Tuesday, November 6, 2007
215-5
The Long-Term Effects of Past N-Fertilization on Subsequent Stands of Douglas-Fir in the Pacific Northwest.
Paul Footen1, Robert Harrison2, Gage Wagoner1, and Brian Strahm1. (1) University of Washington, 27247 NE Union Hill Rd, Redmond, WA 98053, (2) Box 532100, University of Washington, University of Washington, Forest Resources, 218 Bloedel, Seattle, WA 98195-2100
Forest growth in the Pacific Northwest is frequently limited by the supply of plant-available nitrogen (N). To increase productivity, N-fertilization is a commonly utilized silvicultural practice. The long-term effects of this practice have not been investigated to our knowledge. The objective of this study was to learn if subsequent stands of Douglas-fir were affected by previous fertilization. The carryover effects of N-fertilization on understory vegetation and Douglas-fir seedling growth were quantified in five stands in the Puget Sound Region of western Washington. Biomass and N-content of understory vegetation, and average height and diameter at breast height (dbh) of 12-year-old Douglas-fir seedlings were assessed on control (untreated) and N-fertilized plots that had been fertilized with urea 28-30 years ago (total amount of applied N 810-1120 kg/ha). Measurement plots were 0.1 ac with one control and one fertilized plot in each of the five stands. Understory vegetation was sampled to ground level in five random 0.25 m2 subplots on each measurement plot. Samples were dried at 70° C to a constant weight and weighed to obtain biomass estimates. The samples were then finely ground and analyzed for N-content (using a Perkin-Elmer CHN Analyzer). Installation-level values were calculated by averaging all subplots within one installation. Statistical analyses were performed on installation-level data using the paired-sample t-test of these installation means (a = 0.1). Understory vegetation biomass on previously fertilized plots was 73% greater (p = 0.005), and N-content was 97% greater (p = 0.004) than on control plots. In 2006, mean seedling height was 15% greater (p = 0.06) and mean dbh was 29% greater (p = 0.04) than on control plots. These results show that past N-fertilization markedly increased seedling growth, and understory biomass and N-content in a subsequent rotation. Use of N-fertilizer can potentially increase site quality decades after application. This finding has multiple implications for silviculturalists and forest ecosystem managers. The trends in this study should continue to be monitored, and similar studies should be established, to further understand the carryover effects of N-fertilization on forests.