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

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Abstract
• Day-to-day variability in the carbon isotope composition of phloem sap (δ13Chd) and ecosystem respiratory CO2 (δ13CR) were measured to assess the tightness of coupling between canopy photosynthesis (δ13Chd) and ecosystem respiration (δ13CR) 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. δ13Chd was compared with δ13C predicted by an environmentally driven, process-based canopy photosynthesis model. Keeling plots of within-canopy CO2 were used to estimate δ13CR. • By including a lag of 3 d, there was good agreement in the timing and direction of variation in δ13Chd and predictions by the canopy photosynthesis model, suggesting that δ13Chd represents a photosynthesis-weighted, integrative record of canopy photosynthesis and conductance. • Significant day-to-day variability in δ13CR was recorded at one of the two forests. At this site, δ13CR reflected variability in δ13Chd 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.