Robinia pseudoacacia, as an invasive species, can fix N and spread rapidly to N-deficient habitats. An allocation shift of N among defense, growth and fecundity may be involved in its invasive strategy. We hypothesized that investment to reproduction increases with the magnitude of flooding disturbances and tested the hypothesis by studying N allocation among functions. We set up 3 populations along the Mibu River, Nagano, which formed a gradient of disturbances from a most disturbed population by frequent flooding to a less disturbed one. In addition, we added a population on the Kyoto U. campus without flooding. We randomly sampled mature leaves and flowers from 10 to 15 trees in each population in May 2014, and seed pods in Sep. We determined the concentrations of total N, Rubisco and cell-wall N for the leaves and the viability of seeds. N allocation to Rubisco was significantly lower in the least-disturbed population in Nagano, which on the other hand showed higher N allocation to cell wall and higher cell wall proportion. The viable rate of seeds was significantly greater in the most-disturbed population. Our results suggest that R. pseudoacacia invests more resources to reproduction in a highly disturbed habitat, while keeping greater N allocation to cell wall in less-disturbed habitats.