## 3 Results and Discussion
### Declining Ecosystem Services of Acacia  
We evaluated the ecosystem services provided by the _Acacia_ plantations as well as some neighboring species in terms of mitigating climate change and air pollution, which are significant concerns in Hong Kong. Measured values of \(V_{\text{cmax25}}\) and LAI, together with their conventional values from literature, are tabulated in Table 2. The replanted _L. formosana_ has a normal LAI but a roughly doubled \(V_{\text{cmax25}}\) compared with typical tropical broadleaf deciduous trees, while the other two species, _A. confusa_ in the old roadside plantations and the native _T. orientalis_, have smaller LAI (almost halved) but comparable \(V_{\text{cmax25}}\) compared with typical tropical broadleaf evergreen trees and temperate broadleaf deciduous shrubs, respectively. The comparison reveals generally better physiological conditions of the replanted species than the mature _A. confusa_ and mature native _T. orientalis_. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**pH**} & **Bulk Density** & **Ammonium Nitrogen** & **Nitrate Nitrogen** & **Available** & **Phosphorus** & **Soil Organic Matter** & **Total Kjeldahl Nitrogen (\%)** & \multirow{2}{*}{**Na**} & \multirow{2}{*}{**K**} & \multirow{2}{*}{**Mg**} & \multirow{2}{*}{**Ca**} \\  & **(g/cm\({}^{3}\))** & **(mg N/kg)** & **(mg N/kg)** & **(mg N/kg)** & **(mg/kg)** & & & & **Kjeldahl Nitrogen (\%)** & \\ \hline
5.22 & 1.02 & 13.04 & 25.87 & 9.13 & 7.22 & 0.27 & 13.5 & 81.6 & 47.8 & 646 \\ (0.32) & (0.02) & (2.42) & (3.41) & (0.99) & (0.16) & (0.01) & (1.5) & (6.3) & (3.9) & (66) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The physio-chemical properties of soils on _Acacia_-dominated slopes (mean \(\pm\) 1 standard error, N = 40). As we examined the aggregate measured LAI at the four sites (three with replanting, one as control) over the six seasons (Figure S1), we found that LAI of the forest canopies generally follows a seasonal cycle with peak LAI in autumn and lowest LAI in spring. It is particularly noteworthy that all the sites with replanting at various time points experienced an accelerated growth in LAI in the growing season in 2020 that was faster in comparison to the control site. Based on the measured values of \(V_{\text{cmax25}}\) and LAI, the annual GPP and ozone uptake (\(F_{\text{O3}}\)) of _A. confusa_ were estimated to be about 1600 g C m\({}^{-2}\) and 3.1 g O\({}_{3}\) m\({}^{-2}\), respectively (Table 3). By comparison, the replanted species found in the same forest, _L. formosa_, has a significantly larger productivity (2300 g C m\({}^{-2}\)) but slightly smaller ozone uptake (2.5 g O\({}_{3}\) m\({}^{-2}\)) when compared to _A. confusa_. A native shrub species, _T. orientalis_, has a larger productivity (2100 g C m\({}^{-2}\)) and larger ozone uptake (3.5 g O\({}_{3}\) m\({}^{-2}\)) when compared to _A. confusa_. However, when comparing the results from the measurements with those using conventional LAI and \(V_{\text{cmax25}}\) values, we found that the replanted _L. formosa_ has a normal LAI but an almost doubled \(V_{\text{cmax25}}\) compared with the conventional value, while both the original species _A. confusa_ and _T. orientalis_ have an LAI that is about half of the conventional value and a normal \(V_{\text{cmax25}}\). The comparison reveals better health conditions and more active growth of the replanted species than the original species. The subpar performance of the original species is mostly due to the much lower measured LAI than the conventional LAI, which can be explained by the maturity and senescence of the old plants. Climate change itself may have adverse impacts on forests and further constrain the ability of trees to provide ecosystem services [27]. Such impacts are complex, however, and strongly depend on the tree species, climate zone, and the complicating effect of urbanization.