Multidecadal Growth of Western White Pine and Interior Douglas-Fir Following Site Preparation

Site preparation is used to favor seedling regeneration and establishment by enhancing growing conditions and increasing resource availability, yet few studies have compared different site preparation techniques on growth and yield of trees over multiple decades. We destructively sampled 34-year old trees of western white pine (Pinus monticolaDouglas ex D. Don) and Interior Douglas-fir (Pseudotsuga menziesii(Mirb.) Franco var.glauca(Beissn.) Franco planted at two sites using a replicated experiment to test the effectiveness of different site preparation treatments: (1) no site preparation, (2) scalping, (3) bedding, and (4) bedding plus three years of competition control with herbicide. Growth and yield were compared among the treatments, and models of growth were developed for each species and treatment combination. The herbicide treatment was the only treatment that consistently improved growth and yield of both species resulting in 19%-30% gains in height, 43%-63% gains in diameter, and 31%-109% gains in stem volume by age 34. Height growth response to herbicide was sustained until age 14 for white pine and age 12 for Douglas-fir, while the diameter response was sustained until age 23 for white pine and 20 for Douglas-fir. The later peak in growth for white pine suggests a better response to treatment and that the species was able to maintain higher growth following crown closure. Both species exhibited a Type 2 growth response to herbicide, suggesting competition control resulted in sustained gains over time with associated age shifts of 8.5 and 9.7 years for white pine and 7.1 and 10.2 years for Douglas-fir, height and diameter, respectively. This compares to scalping and bedding which produced no detectable difference in growth compared to the control, and in some instances, reduced growth. In the Northern Rocky Mountains, moisture is most limiting. This is likely why trees showed the greatest response to competition control. Interestingly, this growth was sustained well beyond seedling establishment.