Canopy and Soil CO₂ concentration and isotopic measurements were conducted in a slash pine plantation during a progressive drought in the southeastern US (i) to determine variations in d¹³C of foliage (dCf), soil (dCs) and ecosystem-respired CO₂(d Cr) and (ii) to evaluate the usefulness of a two end-member oxygen isotope ratio(d¹⁸O of CO₂) approach to partition nighttime ecosystem respiration into soil and plant components at different heights within the canopy. VCf was enriched by 2.2 + 0.3 (se)%0 during the extreme drought in May relative to September When precipitation was above normal. The enrichment in d Cf exceeded changes in d Cr and d Cs for the same time period by 1.6+0.5%0 and 1.0+ 0.4₀, respectively. Lower variations in the ¹³C of soil respired CO₂ relative to the variations of the autotrophic component can buffer changes in dCr. dCr is linked to vapor pressure deficit (VPD) and its more enriched during periods of high VPD. Intensive measurements conducted at the Duke Forest FACE site confirmed the d Cr- VDP relation. d Cr was more Variable at the pine forest (range: -31 to -27%0) than at the hardwood forest (range -27 to -26%0). Our results from Duke forest suggest that forest age/type exert a significant influence on d Cr.