Impacts of Elevated Atmospheric CO₂ and O₃ on Paper Birch (Betula papyrifera): Reproductive Fitness

Abstract

Atmospheric CO₂ and tropospheric O₃ are rising in many regions of the world. Little is known about how these two commonly co-occurring gases will affect reproductive fitness of important forest tree species. Here, we report on the long-term effects of CO₃ and O₃ for paper birch seedlings exposed for nearly their entire life history at the Aspen FACE (Free Air Carbon Dioxide Enrichment) site in Rhinelander, WI. Elevated CO₂ increased both male and female flower production, while elevated O₃ increased female flower production compared to trees in control rings. Interestingly, very little flowering has yet occurred in combined treatment. Elevated CO₂ had significant positive effect on birch catkin size, weight, and germination success rate (elevated CO₂ increased germination rate of birch by 110% compared to ambient CO₂ concentrations, decreased seedling mortality by 73%, increased seed weight by 17%, increased root length by 59%, and root-to-shoot ratio was significantly decreased, all at 3 weeks after germination), while the opposite was true of elevated O₃ (elevated O₃ decreased the germination rate of birch by 62%, decreased seed weight by 25%, and increased root length by 15%). Under elevated CO₂, plant dry mass increased by 9 and 78% at the end of 3 and 14 weeks, respectively. Also, the root and shoot lengths, as well as the biomass of the seedlings, were increased for seeds produced under elevated CO₂, while the reverse was true for seedlings from seeds produced under the elevated O₃. Similar trends in treatment differences were observed in seed characteristics, germination, and seedling development for seeds collected in both 2004 and 2005. Our results suggest that elevated CO₂ and O₃ can dramatically affect flowering, seed production, and seed quality of paper birch, affecting reproductive fitness of this species.