## 4 Discussion
### Sources of Pb and Cd between Needles and Twigs
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Component Matrix (70.642\% of Total Variance Explained)**} \\ \hline
**Element** & \multicolumn{2}{c}{**Component Matrix**} & \multicolumn{2}{c}{**Rotated Component Matrix**} \\ \hline  & 1(42\%) & 2(16\%) & 3(12\%) & 1(35\%) & 2(19\%) & 3(17\%) \\ V & 0.885 & \(-\)0.233 & \(-\)0.175 & 0.903 & 0.186 & 0.136 \\ Cr & 0.775 & \(-\)0.298 & 0.368 & 0.701 & \(-\)0.105 & 0.567 \\ Co & 0.902 & \(-\)0.105 & \(-\)0.171 & 0.857 & 0.3 & 0.172 \\ Ni & 0.499 & \(-\)0.475 & 0.018 & 0.64 & \(-\)0.233 & 0.109 \\ Cu & 0.682 & \(-\)0.406 & \(-\)0.339 & 0.853 & 0.021 & \(-\)0.126 \\ Zn & 0.63 & 0.048 & 0.116 & 0.481 & 0.23 & 0.358 \\ As & 0.926 & 0.006 & \(-\)0.228 & 0.84 & 0.425 & 0.155 \\ Mo & 0.413 & 0.073 & 0.841 & 0.101 & \(-\)0.087 & 0.93 \\ Sb & 0.424 & 0.588 & \(-\)0.24 & 0.147 & 0.746 & 0.072 \\ Pb & 0.591 & 0.661 & 0.199 & 0.141 & 0.713 & 0.546 \\ Cd & 0.424 & 0.474 & 0.272 & 0.068 & 0.465 & 0.508 \\ Hg & 0.222 & 0.621 & \(-\)0.426 & 0.008 & 0.767 & \(-\)0.166 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Component matrix. Figure 2: Spatial distribution map of Pb and Cd in the needles and twigs in the mining area close to our study area ([[https://www.sohu.com/a/365087860_660942](https://www.sohu.com/a/365087860_660942)]([https://www.sohu.com/a/365087860_660942](https://www.sohu.com/a/365087860_660942))). The red and orange colors on each small map represent relatively high concentration of Pb and Cd, while the green and lime colors represent relatively low concentrations, respectively. The buff color represents the middle values. Pb and Cd had high factor loadings in PC3, which might originate from local sources. To investigate probable anthropogenic sources of Cd and Pb in needles and twigs in detail, the distribution patterns of lead and cadmium mines in the vicinity of the study area  
Figure 4: The 72 h backward trajectories of the sampling period of the study area from January to December 2012 ([[https://ready.arl.noaa.gov/hypub-bin/trajtype.pl?runtype=archive](https://ready.arl.noaa.gov/hypub-bin/trajtype.pl?runtype=archive)]([https://ready.arl.noaa.gov/hypub-bin/trajtype.pl?runtype=archive](https://ready.arl.noaa.gov/hypub-bin/trajtype.pl?runtype=archive))). Figure 3: Pb and Cd in twigs and needles of different ages. were analyzed. Figure 2 shows that relatively high concentrations of Cd were near the mines of Cd, and the highest concentrations of Pb were found in the vicinity of large Pb mines. Mines, an important factor influencing Cd and Pb concentrations in needles and twigs, partly accounted for the distributions of Pb and Cd. However, sites with relatively high concentrations far away from the mines were also observed, which indicated that other factors exist besides mines. Heavy metals are widely used in industrial processes and mining enterprises, which directly discharge wastewater without strict treatment, making the soil around them susceptible to high levels of toxic heavy metals. Pb was always associated with emissions from car traffic [45]. Cd concentrations were elevated near highways [15]. These reports suggested traffic plays an important role for Cd and Pb concentration in needles. Previous literature showed that local sources of heavy metals severely affect mountain forest ecosystems [46]. The results indicated that anthropogenic activities mainly account for lead and cadmium in the eastern Tibet Plateau.