## 3 Results
### Leaf Spectral Reflectance  
Figure 1 shows the characteristics of the leaf spectral reflectance curves for each sampling date, ontogeny, and SO\({}_{2}\) treatment. In general, the green-peak and red-valley reflectance values of _P. cerasifera_ was consistently lower than that of _S. oblate and U. punilla_, and more vulnerable to SO\({}_{2}\) stress. Compared with the CK, the T2 and T3 treatment inhibited the green-peak and red-valley reflectance value of trees. For example, the green-peak and red-valley reflectance value of 10 days leaves of _S. oblate, P. cerasifera_, and _U. punilla_, SO\({}_{2}\) concentration T2 and T3 treatments was less than that of T1, with decreases of 2.62%, 3.16%, and 1.13%, 9.96%, 6.92%, and 4.85%; 2.71%, 3.05%, and 1.66%, 4.41%, 5.80%, and 6.01%, respectively, on 9 th September. The green-peak and red-valley reflectance value of 10-day leaves was consistently lower than that of 40 days leaves. The green-peak and red-valley reflectance of 19 September was consistently lower than that of 1 September and 9 September and was more vulnerable to SO\({}_{2}\) stress. The magnitude of individual differences was influenced by the sampling date and SO\({}_{2}\) treatment.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Time** & **Tree Species** & **Treatment** & **10 Days** & **40 Days** \\ \hline \multirow{8}{*}{**1-Sep**} & \multirow{4}{*}{_S. oblate_} & T1 & 37.83 \(\pm\) 5.53 b & 41.63 \(\pm\) 2.04 b \\  & & T2 & 48.1 \(\pm\) 3.05 a & 49.5 \(\pm\) 2.16 a \\  & & T3 & 20.06 \(\pm\) 7.08 c & 34.73 \(\pm\) 6.35 c \\  & & T1 & 32.26 \(\pm\) 0.40 ab & 47.23 \(\pm\) 2.61 a \\  & & T2 & 36.4 \(\pm\) 1.15 a & 51.9 \(\pm\) 12.5 a \\  & & T3 & 29.93 \(\pm\) 0.49 b & 40.96 \(\pm\) 3.00 a \\  & & T1 & 18.56 \(\pm\) 1.01 a & 23.1 \(\pm\) 0.62 b \\  & _U. punila_ & T2 & 19.23 \(\pm\) 3.72 a & 29.8 \(\pm\) 2.95 a \\  & & T3 & 6.9 \(\pm\) 2.62 b & 18.16 \(\pm\) 0.66 c \\  & & T1 & 45.63 \(\pm\) 1.11 b & 46.8 \(\pm\) 5.53 ab \\  & _S. oblate_ & T2 & 51.2 \(\pm\) 4.08 a & 54.7 \(\pm\) 2.49 a \\  & & T3 & 38.6 \(\pm\) 4.15 c & 42.53 \(\pm\) 3.80 b \\  & & T1 & 33.53 \(\pm\) 0.46 b & 43.76\(\pm\) b \\
9-Sep & _P. cerasifera_ & T2 & 44.4 \(\pm\) 1.80 a & 55.73 \(\pm\) 0.51 a \\  & & T3 & 28 \(\pm\) 1.05 c & 41.4 \(\pm\) 0.96 c \\  & & T1 & 22.93 \(\pm\) 2.55 a & 27.63 \(\pm\) 2.44 a \\  & _U. punila_ & T2 & 26.73 \(\pm\) 1.90 a & 28.13 \(\pm\) 2.10 a \\  & & T3 & 14.56 \(\pm\) 0.60 b & 24.8 \(\pm\) 6.21 a \\  & & T1 & 28.06 \(\pm\) 2.45 b & 31.76 \(\pm\) 5.00 ab \\  & _S. oblate_ & T2 & 34.13 \(\pm\) 1.85 a & 39.26 \(\pm\) 4.74 a \\  & & T3 & 22.26 \(\pm\) 3.00 c & 20.73 \(\pm\) 8.50 c \\  & & T1 & 27.7 \(\pm\) 0.26 a & 35.26 \(\pm\) 4.74 a \\
**19-Sep** & _P. cerasifera_ & T2 & 30.63 \(\pm\) 2.70 a & 36.76 \(\pm\) 1.27 a \\  & & T3 & 18.83 \(\pm\) 2.27 b & 21.5 \(\pm\) 2.26 b \\  & & T1 & 18.9 \(\pm\) 2.78 b & 27.13 \(\pm\) 1.18 b \\  & _U. punila_ & T2 & 26.26 \(\pm\) 2.45 a & 36.23 \(\pm\) 10.8 a \\  & & T3 & 18.53 \(\pm\) 1.00 b & 19.83 \(\pm\) 5.84 b \\ \hline \hline \end{tabular}
\end{table}
Table 3: Leaf chlorophyll SPAD for three different SO\({}_{2}\) treatments (T1–T3) on 1 September, 9 September, and 19 September 2019 for _S. oblate, P. cerasifera_ and _U. punilla_. Bars sharing a common letter are not significantly different (_p_ < 0.05). Bars represent means \(\pm\) SE (_n_ = 9).