Interactive Effects of Soil Temperature and [CO2] on Morphological and Biomass Traits in Seedlings of Four Boreal Tree Species

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 μmol mol-1) and three soil temperatures (T (soil) = 7, 17, and 27°C initially, increased to 10, 20, and 30°C 3 months later) for 4 months. The low T (soil) 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 T (soil) 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 T (soil), and the root biomass of white birch at intermediate T (soil). The data suggest that the [CO2] elevation compensated for the negative effects of low T (soil), e.g., the low T (soil) significantly decreased the height and total biomass of black and white spruce at ambient [CO2], but not at elevated [CO2]. The high T (soil) had much smaller negative effects on growth and biomass than did the low T (soil). Jack pine was the least responsive to T (soil) 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].