Photosynthesis and transpiration ofPinus radiata D. Don under natural conditions in a forest stand.

Gas-exchange ofPinus radiata foliage was measured with climatised cuvettes under natural light in the sun-crown of 8 m tall trees in a forest stand. Measurement began during a period of drought (ψWS -8.2 bar, ψWe -10.5 bar) and continued after elimination of soil moisture-deficit by watering (ψWS -0.5 bar, ψWe -5.5 bar). Soil and air moisture-deficits severely restricted gas-exchange. Watering resulted in an immediate decline in stomatal resistance (r s ) and an increase in net photosynthesis (P N ) of 13%. A slower progressive gas-exchange recovery occurred additionally during the 10 days after watering leading to a further decline inr s to 3 s cm-1 and an ultimate increase inP N of 38% when measured under comparable conditions at 8 mb v.p.d. Radiata pine had a high photosynthetic capacity with a measured maximumP N of 10.2 mg CO2 dm-2 h-1 total needle surface (11.4 mg CO2 g-1 DM h-1).Optimum temperature forP N in March (late summer) occurred at ca. 18°C. Rate ofP N was 95% saturated at irradiance of 900 μE m-2 s-1 and 50% saturated at only 270 μE m-2 s-1. Radiata pine needles responded directly to changes in atmospheric humidity by adjusting their stomatal diffusive resistance. As a result, between 8 and 18 mb v.p.d.P N declined by 2.3% mb-1 increase.