Abstract. Canopy and soil CO₂ concentration and isotopic measurements were conducted in a slash pine forest during a progressive drought in the southeastern US (i) to determine and compare variations in δ¹³C of foliage (δCf) , soil (δCs) and ecosystem-respired CO₂ (δCr) and (ii) to evaluate the usefulness of a two end-member oxygen isotope ratio (δ¹⁸O of CO₂) approach to partition nighttime ecosystem respiration into soil and plant components at different heights within the canopy. δCf was enriched by 2.2 ± 0.3 (se) ‰ during the extreme drought in May relative to September when precipitation was above normal. The enrichment in δCf exceeded changes in δCr and δCs for the same time period (1.6 ± 0.5 ‰ and 1.0 ± 0.4 ‰, respectively). Lower variations in the ¹³C of soil-respired CO₂ relative to the variations of the autotrophic component can buffer changes in δCr.

Application of a two end-member model to canopy δ¹⁸O-CO₂ indicated that soil CO₂ contribution to nighttime CO₂ buildup within the canopy decreased from 100% at the soil surface to 0% within the canopy in September. In May, during the extreme drought period, soil respiration rate was 2.7 times lower than the rate in September despite similar soil temperatures (23 in May vs. 19 ^oC in September). Soil and above-ground respiration equally contributed to the nocturnal CO₂ buildup within the canopy in May. Our model was limited in that it produced an upper limit value for soil respiration and neglected respiration by woody tissue.