Fluxes of carbon and water in a Pinus radiata forest subject to soil water deficit

Abstract

We measured, by eddy covariance, seasonal CO₂ (F_CO2) and water (E) fluxes in an 8-year-old New Zealand Pinus radiata D.Don plantation subject to growing season soil water deficit. Average rates of F_CO2 and E were highest in spring (324 mmol m^-2 d^-1 and 207 mol m^-2 d^-1, respectively) when the abiotic environment was most favourable for surface conductance and photosynthesis. During summer, fluxes were impeded by soil water (θ) deficit and were equal to or smaller than during winter (F_CO2 = 46 mmol m^-2 d^-1 in summer and 115 mmol m^-2 d^-1 in winter; E = 57 and 47 mol m^-2 d^-1, respectively). On particularly hot and dry days, respiration exceeded photosynthetic uptake and the ecosystem was a net carbon source.

Portraying the underlying biochemistry of photosynthesis, daytime half-hourly F_CO2 increased with quantum irradiance absorbed by the canopy (Q_abs) following a non-saturating, rectangular hyperbola. Except for winter, this relation was variable, including hysteresis attributable to diurnal variation in air saturation deficit (D). Daily ecosystem F_CO2, F_CO2/Q_abs and F_CO2/E were inversely proportional to maximum daily D, but in the cases of F_CO2 and F_CO2/Q_abs only after soil moisture deficit became established. Consequently, as the tree growing season progressed, ecosystem carbon sequestration was strongly limited by the co-occurrence of high D at low θ.