Nitrogen Fertilization and Water Effects on Photosynthesis and Earlywood–Latewood Production in Douglas-fir

Abstract

Single tree plots in a 23-year-old stand of Douglas-fir were: (1) fertilized at a rate of 448 kg nitrogen (N) per ha with NH₄NO₃ in April; (2) irrigated in the summer of the same year and in the following year with 25 mm of water per week; (3) given the combined fertilizer and irrigation treatment, or (4) left untreated. Diameter growth at breast height (1.37 m) increased in the first year by 16, 15 and 59%, respectively, above that of the control. Similar effects were obtained at breast height in the second year and also at the one-half tree height position. Only earlywood production was affected significantly; this resulted from an increase in rate and length of time of earlywood production. Treatment effect on nitrogen, phosphorus (P) and potassium (K) concentration of leaves, leaf growth and water stress of soil and trees, was studied. Rate of photosynthesis was reduced similarly by water stress for fertilized and unfertilized trees; an increase in CO₂ diffusion resistance appeared to cause the reduction in rate. Tree water stress had a more adverse effect on stem diameter growth than on rate of photosynthesis. The interaction between irrigation and fertilization indicates that nitrogen fertilization will have the most effect on stem diameter growth in years and on sites with favorable soil-water conditions.