Changes in soil properties and site productivity caused by red alder

Red alder (Alnus rubra Bong.) is well recognized as an effective host plant for the symbiotic fixation of N. While this fixation process leads to the rapid accumulation of N within the ecosystem, it also enhances nutrient accumulation in biomass and soil organic matter and increases nitrification and cation leaching. We hypothesized that changes in soil properties resulting from these processes would decrease site productivity for second rotation red alder. Adjacent stands of 55 yr old alder and Douglas fir (Pseudotsuga menziesii (Mirb.] Franco) were studied at the Thompson Research Center on the Cedar River Watershed in western Washington, USA.The presence of red alder caused the following soil changes: decreased soil solution pH, increased CEC, increased exchangeable acidity accompanied by a decreased soil pH and base saturation. This decreased soil and soil solution pH resulted in increased A1 concentration in the soil solution and on exchange sites as well as decreased P availability. To determine the effect of these changes on the productivity of the 2nd rotation alder forest, a species conversion experiment was initiated 5 yr ago. Results from this conversion study clearly indicated that the first rotation red alder forest has caused a relative decrease in the productivity of the second rotation red alder plantation. Compared to the growth of red alder on the former Douglas fir site, the second rotation red alder on the former red alder site exhibited 339 less height growth and 759, less aboveground biomass accumulation after 5 yr. Future research will focus on identifying those factors causing this lower productivity including P availability, soil acidity and Al toxicity, cation availability, and competition with other vegetation.