Eleven-year growth response of young conifers to biosolids or nitrogen and phosphorus fertilizer on northern Vancouver Island

To test the hypothesis that fertilization with municipal biosolids causes a larger long-term tree growth response than fertilization with conventional chemical fertilizers, we measured the height and diameter of planted western redcedar (Thuja plicata Donn ex D. Don), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and amabilis fir (Abies amabilis (Dougl.) Forbes) 11 years after fertilization with biosolids or nitrogen and phosphorus fertilizer. There were four replicate plots of each tree species and treatment combination. The stand volume in cedar plots doubled in response to both treatments; in treated hemlock and fir plots, the stand volume was about triple that in control plots. There was little difference in tree response between biosolids-amended and fertilized plots. A type 3 response (growth rates remain elevated relative to those in the control) was evident for hemlock and fir. Biosolids appear to be as effective as chemical fertilizers in promoting conifer growth on these nutrient-poor sites, but their effect is neither greater nor more prolonged.