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Forest vegetation response to the application of municipal sewage sludge in the coastal western hemlock zone Coward, Laura Patricia


The disposal of municipal sewage sludge is a major waste management problem for large cities. The Greater Vancouver Regional District is investigating the option of applying sludge to forest lands as a fertilizer for enhanced timber production. A joint project of the University of British Columbia and the Greater Vancouver Regional District is currently looking at the effects of sludge application on managed forest ecosystems. Municipal sewage sludge was applied to Pseudotsuga menziesii stands of four different ages at the Malcolm Knapp Research Forest, British Columbia. The experiment consisted of two phases. Phase I was a preliminary small (100 m2) plot trial involving stands established in 1964, 1980, and 1986. Phase II stands were established in 1975 and the larger plots spanned a small watershed. As part of the larger project, this study examines the effects of sludge application on the minor vegetation. Vegetation parameters which were measured included relative percent cover, height, and biomass of individual species on systematic transects, as well as plot-wide Braun-Blanquet cover-abundance estimates. Relative percent cover was measured using the line-intercept method and plant heights were measured every meter along the transects. Photosynthetically active radiation (PAR) was also measured to establish the light levels available to vegetation in stands of different ages. Phase I was sampled one and two growing seasons following sludge application. Phase II was sampled one growing season prior to and one season following application. To complement the intensive, short-term monitoring of Phases I and II, small plots at the Charles Lathrop Pack Demonstration Forest, Washington were sampled, 11 years following sludge treatment. Relative percent cover and height by species were used as the vegetation response parameters. In general, Phase I shrub cover decreased and herb and fern cover increased with sludge treatment. The cryptogam cover increased with treatment in the youngest stand, but decreased in the 1980 and 1964 stands. At the species level, Gaultheria shallon and Kindbergia oregana decreased while Pteridium aquilinum and Epilobium angustifolium increased with sludge. In Phase II, there was a significant sludge treatment by stand dendity interaction effect on the abundance of Gaultheria and Pteridium. Gaultheria increased with sludge treatment in the unthinned stands and decreased with treatment in the thinned stands. The opposite effect was seen for Pteridium .Also, foliar analyses of Gaultheria leaves revealed significant increases in nitrogen and phosphorus concentrations with sludge treatment. At Pack Forest, Gaultheria, Linnaea borealis, and Kindbergia oregano cover had decreased 11 years after sludge application while Rubus ursinus, Polystichum munitum, Sambucus racemosa, and Galium boreale cover increased with treatment. In both Phases I and II, there were significant effects of soil moisture regime and stand density a number of the species. There were also significant stand density effects on a number of the species at Pack Forest; however, moisture regime was not part of the experimental design. This study demonstrates that soil moisture regime, stand density, and sludge, applied at the one-time rate of 500 KgN/ha, can significantly affect the minor vegetation of forest ecosystems in the CWHdm biogeoclimatic subzone of British Columbia. A heavier and/or more frequent application rate and a long-term study may show a more pronounced effect on species composition and abundance, as indicated by the dramatic shifts in understory vegetation observed at Pack Forest. Continued monitoring of sludge application in this area is recommended to better understand its effects on forest ecosystems.

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