Contribution of plant species
The contributions of P. fortunei and K. bipinnata to relevant indices are compared in Table 2. The two transplanted species were shown to be fast-growing tree species with high growth and metal uptake rates under the improved site conditions. The sum of their number of individual plants in Plot I was only 0.104% of that of the plant community while that of their biomass, Mn uptake and land coverage contributed, respectively, 84.77%, 67.63% and 56.30% to the plant community. The high contribution percentage of the two species to s , h and f showed their importance in matter and energy transformation and conservation (Table 2).
Positive correlations between biomass and uptake of all measured metal elements among species were observed in both Plots I and II at high levels of significance (R2 > 0.95) (Wu et al., 2018), indicating that the accelerated growth of the plant community enhanced its metal uptake rates. Accordingly, the significant contribution of the transplanted species to total metal uptake was accounted for by their high growth rates. In comparison, the average contents of Mn in P. fortunei and K. bipinnata (0.1302, 0.1530 mg/g, calculated using the data in Appendix 1) were much lower than that in Radix Phytolaccae (0.9990 mg/g) in Plot I. The average net dry biomass quantity per plant of the two tree species (31.77, 2.23 kg) was, respectively, 369.5 and 25.92 times higher than that of R. Phytolaccae (0.0860 kg), and their average Mn uptake per plant (4.1373, 0.3410 g) was thus 48.16 and 3.97 times higher than that of this commonly acknowledged hyper-accumulator (0.0859 g) (Xua et al., 2015; Zhang et al., 2015).
The key factor that led to the differences in C Tand N between Plots I and II was the type of applied fertilizers. However, as the organic manure was only addressed to the rooting area of the transplanted species in Plot I for reducing the remediation cost (Wu et al., 2017), the rapid increase in the number of native plant species spreading over the entire area of Plot I suggested that the root growth and metabolic activities of the transplanted species and decomposition of their fallen branches and leaves had also played important roles in promoting the growth of other native species and enhancing the species richness of the plant community particularly in later years.
Despite their important functions as dominant species in the restored plant community, the contributions of P. fortunei andK. bipinnata to the Shannon-Wiener index SI were extremely low and the sum of their percentage values was less than 0.3% (Table 2). The SI index (Eq. 1) is a single function of the number of individual plants m i while the sum of the survived number of individual plants of the two transplanted tree species (1700/hm2, Appendix 1) was negligible compared to the total individual number of the community (1633974.9/hm2), giving thus extremely lowSI i or p i values for these two tree species. This means that the SI i index will not take into account the important roles of the dominant species if they have small individual numbers.
Another phenomenon depicted in Fig. 1. was that the contribution percentage of the two transplanted species to C T(Fig. 1a) and M (Fig. 1b) decreased while that of the other species increased with increasing N . The contribution percentage of a species to C T or M is equal to itsx i (biomass ratio) or p i(individual number ratio) given in percentage. The curves in Fig. 1 thus show clearly the negative effect of species richness (N ) and abundance (M ) on x i andp i of the transplanted tree species.