Tree age affects key fine root morphological traits but does not interfere strongly with (ecto-)mycorrhizal traits

It is assumed that trees should adapt their above- and below-ground organs as they age. However, most studies to date have quantified these trait adjustments in homogeneous forest stands, confounding the effect of stand ageing on soil properties and the intrinsic response of trees to ageing.

Here, we examined 11 morphological, architectural, anatomical and mycorrhizal fine root traits of each of the first five orders for 66 Pinus koraiensis individuals aged 16-285 years old in northeast China, while accounting for soil characteristics (pH and total C, N and P concentrations).

Across all absorptive root orders, Hartig net area and mantle thickness, representative of P. koraiensis reliance on ectomycorrhizal association, displayed an orthogonal pattern to traits describing root economics strategy, specific root length and root tissue density. Hartig net area and mantle thickness were not significantly related to root mycorrhizal colonization intensity and root branching intensity. As trees aged, there was a trade-off between increasing specific root length and decreasing root tissue density in all root orders we measured. Trees facing soil conditions with lower N or P concentrations showed higher dependence on ectomycorrhizal fungi.

This work provides evidence that tree below-ground economics strategy can change significantly throughout their lifetime. In the ectomycorrhizal tree species studied, morphological adjustments are most pronounced, whereas mycorrhizal strategy remains largely consistent. The more conservative root strategy of younger trees than older ones contradicts previous results on the ageing of homogeneous tree stands, suggesting that individual tree responses to ageing in mixed-aged forests might differ strongly from those in homogeneous forest stands experiencing multiple confounding environmental influences.