evidence to suggest that the ongoing rise in the air's CO2 content may well lead to an overall reduction in vegetative isoprene emissions (see Isoprene in our Subject Index), while at the same time enhancing vegetative productivity, which may well lead to an overall increase in vegetative terpene emissions.  As a result, there is reason to believe that the ongoing rise in the air's CO2 content will help to reduce the ongoing rise in the air's O3 concentration, which should be a boon to the entire biosphere.

References
Chameides, W.L., Lindsay, R.W., Richardson, J. and Kiang, C.S.  1988.  The role of biogenic hydrocarbons in urban photochemical smog: Atlanta as a case study.  Science 241: 1473-1475.

Harley, P.C., Monson, R.K. and Lerdau, M.T.  1999.  Ecological and evolutionary aspects of isoprene emission from plants.  Oecologia 118: 109-123.

Kurpius, M.R. and Goldstein, A.H.  2003.  Gas-phase chemistry dominates O3 loss to a forest, implying a source of aerosols and hydroxyl radicals to the atmosphere.  Geophysical Research Letters 30: 10.1029/2002GL016785.

Poisson, N., Kanakidou, M. and Crutzen, P.J.  2000.  Impact of non-methane hydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modeling results.  Journal of Atmospheric Chemistry 36: 157-230.

Schade, G.W. and Goldstein, A.H.  2003.  Increase of monoterpene emissions from a pine plantation as a result of mechanical disturbances.  Geophysical Research Letters 30: 10.1029/2002GL016138.