Effects of soil temperature on growth and biomass allocation in four boreal tree species

Trembling aspen (Populus tremuloides Michx.), black spruce (Picea mariana (Mill.) B.S.P.), white spruce (Picea glauca (Moench) Voss), and jack pine (Pinus banksiana Lamb.) seedlings were subject to seven soil temperature treatments 5,10, 15,20,25,30 and 35°C. At the beginning and end of the experiment, the height and diameter of all seedlings were measured. Ten seedlings from each treatment combination were harvested at the end of the experiment for the determination of biomass allocation. It was found that soil temperature, species and interaction of soil temperature and species had significant effects on biomass and relative growth rate (RGR) of diameter. The relationship between each parameter and soil temperature was modeled using a 3rd order polynomial function. The model showed that the optimum soil temperatures were 19.4°C, 17.3°C, 15.3°C and 21.8°C, respectively, for aspen, black spruce, white spruce and jack pine. The biomass variables showed nonlinear responses to changes in soil temperatures between 5°C and 30°C in all tree species. All the aspen seedlings, about 40 percent of jack pine, 20percent of white spruce and black spruce seedlings survived the 35°C treatment during the experiment. Among the species, aspen had the largest response in total biomass and biomass of different components to soil temperatures. The maximum total biomass for aspen was about 7 times the minimum value, the corresponding values for black spruce, jack pine and white spruce were 2.2, 2.4 and 2.3 times, respectively. The total biomass and biomass of different organs at soil temperature 5°C were smaller than those at temperature 30°C for aspen, black spruce and jack pine. The results were reversed for white spruce. The total biomass at 5°C was 14.3 percent of the value at the optimum soil temperature for aspen, the corresponding values for black spruce, jack pine and white spruce were 45 percent, 42 percent and 42 percent, respectively. Soil temperature did not significantly affect leaf to root ratio, shoot to root ratio, shoot mass to total mass ratio (SMR), leaf mass to total mass ratio (LMR), or stem mass to total mass ratio (SMR). But there were significant differences between species in all the above ratios and different species responded differently to soil temperature treatments. The relative growth rate (RGR) also varied with species. Soil temperature did not affect RGR in jack pine and white spruce. The RGR of black spruce was sensitive to soil temperatures over the whole range while RGR of aspen was only sensitive to soil temperature below 15°C