The impact of black cottonwood on soil fertility of a coniferous forest in the coastal western hemlock zone of British Columbia

Black cottonwood (Populus trichocarpa Torr. and Gray) is a deciduous tree native to coastal and southern British Columbia. We examined the influence of cottonwood on soil fertility within a conifer-dominated forest. Six plots containing cottonwood were paired with six pure conifer plots, and individual pairs were compared for litterfall, early decomposition, properties of the forest floor, properties of the mineral soil, and N mineralization. Cottonwood litter relative to conifer litter had higher concentrations of almost all elements. Twice the proportion of mull humus form was found in cottonwood plots. Higher pH and total N concentrations were found in the forest floor and mineral soil under cottonwood, respectively. The concentration of NO3- was significantly greater under cottonwood within an incubation study. These results suggest a moderate to weak positive effect of cottonwood on soil fertility within temperate coastal forests.