Ecosystem effects of vegetation removal in coastal Oregon Douglas-fir experimental plantations: Impacts on ecosystem production, tree growth, nutrients, and soils

Effects of partial understory removal on belowground and aboveground ecosystem properties were investigated in 5 and 15 yr old Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) experimental plantations in western Oregon. Seedling survival was not affected by understory removal at age 5 yr. But through age 15 yr, understory removal increased seedling survival up to 30% on some of the plots. At plantation age 5 yr, tree biomass accumulation was increased nearly 12-fold with complete shrub and herb removal from the sites. However, total aboveground biomass was the same among treatments at age 5 yr. At plantation age 15 yr, tree biomass accumulation was 336% greater with complete understory removal from the Douglas-fir only plots (DFO). Even though Douglas-fir needle N concentrations were significantly higher on DFC plots at age 5 yr, this difference was not detected at plantation age 15 yr. However, Douglas-fir foliar nutrient concentrations decreased significantly between 5 and 15 yr for: Ca, Mn, Fe, Cu, B, and Zn. The total aboveground N pool was greater for DFC plots due to the greater amount of aboveground biomass present in shrubs and herbs at age 5 yr. From plantation age 5 to 15 yr, the total aboveground N pool dramatically increased. However, the aboveground total N pool was not significantly different among plots. On the other hand, the aboveground P pool was 86% and Ca pool 87% higher on DFO than on DFC plots. At age 15 yr, the upper 15 cm of soil had more than 20% greater bulk density on DFO than on DFC plots. Soil C concentration in the <2 mm soil fraction was reduced by 27% with complete understory removal at age 5 yr. DFO plots soil had 49% less exchangeable Ca than DFC plot soil by age 15 yr. At age 5 yr, net mineralizable NH4+ was the lowest on DFO plots. However, incorporation of organic matter into mineral soil during vegetation removal, and the decreased demand in plant uptake, increased total inorganic N on exchange resins in DFO soil. In the upper 15 cm of soil, delta 15N was enriched due to understory removal.