Environmental changes influence seedling establishment and growth simultaneously and sequentially. However, the complex effects of environmental changes on understory seedlings in temperate forests are not well explored. Thus, a 4-year field experiment was conducted at Rivière à Pierre, Québec, Canada, with two species of yellow birch (Betula alleghaniensis Britton.) and sugar maple (Acer saccharum Marsh.) in three levels of canopy gaps (£50 m², 101-300 m², and 701-1200 m²), two competition patterns (neighbor vegetation weeded and unweeded), and two liming treatments (liming and non-liming). We measured the height and height increment, crown features, and the aboveground biomass of the two species. Our results indicated that seedlings grow taller and represent higher new growth and biomass in large canopy gaps, weeded neighbour vegetation also increased the seedling height and new growths but suppressed stem biomass, and liming had no effect on the two species. The interactions of the canopy gaps, liming, and competition exhibited complex effects on growth and biomass allocation. The interactive effect was found to be positive in a large canopy gap, but not in small and intermediate gaps. It suggested that creation of a large canopy gap accelerates seedling growth and promotes an interactive effect with other factors in the changing environment. In a large canopy gap, the crown length and leaf biomass increased in the weeded and limed plot, but the biomass allocation was different between yellow birch and sugar maple in weeded plots, indicating that weeding the neighbor vegetation leads the two species dynamics in opposite directions.