Chemical, grazing, and manual cutting treatments in mixed herb–shrub communities have no effect on interior spruce survival or growth in southern interior British Columbia

Abstract

Interior spruce ( Picea engelmannii Parry × glauca [Moench] Voss) plantations are commonly weeded of herbs, ferns, and shrubs using foliar glyphosate, livestock grazing, or manual brushing treatments, but it is unknown whether they result in improved conifer survival or growth. We found that despite reducing vegetation for 1–5 years, these treatments applied to mixed herb–shrub communities in southern interior British Columbia had no effect on 5-year survival or growth of interior spruce. Biogeoclimatic zone was the factor that most influenced spruce performance, with greater survival and height growth in the Interior Cedar Hemlock (ICH) zone than in the colder, higher elevation Engelmann Spruce–Subalpine Fir (ESSF) zone. Glyphosate caused larger reductions in vegetation cover and height than did grazing or manual cutting. Glyphosate also reduced plant species richness after 5 years, but none of the treatment methods affected species or structural diversity of the plant community. Structural richness was slightly greater in the ICH than ESSF zone. Our results indicate that the vegetation management treatments operationally applied to herb–shrub communities in southern interior British Columbia have been ineffective at releasing interior spruce.