Julie Sabau 

MSc Candidate, 2006-2009
The Impact of Black Cottonwood on Soil Fertility of a Coniferous Forest

Summary of Research

The function deciduous trees play within an ecosystem, including their effect on soil fertility is expected to differ from that of conifers. Preceding this study, very little was known about the influence of cottonwood on soil fertility. Therefore, this study set out to determine the influence of cottonwood on soil fertility within a CWH subzone forest dominated by Douglas-fir and western hemlock. We sought to quantify the impact of cottonwood by comparison of litterfall, decomposition, properties of the forest floor and mineral soil, and N mineralization rates between plots with and without a cottonwood component.

Cottonwood litter was found to be of higher quality in terms of nutrient concentrations compared to conifer litter. A comparison of early decomposition rates revealed no significant differences between litter types or plot types, although it is possible this will change with increased incubation time. Decomposition results, in combination with no difference in forest floor thickness, or forest floor weight below cottonwood compared to conifers, suggests a lack of influence of cottonwood on litter decomposition within conifer forest.

The proportion of mull humus form under cottonwood was almost double that under conifers. Also, higher total N concentrations in the mineral soil and higher pH in the forest floor were found under cottonwood. These results suggest a positive influence on soil fertility related to the presence of cottonwood.

The net mineralization and net ammonification were lower under cottonwood than under conifers. These results fail to indicate an overall positive influence of cottonwood on N availability within conifer dominated forests.   

This study found less significant differences than expected between plot types. We expected to find more evidence of a positive effect of cottonwood on soil fertility of conifer-dominated stands. The low power encountered in a large number of our statistical tests indicates that the presence of cottonwood may actually have a greater influence than this study was able to capture. Increased statistical differences would likely emerge with an increased sample size.

Results from all components of the project combined suggest a moderate to weak positive effect of cottonwood on soil fertility within conifer dominated forests of coastal British Columbia.