Biogeochemistry of Adjacent Conifer and Alder‐Conifer Stands

To determine the long—term effect of alder on soil fertility, biogeochemical fluxes were measured and calculated for two pairs of adjacent, 55—yr—old stands dominated by conifers, primarily Douglas—fir (Pseudotsuga menziesii), and by conifers and nitrogen—fixing red alder (Alnus rubra). At a low—fertility site in the Wind River Experimental Forest in southwestern Washington, biomass of the alder—conifer stand (289 Mg/ha) exceeded that of the conifer stand (171 Mg/ha), and the aboveground net primary production (ANPP) of the alder—conifer stand (10.3 Mg.ha— ¹ .yr— ¹) was more than twice that of the conifer stand (4.8 Mg.ha— ¹ .yr— ¹). At a more fertile site in the Cascade Head Experimental Forest in western Oregon, both biomass and ANPP were higher than at Wind River, and biomass and ANPP were higher in the conifer stand (584 Mg/ha and 19.2 Mg.ha— ¹ .yr— ¹) than in the alder—conifer stand (342 Mg/ha and 10.7 Mg.ha— ¹ .yr— ¹). Nitrogen accretion in the alder—conifer stand at Wind River averaged 54 kg.ha— ¹ .yr— ¹ for the 52 yr since stand establishment, with a current rate of N fixation of °75 kg.ha— ¹ .yr— ¹. For the alder—conifer stand at Cascade Head, N accretion averaged 73 kg.ha— ¹ .yr— ¹ for 55 yr, with a current N—fixation rate of 85 kg.ha— ¹ .yr— ¹. The cycling of all nutrient appeared very malleable under the influence of alder. At Wind River, return of nutrients in fine litterfall in the alder—conifer stand ranged from 1.5 (P) to 7.9 (N) times those in the conifer stand; whereas at Cascade Head, these ratios ranged from 1.7 (S) to 4.2 (N). Nutrient—use efficiencies (kilograms of ANPP per kilogram of nutrient uptake) were generally lower for the alder—conifer stands at both sites. Denitrification appeared negligible (<0.3 kg.ha— ¹ .yr— ¹) in all stands. Leaching of organic plus inorganic N ranged from °5 kg.ha— ¹ .yr— ¹ for the conifer stand at Wind River, to 50 Kg.ha— ¹ .yr— ¹ for the alder—conifer stand at Cascade Head.