CO2 enrichment of sour orange trees: 13 years and counting

In mid-July of 1987, we began the first phase of what is currently the worl d's longest continuous atmospheric CO2 enrichment experiment by planting ei ght sour orange tree seedlings out-of-doors in undisturbed soil at Phoenix, Arizona, USA, and enclosing them, in pairs, in clear-plastic-wall open-top chambers, two of which have ever since mid-November of that year been main tained at an atmospheric CO2 concentration of 400 ppm and two of which have been similarly maintained at a CO2 concentration of 700 ppm, Over the firs t two years of our ongoing study, this 75% increase in the air's CO2 concen tration caused the CO2-enriched/ambient-treatment ratio of the trees' above ground wood biomass to rise to a value in excess of 3.0. Thereafter, the ra tio experienced exponential decay; but it leveled out at a value of approxi mately 1.8 at the end of seven more years. Fruit production began in the th ird year of the study, with the CO2-enriched/ambient-treatment ratio of yea rly fruit biomass production debuting at a value of 25. This ratio, too, su bsequently experienced rapid exponential decay; but over the last four year s of the study (years 10-13), it has maintained the same average and essent ially constant value as the aboveground wood biomass ratio. We believe thes e findings are indicative of the likelihood that the CO2-enriched trees may have reached an equilibrium condition with respect to the CO2-induced enha ncement of wood biomass and fruit production, and that they will not substa ntially depart from these steady-state responses over the remainder of thei r lifespan. Nevertheless, the experiment continues. Published by Elsevier S cience B.V.