Growth and substrate quality of fine root and soil nitrogen availability in a young Betula ermanii forest of northern Japan: Effects of the removal of understory dwarf bamboo (Sasa kurilensis)

Dwarf bamboos (mainly Sasa species) form dense undergrowth in many forests in Japan and compete with overstory vegetation for soil resources. For a better understanding of the interaction between tree and undergrowth root for soil nutrients, we compared soil N availability, root biomass and its substrate quality in two Betula ermanii stands. In one stand, the aboveground part of undergrowth (Sasa kurilensis) had been removed for 5 years (SR) and in the other, the undergrowth was present (SP). The SR stand had significantly higher NO3-N levels in soil (1.7-5.5 mg kg−1 at different soil depths) than the SP stand (1.2-1.7 mg kg−1), which may affect the amount and substrate quality of fine root. Although the total (live + dead) of the root and rhizome (root-rh) mass did not differ between the two stands, the total production by these categories was about half in the SR stand than the SP stand. Further, the proportion of dead root-rh mass was about two times in the SR stand than the SP stand. In the investigated depth (to 30 cm), the contribution of the C and N inputs to the total in the uppermost layer (0-10 cm deep) was greater in the SR stand (56-59%) than the SP stand (38-42%). Total root-rh of S. kurilensis in the SP stand contributed to about 38-40% of the total C and N input, and in the SR stand the contribution was still about half that of the SP stand. Further, the acid-insoluble C:N ratio in very fine root and live fine root of B. ermanii was significantly less in the SR stand. The contribution of N content in very fine root to the total root-rh was much greater in the SR (45%) stand than the SP stand (36%). These results suggest that the remaining S. kurilensis detritus and the decreased N use by this species after its removal resulted in increased N availability for B. ermanii. Natural regeneration of tree species after simultaneous death of Sasa species, which is very common in Sasa-dominant forests in Japan, may in part, be facilitated by the presence of residues of the root-rh of Sasa species.