Belowground interactions between Kalmia angustifolia and Picea mariana: roles of competition, root exudates and ectomycorrhizal association

Background and aims: Belowground competition and allelopathic interference of neighbouring plants play important roles in shaping plant performance. We assessed the effect of belowground interactions involving root competition, root exudates and ectomycorrhizal (ECM) association between Kalmia and black spruce on above- and belowground biomass increment and foliar nutrient content of black spruce seedlings. We predicted that root competition and root exudates of Kalmia would exert a negative impact on black spruce performance, while ECM in black spruce would exert positive impacts on its performance. Methods: In a greenhouse experiment, using the neutral growth medium of peat-vermiculite, we planted Kalmia and black spruce seedling in pairs. We controlled root competition by placing fine wire mesh between the seedlings and applied benomyl fungicide to control ectomycorrhizae. We used activated charcoal to neutralize the potential allelopathic effects associated with root exudates. After 8 months, we evaluated the performance of black spruce seedlings in terms of biomass, abundance of ectomycorrhizal short roots and foliar nutrient concentrations. To determine treatment effects on soil processes, we also analysed soil pH, total phenols, C:N ratio and bioavailable nitrogen (NO₃− and NH₄+) at the end of the experiment. Results: We found a significant effect of root competition on root-shoot ratio and biomass increment in black spruce seedlings, which achieved the highest root-shoot ratio and biomass increment when major root competition was controlled. Kalmia produced twice the root biomass than black spruce. Surprisingly, charcoal and benomyl treatments—which remove the effects of Kalmia allelopathy on black spruce and suppress ECM growth, respectively—had little or no significant effect on overall biomass of black spruce. In contrast, Kalmia biomass increment and root-shoot ratio were maximized in benomyl treatment. Soil pH and C:N ratio were significantly higher in free competition than root exclusion and benomyl treatments and with activated charcoal. Conclusions: Kalmia root competition mediated by high root biomass is the predominant driver shaping neighbourhood interactions between Kalmia and black spruce, which in turn shape the performance of black spruce seedlings. Root exudates of Kalmia have a very weak effect on black spruce seedling growth.