Effects of Five-Year Vegetation Control On Aboveground Biomass and Nitrogen Allocation In Douglas-Fir Plantations On Three Contrasting Sites.

On three Pacific Northwest sites affiliated with the North American Long-Term Soil Productivity Study, we evaluated effects of presence/absence of five years of annual vegetation control (VC) treatments on allocation of aboveground biomass and nitrogen (N) between planted coast Douglas-fir (Pseudotsuga menziesii var. menziesii) and competing vegetation. Sites differed in soil texture, N content, and water-holding capacity, as well as understory species composition, however, equations for predicting stem, branch, foliar, and total aboveground dry weights based on stem diameter at a height of 15 cm and total tree height did not differ significantly among sites or between VC treatments. Estimated whole-tree biomass among the six site/VC combinations at plantation-year 5 ranged from 0.8 to 7.5 Mg per ha. Increases in tree biomass associated with VC ranged from 62 to 173 percent among the three sites. Among the sites, there were positive, linear relationships between soil total N content to a depth of 60 cm and both N content of aboveground vegetation (trees plus competing vegetation) and Douglas-fir foliar N concentration; this supports the premise that soil N content is strongly linked to N uptake and plant growth at the study sites. Tree N content increased by 8.4, 8.2, and 40.0 kg per ha with VC at the three sites, whereas competing vegetation N content decreased with VC by 0.9, 18.8, and 32.0 kg per ha, respectively, at the same sites. In addition to the differences in N availability among sites, trends in biomass and N allocation were clearly influenced by species composition of the understory vegetation community and efficacy of the VC treatments.