A Field Technique for Measuring ¹⁴CO₂ Absorption by Excised or Intact Conifer Needles

A small gas exchange chamber has been used to measure photosynthesis with 14C-labelled carbon dioxide on young Douglas-fir and Corsican pine needles exposed in four different ways: intact needles on the plantlet, needles left intact on an excised shoot, needles detached at the base, and cut portions of needles. A nonlinear adjustment of the light-response curve of photosynthesis has shown that at light saturation carbon dioxide absorption may be reduced by 35 percent if the needles are detached before the measurement. Excision of a shoot has no effect. Some advantages and drawbacks of the isotope method are discussed in connection with the variability of results obtained on trees. Forest Sci. 25:72-80.