Drought resistance of Pinus sylvestris seedlings conferred by plastic root architecture rather than ectomycorrhizal colonisation

Context Increased summer drought is considered as a threat to the regeneration of Pinus sylvestris in the Central Alps. To a certain degree, seedlings are able to mitigate negative effects of drought by altering root/shoot ratios. But, seedlings may also enhance access to water and nutrients by cooperation with ectomycorrhizal fungi. Aims We tested the importance of both mechanisms for drought resistance of P. sylvestris seedlings during early establishment and assessed whether differences occur between topsoil and deeper soil layers. Methods Biomass allocation and colonisation of fine roots by ectomycorrhizal fungi were assessed in seedlings grown for 6 months in a common garden under different precipitation scenarios: constant drought (March-September), summer drought (June-September), and wet conditions. Results Root/shoot ratios increased from 0.6 under wet conditions to 0.8 under drought conditions, irrespective of the onset of the drought (March vs. June). In both drought scenarios, seedlings had shorter roots in the topsoil, increased the number of root tips per root length in both soil layers, but did not alter the colonisation rate of root tips by ectomycorrhizal fungi. Conclusion We conclude that plasticity in root architecture is an important mechanism for drought resistance of P. sylvestris during early seedling establishment.