Abstract: Fixed-point sampling was conducted in the field at different periods to explore the change of carbon, nitrogen, phosphorus, potassium and the stoichiometry characteristics in the roots, stems and leaves in different phenological periods of the Alchornea trewioides which is known as the pioneer plant in karst area. The contents of soil carbon, nitrogen, phosphorus and potassium were measured and the content of total carbon (TC), total nitrogen (TN), total phosphorus (TP) and total potassium (TK) in plants and their relationships with soil nutrients were analyzed, and the correlation between plant element content and basic soil nutrients content were analyzed. The results showed that: (1) From florescence to abscission period, the variation range of TC, TN, TP and TK are 428.42~460.64 g/kg, 8.43~25.81 g/kg, 0.37~1.13 g/kg, and 2.82~6.32 g/kg, respectively The contents of TC, TN, TP and TK in leaves are the highest in all organs. At florescence, the contents of C, N, P and K in leaves were 446.75, 24.12, 1.13 and 6.32 g/kg, respectively. At fruit stage, the contents of C, N, P and K in leaves were 451.405、25.805、0.88 and 5.54 g/kg, respectively. In the abscission period, the contents of C, N, P and K in leaves were 460.635、19.02、1.07 and 5.52 g/kg, respectively. The contents of C, N and K in roots are more stable than those in stems and leaves, and the fluctuation of P content in roots is larger than that in stems and leaves. The variation coefficient of TC, TN, TK in different organs is stem >leaf > root, and the variation coefficient of TP is root > leaf > stem. The variation coefficient of TP in root is 39.1%, 250.04% higher than that of stem and 206.67% higher than that of leaf. (2) From florescence to abscission period, the variation range of m_C∶m_N, m_C∶m_P, m_C∶m_K and m_N∶m_P in roots, stems and leaves were 17.53~51.99, 396.86~1 179.7,71.16~154.88 and 13.28~29.59, respectively. Among all organs, the mass ratio of m_C∶m_N, m_C∶m_P and m_C∶m_K of leaves is the lowest. At florescence, the mass ratio of m_N:m_P is root > leaf > stem, in fruit and abscission period it is leaf > root and stem. The mass ratio of m_N∶m_P of roots, stems and leaves approaches 16 in abscission period.The m_N∶m_P mass ratio of the leaves of Alchornea trewioides were higher than 16, and the growth of the Alchornea trewioides is limited by P. The interaction effects of different organs and phenophases on m_C∶m_N, m_C∶m_P, m_C∶m_K and m_N∶m_P were significant. (3) Soil total nitrogen had an important effect on plant TN content, and soil available potassium had an important effect on plant TC and TK content, while soil total carbon and available phosphorus content had no significant effect on plant TC, TN, TP and TK.

Key words: ecological stoichiometry, phenology, karst areas, Alchornea trewioides