Nutrient supply and belowground interaction alter responses to CO2 elevation in black spruce and white spruce

Abstract

The carbon dioxide concentration ([CO2]) in the atmosphere has been increasing since the start of the industrialization. The impact of increasing [CO2] on the physiology and growth of plants and on the composition, dynamics and productivity of plant communities have been studied extensively in the past few decades. However, the responses to CO2 elevation can be altered by other environmental and biological factors. Such interactions, particularly those involving multiple factors, are complex and not well understood. In this study, we investigated the interactive effects of CO2 elevation, nutrient availability and root interaction between plants on the total seedling photosynthesis, growth, and biomass allocation of two boreal conifer tree species, black spruce and white spruce. Seedlings were exposed to two [CO2] (380 vs. 720 µmol mol−1) and two nutrient levels (low vs. high) with or without root interaction. We found that each of the three treatments influenced the effects of others and the two species also responded differently to those interactions. Root interactions reduced seedling growth only when nutrient supply was high. White spruce was more sensitive root interaction than black spruce while black spruce was more sensitive to the negative impact of low nutrient supply than white spruce. Surprisingly, CO2 elevation and its interactions did not significantly affect seedling growth. The CO2 elevation significantly increased total seedling photosynthesis while root interaction reduced it and the reduction was greater under the elevated CO2. The photosynthetic response to CO2 elevation was more sensitive to nutrient availability in white spruce than in black spruce. Seedling biomass was more sensitive to the negative impact of root interaction and low nutrient supply in black spruce than white spruce. Root interaction reduced biomass allocation to leaf and the impact was reduced by low nutrient. Biomass allocation to leaf was more sensitive to low nutrient supply in black spruce than white spruce. CO2 elevation reduced but root interaction increased root/leaf mass ratio, partially offsetting each other when both treatments were applied. The root/leaf ratio of white spruce was more sensitive to CO2 elevation while that of black spruce was more sensitive to root interaction and nutrient supply. Low nutrient supply and root interaction both had positive effect on root/leaf ratio and their effects were confounded when both were applied simultaneously. Therefore, their effects were observed only when one of them was absent.