Aims In relation to global climate changes, the issue of how forest ecosystems could affect biomass and soil carbon sequestration is essential. Materials & Methods To do this research, ailanthus (Ailanthus altissima Mill.) and Arizona cypress (Cupressus arizonica Greene) plantations were selected each one with an area of 20 hectare in forest park of Malayer, Western Iran. An adjacent area with no tree was selected as control. In each of the plantations and control area, ten plots of 20 × 20 m2 deployed and biomass of trees, biodiversity indices (Shannon–Wiener, Simpson, Menhinick, and Margalef indices), and carbon sequestration of aboveground tree biomass, belowground biomass, leaf litter, grass, and soil were measured. Findings The results showed that the carbon sequestration in Arizona cypress plantation (32.32 t ha−1) and the soil under it (11.15 t ha−1) was higher than that in ailanthus plantation and the soil under it (17.99 and 7.6 t ha−1, respectively). However, the soil carbon sequestration under both plantations was higher than that in control area (5.28 t ha−1). According to the results, it was found that herbaceous understory of ailanthus plantation had stored carbon more than arizona cypress plantation. Furthermore, the results indicated that there is a significant difference between two plantations from the point view of the understory plant diversity (Menhinick index in ailanthus and Arizona cypress plantations was 3.17 and 2.44, respectively). Conclusion This research confirms that plantation with Arizona cypress tree is more efficient in soil and tree biomass carbon sequestration than plantation with ailanthus trees. Furthermore, according to the results, the understory plant richness in ailanthus plantation was higher than that in arizona cypress.
