Ecosystem production in Douglas-fir plantations: Interaction of red alder and site fertility

Abstract

Pure stands of Douglas-fir ( Pseudotsuga menziesii [Mirb.] Franco) and mixed stands of Douglas-fir and naturally established red alder ( Alnus rubra Bong.) were examined on two different sites for soil properties, tree growth and nutrition, and aboveground ecosystem biomass and net primary production. One site (Mt. Benson, Nanaimo, B.C.) was nitrogen (N)-deficient and had a low site index (expected Douglas-fir height of 24 m at 50 years). The other site (Skykomish, western Washington) was N-rich and had a site index of 45 m at 50 years. Soil N accretion on the red alder units was estimated at 65 (Mt. Benson) and 42 (Skykomish) kg ha −1 year −1 for 23 years to a soil depth of 50 cm. At the current stage of plantation development, presence of red alder at the infertile Mt. Benson site increased average Douglas-fir diameter but did not affect its basal area and basal area growth rate; including alder stem biomass increased total stand basal area and basal area growth 2.5 fold. Presence of red alder at the fertile Skykomish site decreased average diameter, basal area, and basal area growth of Douglas-fir; including alder biomass left total stand basal area and basal area growth unchanged. Douglas-fir foliar N concentrations on Mt. Benson increased from 0.93 without alder to 1.41% on the red alder unit but were 1.55% for both units at Skykomish. Although alder did not affect Douglas-fir aboveground biomass and net primary production on Mt. Benson, total ecosystem biomass doubled and production tripled when alder biomass was included. Conversely, at Skykomish, Douglas-fir biomass and production decreased, and total ecosystem values were essentially unchanged. Mixing red alder and Douglas-fir seems to have great potential for increasing Douglas-fir growth and ecosystem production on infertile, N-deficient sites but probably has limited value on fertile, N-rich sites.