Effects of Nitrogen Fertilization on Photosynthesis and Respiration in Douglas-Fir

Following a spring application of ammonium nitrate at the rate of 400 lb N/acre, diameter growth of 24-yr-old Douglas-fir trees was increased by 51 and 54 percent in the first and second growing season, respectively. Except for brief periods, the rate of net photosynthesis, measuredon detached shoots, was higher for treated trees from early July of the first season until the end of July of the second season, but not thereafter. In the treatment year, the rate was increased only for current shoots, but photosynthesis was also increased for 1-yr-old shoots in June andJuly of the second season. Nitrogen application increased photosynthesis only when light intensities were 2000 ft-c or above; the increase was 78 percent at 5000 ft-c with current-year shoots in the year of fertilization. Fertilization increased dark respiration rates of current and 1-yr-oldshoots. Forest Sci. 17:407-414.