Drought stress and artificial mycorrhization: divergent growth responses in Pedunculate oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.) seedlings

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ABSTRACT Understanding the interactions between mycorrhizal fungi and trees in drought conditions is increasingly important for forest sustainability. We conducted a water manipulation experiment with Quercus robur (oak) and Pinus sylvestris (pine) seedlings, both inoculated with the ectomycorrhizal fungus Hebeloma crustuliniforme. Over three years, we examined how varying water regimes (sublethal drought, water deficit, optimal supply) impacted the above – and belowground growth and allometry of mycorrhized and non-mycorrhized seedlings to assess the effects of mycorrhization under different water availability conditions. Responses to water regimes were species-specific and influenced by mycorrhization. Pine exhibited significant effects on aboveground parameters, particularly diameter, while belowground traits and allometry under drought showed smaller differences between mycorrhized and non-mycorrhized seedlings. Drought stress in mycorrhized pine resulted in a less favorable, lower root-to-leaf ratio. In contrast, oak responded to mycorrhization with greater height, total biomass, and root architecture, compared to non-mycorrhized seedlings, especially under optimal conditions. Our findings suggest that the growth mechanisms in young oak may enhance rooting, potentially leading to improved efficiency in mycorrhizal utilization. However, drought negatively affected seedlings of both species and their symbiont, highlighting the limited effectiveness of artificial mycorrhization in mitigating the challenges posed by climate change.