Paper birch competitive effects vary with conifer tree species and stand age in interior British Columbia forests: implications for reforestation policy and practice

Abstract Paper birch ( Betula papyrifera Marsh) is considered the most important competitive threat to commercially valuable conifers in the highly productive interior cedar–hemlock forests of British Columbia. It is routinely removed from conifer plantations at high cost to increase conifer growth rates and meet reforestation policy regulations. Competitive effects of paper birch and other neighbours on conifer growth were measured in mixed forests to test the hypotheses that the importance and intensity of interspecific competition were (a) high in young plantations; (b) decrease with stand age; (c) decrease with conifer species shade tolerance. Neighbourhood analysis was applied to 748 target conifer trees in 11 stands that included three stand age classes (11, 25, and 50 years-old) and three target conifer species (Douglas-fir ( Pseudotsuga menziesii Mirb. Franco), western redcedar ( Thuja plicata D. Don), and western larch ( Larix occidentalis Nutt.)). All sites had similar site characteristics and disturbance histories. Regression models were used to examine the importance (model adjusted r 2 ) and intensity (model β 1 ) of neighbour competition on target conifer size. Double logarithmic regression models explained on average 38±3% of the variation in target conifer diameter from the neighbourhood index, NRI ( neighbour relative height index )=∑(( neighbour basal area × height )/ target conifer height ) . The models indicated that broadleaf neighbours were more important and intense competitors in the 11- and 25-year-old stands, and conifers were more important and intense competitors in the 50-year-old stands. When all broadleaf and conifer neighbours were included in the models, the intensity and/or importance of neighbour competition decreased in the order of 25>50>11-year-old stands, and western larch>Douglas-fir>western redcedar. Instead of encouraging uniform broadleaf removal across most conifer plantations in the BC interior, reforestation policies should take into account the variable effects of broadleaf competition within stands, across stand ages, and with different tree species compositions.