In this research, we developed a new approach of using the species/carbon (S/C) value to numerically analyze the properties of understory plants under Casuarina and Acacia confuse plantations in Taiwan. The difference between the S/C values in Taichung Harbor and the Sihhu area was the largest, at 10.12. The ratio of Taichung Harbor to Sihhu was as great as 11.12. The difference of Tongsiao and the Hengchun Peninsula for Acacia confusa was 2.66. The S/C value obviously decreased as the C value was increased. Meanwhile, the variety of species was reduced with the increasing C value and showed a negative correlation. The correlation between biomass and species was not significant, with a correlation coefficient at -0.136.