Impacts of Elevated Carbon Dioxide and Tropospheric Ozone on the Growth and Productivity of Trees

181 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008.Forests are an integral part of the global carbon cycle. Any change in their capacity to assimilate carbon through photosynthesis has significant implications for global climate change in the 21st Century. The concentrations of two rising greenhouse gases, carbon dioxide ([CO2]) and tropospheric ozone ([O3]) have this potential through direct alteration of photosynthesis. This thesis aims to provide quantitative estimates of the magnitude of the impact of these gases on tree growth and productivity. Free-Air Concentration Enrichment (FACE) technology was employed at PopFACE to fumigate three fast growing Populus species to elevated [CO2] of 550 under open-air conditions from planting through canopy closure. Extensive measurements taken at PopFACE detailing photosynthesis, canopy leaf area development and microclimate created an unparalleled opportunity to provide the first estimates the gross primary productivity (GPP) of trees grown under FACE from planting through to canopy closure. Results suggest that GPP is stimulated by elevated [CO2] throughout but that the magnitude of the stimulation diminishes markedly as the canopy closes. It is shown that this is not due to physiological "down-regulation," but simply the result of the diminished response of leaves in low light to elevated [CO 2]. While a few FACE studies have examined the effect of [CO2] on forests, [O3] has received less attention, and chamber studies have shown a wide range of responses. To determine the magnitude [O3 ] impacts, the extensive, yet disparate evidence compiled in the peer-reviewed literature over more than 50 years was examined using meta-analytic techniques providing the first quantitative summary of the entirety of the O3 literature to date. The meta-analysis summarized 8589 independent measurements of tree biomass, growth, physiology and biochemistry. Results show that current [O3] is depressing light-saturated photosynthesis by 11% relative to pre-industrial concentrations. This indicates that the carbon sink represented by forest CO2 uptake is already diminished by [O3]. By 2050, the reductions could reach 18% with further reductions by 2100. It is also shown that there is a significant parallel decrease in biomass under current [O3] (7%) and if [O3] continues to rise, the reductions will become progressively more severe reaching 11% by 2050 and 17% by 2100. The study also revealed that gymnosperms were affected significantly less than angiosperms, suggesting that [O3] also has the potential to alter the composition of mixed forests. Root to shoot ratio was also decreased suggesting that beside decreased biomass, forests might become more vulnerable to drought and storms. Underlying decreased photosynthesis are significant decreases in stomatal conductance, transpiration, leaf area, chlorophyll content and Rubisco content. The results suggest that even in production forests the increased sink due to rising [CO2] will be greatly decreased at canopy closure, while rising [O3] will greatly decrease biomass. This implies that the northern hemisphere forest carbon sink is unlikely to keep pace with the rise in [CO2] and could diminish as a result of rising [O3].U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD