Trees grown in CO2-enriched air nearly always exhibit a number of positive responses to what we call the "elixir of life," while trees grown in O3-enriched air tend to experience a number of negative responses. So what happens when both of these trace constituents of the atmosphere increase in unison? We here review what some of the studies that have explored this question as it pertains to yellow-poplar (Liriodendron tulipifera L.) trees have learned about the subject.
Scherzel et al. (1998) grew yellow-poplar seedlings in open-top chambers for four years at three different combinations of atmospheric O3 and CO2 -- (1) ambient O3 and ambient CO2, (2) doubled O3 and ambient CO2, and (3) doubled O3 and doubled CO2 -- to study the interactive effects of these gases on leaf-litter decomposition. This experiment revealed that the decomposition rates of yellow-poplar leaves were similar for all three treatments for nearly five months, after which time litter produced in the elevated O3 and elevated CO2 air decomposed at a significantly slower rate, such that even after two years of decomposition, litter from the elevated O3 and elevated CO2 treatment still contained about 12% more biomass than litter produced in the other two treatments. This reduced rate of decomposition under elevated O3 and CO2 conditions will likely result in greater carbon sequestration in soils supporting yellow-poplar trees over the next century or more.
Loats and Rebbeck (1999) grew yellow-poplar seedlings for ten weeks in pots they placed within growth chambers filled with ambient air, air with twice the ambient CO2 concentration, air with twice the ambient O3 concentration, and air with twice the ambient CO2 and O3 concentrations to determine the effects of elevated CO2 and O3 on photosynthesis and growth in this deciduous tree species. In doing so, they found that doubling the air's CO2 concentration increased the rate of net photosynthesis by 55% in ambient O3 air, and that at twice the ambient level of O3 it stimulated net photosynthesis by an average of 50%. Similarly, the doubled CO2 concentration significantly increased total biomass by 29%, while the doubled O3 concentration had little impact on growth.
Last of all, Rebbeck et al. (2004) grew yellow poplar seedlings for five years within open-top chambers in a field plantation at Delaware, Ohio, USA, exposing them continuously from mid-May through mid-October of each year to either (1) charcoal-filtered air to remove ambient O3, (2) ambient O3, (3) 1.5 times ambient O3, and (4) 1.5 times ambient O3 plus 350 ppm CO2 above ambient CO2, while they periodically measured a number of plant parameters and processes. Throughout the study, the trees were never fertilized, and they received no supplemental water beyond some given in the first season.
Averaged over the experiment's five growing seasons, the midseason net photosynthetic rate of upper canopy foliage at saturating light intensities declined by 10% when the trees were grown in ambient O3-air and by 14% when they were grown in elevated O3-air, when compared to the trees that were grown in the charcoal-filtered air, while seasonal net photosynthesis of foliage grown in the combination of elevated O3 and elevated CO2 was 57-80% higher than it was in the trees exposed to elevated O3 alone. There was also no evidence of any photosynthetic down regulation in the trees exposed to the elevated O3 and CO2 air, with some of the highest rates being observed during the final growing season. Consequently, Rebbeck et al. concluded that "elevated CO2 may ameliorate the negative effects of increased tropospheric O3 on yellow-poplar." In fact, their results suggest that a nominally doubled atmospheric CO2 concentration more than compensates for the deleterious effects of a 50% increase in ambient O3 levels ... and by several times over.
As the air's CO2 content continues to rise, therefore, earth's yellow-poplar trees will likely display substantial increases in photosynthetic rate and biomass production, even under conditions of elevated O3 concentrations; and the soils in which the trees grow should sequester ever greater quantities of carbon.
References
Loats, K.V. and Rebbeck, J. 1999. Interactive effects of ozone and elevated carbon dioxide on the growth and physiology of black cherry, green ash, and yellow-poplar seedlings. Environmental Pollution 106: 237-248.
Rebbeck, J., Scherzer, A.J. and Loats, K.V. 2004. Foliar physiology of yellow-poplar (Liriodendron tulipifera L.) exposed to O3 and elevated CO2 over five seasons. Trees 18: 253-263.
Scherzel, A.J., Rebbeck, J. and Boerner, R.E.J. 1998. Foliar nitrogen dynamics and decomposition of yellow-poplar and eastern white pine during four seasons of exposure to elevated ozone and carbon dioxide. Forest Ecology and Management 109: 355-366.
Last updated 30 April 2008