Effects of soil warming, nutrient supply, and inter-specific interaction on physiological traits of black spruce, and white spruce under current, and doubled atmospheric CO2 concentration

Background/Question/Methods: The current atmospheric CO₂ concentration ([CO₂]) is higher than pre-industrial levels. The effects of increasing [CO₂] on plants are generally positive but influenced by other factors: soil temperature and nutrient supply. Increases in [CO₂] may be accompanied by an increase in soil temperature, particularly in the boreal region. However, it is unknown how soil warming will affect trees’ response to the CO₂ elevation. Soil temperature in the boreal forest may increase by 5 ^oC by the end of the century. Nutrient supply is found to influence physiological responses to [CO₂] and soil temperature. Black spruce (Picea mariana [Mill.] B.S.P.) and white spruce (Picea glauca [Moench] Voss.) are congeneric species and co-exist on upland sites, but it is not clear whether they would respond differently under elevated [CO₂], higher soil temperature and different nutrient regimes. In this study, young seedlings of the two species were exposed to two soil temperatures and two nutrient regimes under ambient and doubled [CO₂] for 4.5 months. To examine inter-species interactions, they were planted in large pots under two scenarios: monoculture and 50/50% mixture of the two species.

Results/Conclusions:

It was found that the CO₂ elevation stimulated photosynthesis to a much larger extent in both species when they grew together as compared to growing separately. There was a significant Temperature x Nutrient x Species effect on photosynthetic rate in the mixture experiment. White spruce responded positively to warmer soil temperature and higher nutrient supply by increasing photosynthetic rate while the photosynthesis of black spruce did not show a significant response. In the mono experiment, the 4 factors had significant interactive effect on nitrogen use efficiency (NUE). Overall, elevated [CO₂] enhanced NUE in black spruce irrespective of the temperature-nutrient treatment while it increased NUE in white spruce only in the high nutrient treatment. In the mixture experiment, there were significant CO₂ x Nutrient and CO₂ x Species effects. CO₂ elevation increased NUE in seedling grown in the low nutrient treatment. CO₂ elevation enhanced NUE more in black spruce than in white spruce. The species responded differently to nutrient supply, with black spruce being more efficient in utilising nutrients. Growth in mixture increased CO₂ elevation enhancement on photosynthetic rate indicating more efficient resource utilisation.