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

Abstract

• Day-to-day variability in the carbon isotope composition of phloem sap (δ13Chd) and ecosystem respiratory CO 2 (δ13CR) were measured to assess the tightness of coupling between canopy photosynthesis (δ13C hd) 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. © New Phytologist (2005).