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 (delta C-13(hd)) and ecosystem respiratory CO2 (delta C-13(R)) were measured to assess the tightness of coupling between canopy photosynthesis (delta C-13(hd)) and ecosystem respiration (delta C-13(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. delta C-13(hd) was compared with delta C-13 predicted by an environmentally driven, process-based canopy photosynthesis model. Keeling plots of within-canopy CO2 were used to estimate delta C-13(R). By including a lag of 3 d, there was good agreement in the timing and direction of variation in delta C-13(hd) and predictions by the canopy photosynthesis model, suggesting that delta C-13(hd) represents a photosynthesis-weighted, integrative record of canopy photosynthesis and conductance. Significant day-to-day variability in delta C-13(R) was recorded at one of the two forests. At this site, delta C-13(R) reflected variability in delta C-13(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.