Timing and duration of herbaceous vegetation control in northern conifer plantations: 15th-year tree growth and soil nutrient effects

Abstract

A 15-year re-measurement of a study designed to identify the optimum timing and duration of herbaceous vegetation control in plantations of four commercial conifer species was completed in northern Ontario. Few differences were revealed in conifer growth when contrasting early and delayed timing of vegetation control. Conversely, each conifer species responded positively to increased duration of vegetation control, with stand volume gains of up to 209% achieved with four to five years of vegetation control following planting. Compared to earlier assessments, the timing of vegetation control appeared less important than duration. Diminishing returns in the fastest-growing species (jack pine [Pinus banksiana Lamb.] and red pine [Pinus resinosa Ait.]) are consistent with intraspecific competition related to the onset of crown closure in these stands. Quantification of a suite of soil nutrient pools along the gradient of increased duration of vegetation control indicated that the more intensive levels of vegetation control did not adversely affect the assessed soil nutrient pools in red pine or jack pine, but a cautionary approach should be considered for white pine (Pinus strobus L.) and black spruce (Picea mariana [Mill.] BSP), where some declines were evident. Vegetation control for two to three years following planting should maximize early conifer growth potential without adverse longer-term effects on soil nutrient pools.