Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2007
Publication Date: 10/23/2007
Citation: Kimball, B.A., Idso, S.B., Johnson, S.M., Rillig, M.C. 2007. Seventeen Years of CO2 Enrichment of Sour Orange Trees: Final Results. Global Change Biology 13: 2171-2183. Interpretive Summary: The CO2 concentration of the atmosphere is 35% higher now than it was in pre-industrial times due to fossil fuel burning, and it continues to rise 1-2 ppm per year. The long-term responses of trees to elevated CO2 are especially crucial (1) to mitigating the rate of atmospheric CO2 increase, (2) to determining the character of future forested natural ecosystems and their spread across the landscape, and (3) to determining the productivity of future agricultural tree crops. Therefore, a long-term experiment was started 1987 with sour orange trees in open-top CO2 enrichment chambers, and it was terminated in 2005 with a final massive harvest. Despite some acclimation or down-regulation of photosynthetic rates at 14 years into the experiment, the ratios of annual increments of wood plus fruit of the elevated-CO2 trees to those of the control trees plateaued at about 1.7 for the last past 9 years, and summing the total fruit production plus prunings over the 17 years plus the final standing biomass resulted in an 80% overall stimulation of growth of the CO2-enriched trees relative to the controls. Thus, so long as climate change effects do not affect growth, the increasing atmospheric CO2 concentration should be highly beneficial to future citrus production. These data also indicate that trees can sequester significantly more carbon at higher CO2 concentrations, which will help to mitigate the rate of rise of CO2 concentration in the atmosphere. Thus, while this research especially benefits growers and consumers of citrus products, it also benefits all plants and animals on Earth.
Technical Abstract: The long-term responses of trees to elevated CO2 are especially crucial (1) to mitigating the rate of atmospheric CO2 increase, (2) to determining the character of future forested natural ecosystems and their spread across the landscape, and (3) to determining the productivity of future agricultural tree crops. Therefore, a long-term CO2-enrichment experiment on sour orange trees was started in 1987, and the final results after 17 years are reported herein. Four sour orange trees (Citrus aurantium L.) were grown from seedling stage at 300 'mol mol-1 CO2 above ambient in open-top, clear-plastic-wall chambers at Phoenix, Arizona. Four control trees were similarly grown at ambient CO2. All trees were supplied ample water and nutrients comparable to a commercial orchard. After a peak 2-4 years into the experiment, there was a productivity plateau at about a 58 % enhancement of annual fruit and incremental wood production over the last several years of the experiment. When summed over the duration of the experiment, there was an overall enhancement of 80% of total biomass production. Much of the enhancement came from greater numbers of fruits produced, with no change in fruit size. Thicker trunks and branches and more branches and roots were produced, but the root/shoot ratio was unaffected. Also, there was almost no change in the elemental composition of the biomass produced, perhaps in part due to the minimal responsiveness of root-symbiotic arbuscular mycorrhizal fungi to the treatment.