The National Natural Science Foundation of China（31670406），Guangxi Key R & D Program Project（AB16380254），Guangxi BaGui Scholars Talent Project（C33600992001）
In this study, the element contents and chemometric characteristics of the leaves of two tropical karst forests, a tropical mangrove forest, and a subtropical evergreen broad-leaved forest (SEBF) from the literature were compared. The results showed that the plant leaves from karst forests in Xishuangbanna and Nonggang were generally rich in Ca and Mg due to the influence of carbonate rocks. The rocks of Nonggang area are composed of a certain amount of dolomite, which makes plants contained much higher Mg. The karst forest in Xishuangbanna was lack of K, Fe, Na and Zn. Due to the composition of certain percentage of deciduous trees in Xishuangbanna karst forest, the mean foliar content was relatively lower. However, the plants in Nonggang karst forest had a higher content of K, Zn and S. Mangrove plants were rich in P, Ca, Mg, Na and S. This is likely because of the relatively rich mineral elements in the seawater that entered the soil and were absorbed and by the mangrove plants. However, the mangrove plants were deficit of Fe, Si and Zn. Plants in subtropical evergreen broad-leaf forest were rich of Mn but deficit of P and Na. Compared with other two types of forests, the evergreen broad-leaf forest had a lower content of N, P, Ca and Mg. The foliar N/P ratios of the plants in karst forest of Xishuangbanna and Nonggang were 14.27 and 18.26, respectively, indicating the nutrient limitation of both N and P in the former or P in the latter. The N/P ratio of mangrove plants was 13.12, indicating the limitation of N. The evergreen broad-leaf forest plants were seriously deficient of P as indicated by their mean N/P ratio of 26.27. There was a significantly positive correlation between N and P elements in the plant leaves from all four forests, revealing a general rule of plant adaptation to the environment. Two divalent cations, Ca++ and Mg++ had synergic relationship in the karst forests and evergreen broad-leaf forest, but there was no correlation between them in the mangrove forest. The analysis showed that the absorption of Mg of the mangrove plants was increased due to the stress of sodium salt, which changed the balance of Ca and Mg. N and K showed a synergic relationship in mangrove forest and evergreen broad-leaf forest, but the synergy of Ca and Mg in karst environment affected the absorption of K, which changed the synergic relationship between N and K. P and Zn showed a synergic relationship in the karst forests and mangrove forest because these two elements were involved in the enzyme synthesis in plant metabolism. The antagonistic relationship between K and Mn in mangrove plants may be helpful for mangrove plants to avoid excessive absorption of Mn, but there was no correlation between these two elements in karst forest and evergreen broad-leaf forest. S and P showed positive correlation in Nonggang karst forest, which could help the karst plants to alleviate P deficiency symptoms, but there was no correlation in mangrove plants with abundant P. The present results have important implications for the modelling of biogeochemical cycle and management of forest ecosystems.