## 3 Results
### Chlorophyll Fluorescence  
Table 5 shows the leaf chlorophyll fluorescence (Fv/Fm) characteristics for each sampling date, ontogeny, and SO\({}_{2}\) treatment. In general, the Fv/Fm value of _U. pumila_ was consistently lower than that of _S. oblate_ and _P. cernsifera_, more vulnerable to SO\({}_{2}\) stress. Compared with the CK, the T2 treatment concentration promoted the Fv/Fm value of trees, T3 treatment concentration inhibited the Fv/Fm value of trees. For example, the Fv/Fm value of 10 days leaves of _S. oblate_, _P. cernsifera_, and _U. pumila_ SO\({}_{2}\) concentration T2 treatments was more than that of T1, with increases of 7.5%, 6%, and 12%, respectively. The T3 treatments was less than that of T1, with decreases of 0.4%, 7%, and \(-\)3%, respectively, on 9 September. The Fv/Fm value of 10-day leaves was consistently lower than that of 40-day leaves. The Fv/Fm value of 19 September was consistently lower than that of 1 September and 9 September, more vulnerable to SO\({}_{2}\) stress.  
### Chlorophyll Fluorescence  
Table 5 shows the leaf chlorophyll fluorescence (Fv/Fm) characteristics for each sampling date, ontogeny, and SO\({}_{2}\) treatment. In general, the Fv/Fm value of _U. pumila_ was consistently lower than that of _S. oblate_ and _P. cernsifera_, more vulnerable to SO\({}_{2}\) stress. Compared with the CK, the T2 treatment concentration promoted the Fv/Fm value of trees, T3 treatment concentration inhibited the Fv/Fm value of trees. For example, the Fv/Fm value of 10 days leaves of _S. oblate_, _P. cernsifera_, and _U. pumila_ SO\({}_{2}\) concentration T2 treatments was more than that of T1, with increases of 7.5%, 6%, and 12%, respectively. The T3 treatments was less than that of T1, with decreases of 0.4%, 7%, and \(-\)3%, respectively, on 9 September. The Fv/Fm value of 10-day leaves was consistently lower than that of 40-day leaves. The Fv/Fm value of 19 September was consistently lower than that of 1 September and 9 September, more vulnerable to SO\({}_{2}\) stress.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Time** & **Tree Species** & **Treatment** & **10 Days** & **40 Days** \\ \hline \multirow{8}{*}{9-Sep} & \multirow{8}{*}{_S. oblate_} & T1 & 28.87 \(\pm\) 0.48 c & 25.39 \(\pm\) 0.92 c \\  & & T2 & 30.77 \(\pm\) 0.50 b & 26.99 \(\pm\) 0.40 b \\  & & T3 & 31.77 \(\pm\) 0.15 a & 28.10 \(\pm\) 0.42 a \\  & & T1 & 29.22 \(\pm\) 0.80 c & 25.76 \(\pm\) 0.53 c \\  & & T2 & 30.06 \(\pm\) 0.48 b & 29.10 \(\pm\) 1.04 b \\  & & T3 & 31.17 \(\pm\) 1.30 a & 30.44 \(\pm\) 0.77 a \\  & & T1 & 28.49 \(\pm\) 2.52 c & 25.92 \(\pm\) 0.47 c \\  & _U. pumila_ & T2 & 29.31 \(\pm\) 0.44 b & 28.40 \(\pm\) 0.48 b \\  & & T3 & 30.35 \(\pm\) 0.88 a & 30.18 \(\pm\) 0.18 a \\  & & T1 & 24.38 \(\pm\) 0.43 c & 22.22 \(\pm\) 0.22 c \\  & _S. oblate_ & T2 & 29.50 \(\pm\) 0.81 b & 23.09 \(\pm\) 0.48 b \\  & & T3 & 30.48 \(\pm\) 0.46 a & 24.45 \(\pm\) 0.39 a \\  & & T1 & 25.47 \(\pm\) 0.87 c & 24.15 \(\pm\) 0.73 c \\
**19-Sep** & _P. cernsifera_ & T2 & 27.43 \(\pm\) 0.14 b & 25.05 \(\pm\) 1.35 b \\  & & T3 & 28.79 \(\pm\) 0.40 a & 26.79 \(\pm\) 1.00 a \\  & & T1 & 25.36 \(\pm\) 0.21 c & 22.33 \(\pm\) 1.92 c \\  & _U. pumila_ & T2 & 26.85 \(\pm\) 0.77 b & 22.8 \(\pm\) 0.43 b \\  & & T3 & 27.97 \(\pm\) 0.74 a & 23.85 \(\pm\) 1.20 a \\ \hline \end{tabular}
\end{table}
Table 4: Leaf temperature for three different SO\({}_{2}\) treatments (T1—T3) on 9 September and 19 September 2019 for _S. oblate_, _P. cernsifera_ and _U. pumila_. Bars sharing a common letter are not significantly different (\(p<0.05\)). Bars represent means \(\pm\) SE (\(n=9\)).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Tree Species** & **Treatment** & \multicolumn{2}{c}{**1-Sep**} & \multicolumn{2}{c}{**9-Sep**} & \multicolumn{2}{c}{**19-Sep**} \\ \cline{3-8}  & & **10 Days** & **40 Days** & **10 Days** & **40 Days** & **10 Days** & **40 Days** \\ \hline \multirow{8}{*}{_S. oblate_} & T1 & 0.61 \(\pm\) 0.11 b & 0.65 \(\pm\) 0.08 b & 0.66 \(\pm\) 0.10 b & 0.71 \(\pm\) 0.04 b & 0.61 \(\pm\) 0.11 b & 0.65 \(\pm\) 0.08 b