Eight-year responses of light interception, effective leaf area index, andstemwood production in fertilized stands of interior Douglas-fir (Pseudotsuga menziesii var. glauca)

Data from 72 Douglas-fir (Pseudotsuga,menziesii var. glauca (Beissn.) Franc e) plots across the Interior Northwest were used to determine whether (i) i ncreased intercepted photosynthetically active radiation (%IPAR), effective leaf area index (LAI(e)), and stemwood volume production (cubic metres ste mwood per hectare per year) could still be detected 7 or 8 years after nitr ogen fertilization and (ii) fertilization would increase the efficiency by which light is converted into stemwood volume. Projected LAI(e) varied from 1.82 to 6.07 m(2).m(-2) on the control plots. The fertilized plots interce pted 9-11% more light than the control plots (P < 0.001); they also had 22- 25% higher LAI(e) than the control plots (P < 0.001). Stemwood volume produ ction increased by 25-29% relative to the control (P < 0.001) and increased exponentially with %IPAR across all study plots (R-2 = 0.57). Stemwood gro wth efficiency averaged 12.0 +/- 0.4 (mean +/- SE), 13.5 +/- 0.4, and 13.7 +/- 0.4 m(3).ha(-1).a(-1) per IPAR for the control, low fertilization, and high fertilization plots, respectively (P < 0.01). Fertilization thus induc ed increases in both light interception and the efficiency with which inter cepted light was converted to stemwood across the region.