What was done
Seedlings of yellow-poplar (Liriodendron tulipifera) and two eastern white pine (Pinus strobus) genotypes were grown in open-top chambers for four years at various atmospheric concentrations of O3 (ozone) and CO2 --either ambient or twice ambient-- to study the interactive effects of these gases on foliar nitrogen content and litter decomposition in these species. Unfortunately, because elevated CO2 was not tested as an isolated variable, no direct effects of elevated CO2 on these parameters were obtained.  However, the interactive effects of elevated CO2 and O3 on these species were elucidated from this study.

Rates of litter decomposition were determined by weighing senesced leaves collected from each treatment combination before and after putting them in bags placed in natural forest soils in the nearby woods for various lengths of time.

What was learned
Elevated O3, alone, had little impact on foliar nitrogen contents in either tree species, while the combination of elevated O3 and CO2 tended to significantly reduce it (range of 8 to 28%).  Thus, the observed reductions in leaf nitrogen content that occurred in this interactive treatment were likely due to the effects of elevated CO2 alone (even though they were not directly isolated by experimentation).

Rates of litter decomposition of yellow-poplar were similar for all treatment combinations for nearly five months, after which time litter produced in the elevated O3 and CO2 treatment combination decomposed at a significantly slower rate.  Even after two years of decomposition, for example, litter from the elevated O3 and CO2 treatment still contained about 12% more biomass than litter produced from any other treatment.  In contrast, rates of litter decomposition of eastern white pine were not significantly affected by any treatment, regardless of genotype.

What it means
As the CO2 content of the air rises, it is likely that its O3 content will also increase.  Under such atmospheric conditions, it is likely that foliar nitrogen concentrations in yellow-poplar and eastern white pine seedlings will decrease, as this limiting nutrient is translocated away from photosynthesizing foliage to other areas that require it within the plants.  However, it is not likely that this reduction in leaf nitrogen content will reduce overall seedling growth and biomass.  In addition, the results from this study indicate that future rates of litter decomposition will not be affected in eastern white pine, while they will be slightly reduced in yellow-poplar.  Thus, reduced rates of decomposition under elevated O3 and CO2 conditions will likely result in greater carbon sequestration in yellow-poplar litter during the next century.