Effects of soil temperature on ecophysiological traits in seedlings of four boreal tree species

One-year-old seedlings of black spruce (Picea Mariana (Mill.) B.S.P.), white spruce (Picea glauca (Moench) Voss.), jack pine (Pinus banksiana Lamb.) and aspen (Populus tremuloides Michx.) were exposed to 5, 10, 15, 20, 25, 30 and 35 T soil temperatures for 4 months. The rate of net photosynthesis (A), stomatal conductance, mid-day xylem water potential, the internal to ambient CO2 concentration ratio (C-i/C-a) and specific leaf area (SLA) were evaluated 3 months after the initiation of the treatments. The responses of the above variables generally showed a bell-shaped curve except the C-i/C-a ratio. The relationships between soil temperature and the ecophysiological variables (except C-i/C-a) were modeled using third-order polynomials. The models suggest that jack pine generally had the highest optimal soil temperature among the four species, followed sequentially by black spruce, trembling aspen and white spruce. Aspen was generally more sensitive to low soil temperatures than the conifers while black spruce was more sensitive to high soil temperatures than other species. Jack pine and trembling aspen generally showed different response patterns than black and white spruce. The photosynthesis in as en and jack pine was p progressively less limited by stomatal conductance (relative to mesophyll limitation) as soil temperature increased from 5 to 35 degreesC whereas in the spruces, stomatal conductance was the least limiting to photosynthesis at moderate soil temperatures (10-15degreesC for white spruce, 15-20degreesC for black spruce). The xylem water potential of jack pine and aspen was more sensitive to low soil temperatures while that of the spruces was more sensitive to high soil temperatures. The specific leaf area of jack pine and aspen showed large variations with changes in soil temperature while that of the spruces was relatively constant throughout the range of soil temperatures. Most of the conifer seedlings in the 35 T soil temperature treatment died at the end of the experiment while all aspen seedlings survived. (C) 2004 Elsevier B.V. All rights reserved.