Plant physiological traits and their relation to soil N availability was investigated as regulators of the distribution of understory shrub species along a slope in a Japanese cedar (Cryptomeria japonica) plantation in central Japan.At the study site, previous studies demonstrated that both net and gross soil nitrification rates are high on the lower slope and there are dramatic declines in different sections of the slope gradient. We examined the distributions of understory plant species and their nitrate (NO (3) (-) -N) use traits, and compared the results with the soil traits.Our results show that boundaries between different dominant understory species correspond to boundaries between different soil types. Leucosceptrum stellipilum occurs on soil with high net and gross nitrification rates. Hydrangea hirta is dominant on soil with high net and low gross nitrification rates. Pieris japonica occurs on soil with very low net and gross nitrification rates. Dominant understory species have species-specific physiological traits in their use of NO (3) (-) -N. Pieris japonica lacks the capacity to use NO (3) (-) -N as a N source, but other species do use NO (3) (-) -N. Lindera triloba, whose distribution is unrelated to soil NO (3) (-) -N availability, changes the extent to which it uses NO (3) (-) -N in response to soil NO (3) (-) -N availability.Our results indicate that differences in the physiological capabilities and adaptabilities of plant species in using NO (3) (-) -N as a N source regulate their distribution ranges. The identity of the major form of available soil N is therefore an environmental factor that influences plant distributions.