Variation in the degree of coupling between δ¹³C of phloem sap and ecosystem respiration in two mature Nothofagus forests

• Day-to-day variability in the carbon isotope composition of phloem sap (δ¹³C_hd) and ecosystem respiratory CO ₂ (δ¹³C_R) were measured to assess the tightness of coupling between canopy photosynthesis (δ¹³C _hd) and ecosystem respiration (δ¹³C_R) in two mature Nothofagus solandri (Hook. f.) forests in New Zealand. • Abundant phloem-tapping scale insects allowed repeated, nondestructive access to stem phloem sap 1-2 m above ground. δ¹³C_hd was compared with δ¹³C predicted by an environmentally driven, process-based canopy photosynthesis model. Keeling plots of within-canopy CO₂ were used to estimate δ¹³C_R. • By including a lag of 3 d, there was good agreement in the timing and direction of variation in δ¹³C_hd and predictions by the canopy photosynthesis model, suggesting that δ¹³C_hd represents a photosynthesis-weighted, integrative record of canopy photosynthesis and conductance. • Significant day-to-day variability in δ¹³C_R was recorded at one of the two forests. At this site, δ¹³C_R reflected variability in δ¹³C_hd only on days with <2 mm rain. We conclude that the degree of coupling between canopy photosynthesis and ecosystem respiration varies between sites, and with environmental conditions at a single site.