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 ecosys tem carbon uptake (F-CO2) in an 8-year-old, closed canopy Pinus radiata D.D on plantation in New Zealand (42 degrees 52' S, 172 degrees 45' E). The ana lyses utilized a biochemically based, big-leaf model of tree canopy photosy nthesis (A(c)), coupled to multiplicative environmental-constraint function s of canopy stomatal conductance (G(c)) via environmental measurements, a t emperature-dependent model of ecosystem respiration (R-eco), and a soil wat er balance model. Available root zone water storage capacity at the measure ment site is limited to about 50 mm for the very stony soil, and annual pre cipitation is only 660 mm, distributed evenly throughout the year. Accordin gly 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 plent iful soil water supply was associated with sufficient quantum irradiance (Q (abs)), and moderate air saturation deficit (D) and temperature (T). From l ate spring onwards, soil water deficit and D confined G(c) and A(c) congruo usly, which together with the solely temperature dependency of R-eco result ed 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 sens itive to changes in Qabs However, Qabs did not vary significantly between y ears, 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-a verage rainfall year, exceeding by one third the net uptake in a year with 20% less than average rainfall (515 g C m(-2)).