Volume 10, Number 49: 5 December 2007
Ozone (O3) is the primary air pollutant responsible for visible foliar injury and reduced growth in trees the world over. Most studies of the subject suggest it gains entrance to leaves through their stomata, whereupon it interferes with the process of photosynthesis and thereby reduces plant productivity. The global significance of the phenomenon was indicated by Fowler et al. (1999), who estimated O3 to have been negatively impacting a quarter of earth's forests at the close of the 20th century, and who calculated it to have the potential to negatively impact fully one-half of the planet's forests by 2100.
What effect has this phenomenon had on earth's trees since the inception of the Industrial Revolution? And what will be its likely impact over the course of the current century?
In a meta-analysis of experiments conducted since the late 1960s in ambient-O3 and charcoal-filtered air, Wittig et al. (2007) calculate that the rise in the atmosphere's O3 concentration since the start of the Industrial Revolution has caused a mean decrease of 11% in the leaf photosynthetic CO2 uptake of earth's temperate and boreal forests. In addition, based on projections derived from the A2 storyline of the Special Report on Emissions Scenarios included in IPCC Assessment Report Four (which indicate that atmospheric O3 concentrations could rise 20-25% between 2015 and 2050, and that they could further increase by 40-60% by 2100 if current emission trends continue), they calculate that temperate and boreal forest photosynthetic rates could decline by an additional 8-16% by the end of the century.
Fortunately, the stomatal-aperture-constricting effect of concomitant past increases and anticipated future increases in the air's CO2 content tend to counter the negative influence of rising O3 concentrations by retarding O3 entry into plant leaves. In addition, the CO2-induced increase in leaf photosynthesis (its "aerial fertilization effect") has been shown to often more than compensate for the negative influence of ozone on leaf photosynthesis rates (see Ozone (Effects on Plants) in our Subject Index). What is more, these welcome findings comprise only half of the good news about rising CO2 concentrations and their impact on the ozone problem, as we describe in what follows.
First of all, it is a well-established fact that vegetative isoprene emissions are responsible for the production of vast amounts of tropospheric ozone (Chameides et al., 1988; Harley et al., 1999). In fact, it has been calculated by Poisson et al. (2000) that current levels of non-methane hydrocarbon (NMHC) emissions (the vast majority of which are isoprene, accounting for more than twice as much as all other NMHCs combined) likely increase surface ozone concentrations from what they would be in their absence by up to 50-60% over land. In addition, although little appreciated, it has been known for some time now (see Isoprene in our Subject Index) that atmospheric CO2 enrichment typically leads to large reductions in isoprene emissions from plants; yet this phenomenon has typically not been factored into projections of future atmospheric O3 concentrations.
This glaring omission was recently addressed by Arneth et al. (2007), who note that future vegetative isoprene emissions have typically been modeled to rise in tandem with projected increases in vegetative biomass and productivity (driven by projected changes in various environmental factors), which protocol, in an anticipated warmer and CO2-enriched world of the future, has generally led to predictions of significant increases in isoprene emissions and, therefore, significant increases in future atmospheric O3 concentrations, as have been anticipated to occur by Wittig et al. However, Arneth et al. convincingly demonstrate that "a quite different result is obtained when the direct CO2 effect on isoprene emissions is included," noting that in this more realistic situation a properly-forced model "maintains global isoprene emissions within ± 15% of present values."
In light of these important findings, the team of seven Swedish and UK researchers correctly concludes that "predictions of high future tropospheric O3 concentrations partly driven by isoprene emissions may need to be revised." And for a no-net-change in vegetative isoprene emissions between now and the end of the current century, the fears of Wittig et al. should fail to materialize. In fact, just the opposite should occur, as the greening of the earth continues.
Sherwood, Keith and Craig Idso
References
Arneth, A., Miller, P.A., Scholze, M., Hickler, T., Schurgers, G., Smith, B. and Prentice, I.C. CO2 inhibition of global terrestrial isoprene emissions: Potential implications for atmospheric chemistry. Geophysical Research Letters 34: 10.1029/2007GL030615.
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.
Fowler, D., Cape, J.N., Coyle, M., Flechard, C., Kuylenstierna, J., Hicks, K., Derwent, D., Johnson, C. and Stevenson, D. 1999. The global exposure of forests to air pollutants. Water, Air & Soil Pollution 116: 5-32.
Harley, P.C., Monson, R.K. and Lerdau, M.T. 1999. Ecological and evolutionary aspects of isoprene emission from plants. Oecologia 118: 109-123.
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.
Wittig, V.E., Ainsworth, E.A. and Long, S.P. 2007. To what extent do current and projected increases in surface ozone affect photosynthesis and stomatal conductance of trees? A meta-analytic review of the last 3 decades of experiments. Plant, Cell and Environment 30: 1150-1162.