Critical period of interspecific competition for four northern conifers: Ten-year growth and associated vegetation dynamics
The influence of the timing and duration of interspecific competition on planted jack pine (Pinus banksiana Lamb.), red pine (Pinus resinosa Ait.), eastern white pine (Pinus strobus L.), and black spruce (Picea mariana (Mill.) BSP) was assessed using 10-year growth responses in a northern Ontario experiment. Stand volume was 117%, 208%, 224%, and 343% higher for jack pine, red pine, white pine, and black spruce, respectively, with 5 years of vegetation control than with no vegetation control. Stand volume increased linearly with number of years of vegetation control, and the slope of the relationship varied among conifer species. Change-point regression analysis was used to derive segmented weed-free and weed-infested curves, and to simultaneously estimate key critical-period parameters. Weed-free and weed-infested curves in the 10th year were similar to those derived in year 5, indicating that the patterns established during the first few years after planting were relatively robust for the first decade. The critical-period was 2 and 3 years after planting for jack pine and red pine, respectively, and occupied most of the 5-year period for white pine and black spruce. Principal components analysis of the vegetation community indicated that repeated herbicide applications caused differential shifts in the relative abundance of shrub, fern, and moss species through the 10th year. Species richness, however, was not substantially different between the untreated control and the most intensive treatments. Difference modeling was used to quantify how annual volume increment during the first decade varied with time, conifer species, cover of woody and herbaceous vegetation, and stage of development.