GROWTH AND MAINTENANCE RESPIRATION IN STEMS OF QUERCUS-ALBA AFTER 4 YEARS OF CO2 ENRICHMENT

Atmospheric CO2 enrichment is increasingly being reported to inhibit l eaf and whole-plant respiration. It is not known, however, whether thi s response is unique to foliage or whether woody-tissue respiration mi ght be affected as well. This was examined for mid-canopy stem segment s of white oak (Quercus alba L.) trees that had been grown in open-top field chambers and exposed to either ambient or ambient + 300 mu mol mol(-1) CO2 over a 4-year period. Stem respiration measurements were m ade throughout 1992 by using an infrared nas analyzer and a specially designed in situ cuvette. Rates of woody-tissue respiration were simil ar between CO2 treatments prior to leaf initiation and after leaf sene scence, but were several fold greater for saplings grown at elevated c oncentrations of CO2 during much of the growing season. These effects were most evident on 7 July when stem respiration rates for trees expo sed to elevated CO2 concentrations were 7.25 compared to 3.44 mu mol C O2 m(-2) s(-1) for ambient-grown saplings. While other explanations mu st be explored, greater rates of stem respiration for saplings grown a t elevated CO2 concentrations were consistent with greater rates of st em growth and more stem-wood volume present at the time of measurement . When rates of stem growth were at their maximum (7 July to 3 August) , growth respiration accounted for about 80 to 85% of the total respir atory costs of stems at both CO2 treatments, while 15 to 20% supported the costs of stem-wood maintenance. Integrating growth and maintenanc e respiration throughout the season, taking into account treatment dif ferences in stem growth and volume, indicated that there were no signi ficant effects of elevated CO2 concentration on either respiratory pro cess. Quantitative estimates that could be used in modeling the costs of woody-tissue growth and maintenance respiration are provided.