Long-term effects of elevated CO2 on sour orange trees

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 mu mol 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.