The impact of black cottonwood on forest floor and mineral soil properties within a conifer-dominated forest in British Columbia

Background/Question/Methods

Black cottonwood (Populus trichocarpa Torr. & A. Gray) is a shade intolerant deciduous tree native to coastal and southern British Columbia, and a common species of mixed coastal forests. Traditionally forest managers have viewed black cottonwood as a competitor to economically important conifers. Little information is available concerning the influence of black cottonwood on the mixed forest ecosystem. This study aims to fill part of that knowledge gap by examining the influence of black cottonwood on soil properties within a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) dominated forest. The study was undertaken at the Malcolm Knapp Research Forest located in Haney, British Columbia. Seven plots with a cottonwood component were compared to seven pure conifer plots on a paired plot basis. These were used to examine the influence of black cottonwood on the following: depth and humus form of the forest floor, physical and chemical properties of the forest floor and mineral soil, and rate of nitrogen mineralization within the forest floor.

Results/Conclusions

Compared to conifer plots black cottonwood plots had a higher percentage of the vermimull humus form coupled with a lower percentage of the mormoder humus form. This higher abundance of the mull humus form suggests that nutrients may cycle faster and be more readily available in conifer forest where black cottonwood is allowed to persist. The following significant differences were found within the forest floor of black cottonwood plots when compared to pure conifer plots: thicker Ah horizons, a higher bulk density, lower acidity, higher total carbon and lower exchangeable potassium and iron. The mineral soil differed in the following significant ways in plots with a cottonwood component: higher total nitrogen and sulfur, higher available phosphorous and a higher base saturation, lower C:N ratio, lower available iron and lower available aluminum. The lower acidity of the forest floor beneath cottonwood may indicate a faster rate of decomposition and increased nutrient availability. The lower C:N ratio under cottonwood suggests that nitrogen availability may be greater where cottonwood is a component of the stand. Overall these differences suggest that the presence of black cottonwood may have a positive influence on soil chemistry and nutrition within conifer dominated stands.