FLUXES OF CARBON AND WATER IN A PINUS-RADIATA FOREST SUBJECT TO SOIL-WATER DEFICIT

We measured, by eddy covariance, seasonal CO2 (F-CO2) and water (E) fl uxes in an 8-year-old New Zealand Pinus radiata D.Don plantation subje ct 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 sur face conductance and photosynthesis. During summer, fluxes were impede d by soil water (theta) deficit and were equal to or smaller than duri ng 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 partic ularly hot and dry days, respiration exceeded photosynthetic uptake an d the ecosystem was a net carbon source. Portraying the underlying bio chemistry of photosynthesis, daytime half-hourly F-CO2 increased with quantum irradiance absorbed by the canopy (Q(abs)) following a non-sat urating, rectangular hyperbola. Except for winter, this relation was v ariable, 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 establ ished. Consequently, as the tree growing season progressed, ecosystem carbon sequestration was strongly limited by the co-occurrence of high D at low theta.