Assessment of annual carbon exchange in a water-stressed Pinus radiata plantation: an analysis based on eddy covariance measurements and an integrated biophysical model

We used a combination of eddy flux, chamber and environmental measurements with an integrated suite of models to analyse the seasonality of net ecosystem carbon uptake (F_CO2) in an 8-year-old, closed canopy Pinus radiata D.Don plantation in New Zealand (42°52′ S, 172°45′ E). The analyses utilized a biochemically based, big-leaf model of tree canopy photosynthesis (A_c), coupled to multiplicative environmental-constraint functions of canopy stomatal conductance (G_c) via environmental measurements, a temperature-dependent model of ecosystem respiration (R_eco), and a soil water balance model. Available root zone water storage capacity at the measurement site is limited to about 50 mm for the very stony soil, and annual precipitation is only 660 mm, distributed evenly throughout the year. Accordingly the site is prone to soil moisture deficit throughout the summer. G _c and A_c obtained maximum rates early in the growing season when plentiful soil water supply was associated with sufficient quantum irradiance (Q_abs), and moderate air saturation deficit (D) and temperature (T). From late spring onwards, soil water deficit and D confined G_c and A_c congruously, which together with the solely temperature dependency of R_eco resulted in the pronounced seasonality in F_CO2. Reflecting a light-limitation of A_c in the closed canopy, modelled annual carbon (C) uptake was most sensitive to changes in Q_abs. However, Q_abs did not vary significantly between years, and changes in annual F_CO2 were mostly due to variability in summer rainfall and D. Annual C-uptake of the forest was 717 g C m^–2 in a near-average rainfall year, exceeding by one third the net uptake in a year with 20% less than average rainfall (515 g C m^–2).