Critical period of interspecific competition for northern conifers associated with herbaceous vegetation

Using critical-period analysis, we examined the temporal effects of intersp ecific competition from herbaceous vegetation on seedlings of jack pine (Pi nus banksiana Lamb.), red pine (Pinus resinosa Ait.), eastern white pine (P inus strobus L.), and black spruce (Picea mariana (Mill.) BSP) during the f irst 5 years after planting. The critical period is the time period during stand development when interspecific competition reduces tree growth. We fo und both similarities and differences in responses among tree species. Gain s in stem volume index associated with increasing duration of vegetation co ntrol (expressed by weed-free curves) differed among species. In contrast, declines in stem volume index with increasing duration of competition after planting (expressed by weed-infested curves) were equal among species. Cri tical periods for stem volume index were shorter for shade-intolerant jack and red pine (1 and 2 years after planting) than for more shade-tolerant wh ite pine and black spruce (1-3 years for spruce and 1-4 years for white pin e). Intolerant species had greater absolute stem volume growth, but smaller relative declines from continuous association with herbaceous vegetation ( 85, 81, 78, and 67% for white pine, black spruce, red pine, and jack pine, respectively). Herbaceous vegetation did not affect survival and had a vari able influence on height growth of all species.