Aims We studied root morphology across the first five branch orders of 20 hardwood tree species from secondary forest in northeastern China. Our objectives were to determine how root morphology changes across root branch orders and how such variations of root morphology differ between ectomycorrhizae (EM)-dominated and arbuscular mycorrhizae (AM)-dominated fine roots.

Methods The fine roots were sampled in secondary forest in August 2006. We chose a random location in a plot under a randomly chosen individual tree and used a shovel to remove six soil blocks, 20 cm×20 cm×10 cm depth, from which the intact root segments were collected and placed in a Whirl-pac bag on ice in a cooler and then transported to the lab and frozen for dissection at a later date. In the laboratory, we dissected each individual root by branching order beginning with the distal end of a root system. After the dissection, we verified the type of mycorrhizal infection by staining and direct microscopic observation and determined diameter, length, and specific root length (SRL) of a given order.

Important findings Root diameter and length increased, and SRL decreased from the first to fifth order roots, except in Ulmus pumila, Acer ginnala, Alnus sibirica, A. mandshurica and U. laciniata, in which first order roots had larger (or equal) diameter and lower SRL compared to second order roots. The first three order roots accounted for 80% of the total length, suggesting that lower order roots would have similar physiological functions despite their differences in morphology. Eleven species were EM, and nine were AM. Average diameter, length and SRL of each root order were larger for AM than EM species.