Root order- and root age-dependent response of two poplar species to belowground competition

Morphology and functioning of fine roots vary with distance from the distal root ending reflecting influences of root order and root age, but changes in tree fine root systems as a response to the presence of conspecific or allospecific competitors have rarely been studied. Our aim was to identify the effect of belowground competition on the morphology and chemical properties of tree fine roots of different order and age. We grew saplings of a fast-growing (Populus trichocarpa) and a relatively slow-growing poplar species (P. tremula) in monoculture (intraspecific competition) and mixture (interspecific competition) in rhizoboxes, enabling the continuous monitoring of root growth, age determination of root segments, and a harvest-based analysis of root orders. Interspecific root competition was highly asymmetric in favor of P. trichocarpa. Species comparison in terms of root traits in four root order classes and eight age classes revealed thinner and longer 1st and 2nd order roots in fast-growing P. trichocarpa, whereas root N concentration was higher in P. tremula despite its much lower fine root productivity. The competition treatment had only a weak effect on fine root morphology. Comparison of the harvest and window observation data revealed considerable root polymorphism in both species (thicker young pioneer roots with low tissue density at the windows vs. thinner fibrous 1st-order roots without window contact in the soil) suggesting a marked alteration of fine root morphology by the presence of observation windows. Our results on the effects of intraspecific and interspecific competition suggest that modification in fine root traits plays only a minor role in the plants' belowground competitive strategies. Root polymorphism has to be accounted for when interpreting root window and likely also rhizoscope data of future experiments.