To better understand how soil temperature will influence the responses of boreal trees to increasing atmospheric [CO2], one-year-old jack pine (Pinus banksiana lamb.), black spruce (Picea mariana [Mill.] B.S.P.), white spruce (Picea glauca [Moench] Voss), and current-year white birch (Betula paperifera Mash) seedlings were subjected to two [CO2] (360 versus 720 mol mol−1) and three soil temperatures (Tsoil = 7, 17, and 27°C initially, increased to 10, 20, and 30°C 3 months later) for 4 months. The low Tsoil significantly suppressed height growth, stem biomass, and total biomass in white birch, black and white spruce, root collar diameter (RCD), and foliage biomass in white birch and white spruce, as well as root biomass in white birch under both ambient and elevated [CO2] and in white spruce under ambient [CO2]. This low Tsoil effect was much more significant in white birch than in the conifers. The [CO2] elevation significantly increased RCD, foliar biomass, and total biomass of the four species at all soil temperatures, stem biomass of all the species at the low Tsoil, and the root biomass of white birch at intermediate Tsoil. The data suggest that the [CO2] elevation compensated for the negative effects of low Tsoil, e.g., the low Tsoil significantly decreased the height and total biomass of black and white spruce at ambient [CO2], but not at elevated [CO2]. The high Tsoil had much smaller negative effects on growth and biomass than did the low Tsoil. Jack pine was the least responsive to Tsoil and [CO2]. In general, the ratios of stem, foliage, and root mass to total mass were much less responsive to the treatments than total or component biomass. Neither treatment significantly affected the volume/mass ratio of the stem in any of the four species. The data suggest that white birch and white spruce will benefit the most and jack pine will benefit the least from the increasing atmospheric [CO2].