LONG-TERM EFFECTS OF ELEVATED CO2 ON SOUR ORANGE TREES

Authors: Kimball, Bruce; IDSO, SHERWOOD - CTR FOR STUDY CO2 & GC

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 9/15/2004
Publication Date: 7/19/2005
Citation: Kimball, B.A., Idso, S.B. 2005. Long-term effects of elevated CO2 on sour orange trees. In Abstracts of 6th International Symposium on Plant Responses to Air Pollution and Global Changes from Molecular Biology to Plant Production and Ecosystem. Yatabe Printing Co., Ltd, Tsukuba, Japan. p 80.

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. Despite some acclimation or down-regulation of photosynthetic rates at 14 years into the experiment, the ratios of annual increments of both wood and fruit of the elevated-CO2 trees to those of the control trees have plateaued at about 1.75 for the past 9 years. 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, we initiated a long-term CO2-enrichment experiment on sour orange trees in 1987. Four sour orange trees (Citrus aurantium L.) have been grown from seedling stage at 300 umol mol-1 CO2 above ambient in open-top, clear-plastic-wall chambers at Phoenix, Arizona. Four control trees have been similarly grown at ambient CO2. All trees have been given ample water and nutrients. The ratios of wood plus fruit annual biomass increments of the elevated-CO2 trees to those of the control trees reached a peak of about 3.0 two years into the experiment, declined from about year 2 to year 8, and has plateaued at about 1.75 for the past 9 years. The enhancement ratio for net photosynthesis was about 2.8 in year 2 but had declined to 1.3 by the 14th year, indicating some acclimation to the elevated CO2. Initial reductions in leaf N concentrations disappeared by year 7, while carbohydrate concentrations remained higher. Intrinsic water use efficiency determined from carbon isotope ratios was increased under elevated-CO2 by the same amount as biomass, which implies water use was unchanged. Storage proteins have been detected in the leaves of the enriched trees, which might be a mechanism that enables a six-fold increase in the rate of bud burst in the spring.