## 2 Methods  
We illustrated the various problems of _Acacia_-dominated slopes by combining the results of a targeted literature review of peer-reviewed scientific articles, research reports and local research theses published in the last 25 years and a cross-disciplinary survey of _Acacia_-dominated slopes at Tai Tong, Hong Kong (China). We studied four sites that are located adjacent to the roadside with high pedestrian and traffic flows particularly during the daytime. These sites were selected since they represented some _Acacia_-dominated slopes that were covered under the Government's \"Enhancement of Vegetated Slopes Programme\" in which the senesced _A. confusa_ trees would be felled and replaced with the planting of other native species. The study area is located in close proximity (around 100 m or less) to natural regenerated forests and plantations the Tai Lam Country Park, which is a legally protected area in Hong Kong for nature conservation, recreational and educational purposes. The size of the four sites ranges from 959.5 m\({}^{2}\) to 3248 m\({}^{2}\) and with a total of 262 numbers of _A. confusa_ trees at the beginning of the survey. We conducted the following measurements: monitoring of soil conditions, measurement of canopy ecophysiological parameters for evaluating climate and air quality regulating services, and assessment of tree health. To monitor the soil conditions, surface (0-10 cm) soil samples were collected with a soil auger seasonally in 5 replicate locations at each of the four sites in Tai Tong from July 2018 to April 2020. The samples were immediately brought back to the laboratory and stored at 4 \({}^{\circ}\)C in a refrigerator until further analysis of their physicochemical properties. Soil pH was measured by the glass electrode method using an Orion Expandable Ion Analyzer after shaking the soil slurry with a soil to water ratio of 1:2.5 (\(w/v\)). Soil bulk density was determined by collecting a fixed volume of top soils with aluminum cores, drying in the oven at 105 \({}^{\circ}\)C for 48 h, and measuring the total weight of the oven-dry soils. Soil ammonium and nitrate concentrations were determined by extracting the soil samples with potassium chloride for 1 h and analyzing the extracts for ammonium and nitrate concentrations spectrophotometrically by flow injection analyzer. Available phosphorus concentration was determined by extracting soil samples with ammonium lactate solution and analyzing the extracts for phosphate concentration by flow injection analyzer. Soil organic matter was determined by the loss on ignition method by combusting the soil samples at 550 \({}^{\circ}\)C for 4 h. Total Kjeldahl nitrogen in soils was determined by the Kjeldahl oxidation method. The concentrations of extractable cations (Na, Mg, K, Ca) were determined by atomic absorption spectroscopy after extracting the soils with ammonium acetate solution for one hour. To evaluate climate and air quality regulating services, we first measured important climate- and air quality-relevant ecophysiological parameters at the four sites over six seasons from spring 2019 to summer 2020, combined with reference data representing typical \"conventional\" values for the corresponding vegetation types from [PERSON] et al. [19]. Specifically, we collected two variables, namely, species-specific maximum carboxylation rates at 25 \({}^{\circ}\)C (\(V_{\text{cmx25}}\)) using the LICOR-6800 photosynthesis system and following the established protocol of [PERSON] et al. [20], and leaf area index (LAI) using the LICOR-2200C LAI meter, of three plant species: _A. confusa_ (old, exotic plantation), _Liquidambar formosana_ (replanted native) and _Trema orientalis_ (native). The latter two species were chosen because _L. formosana_ is the most prevalent native tree used in the replanting program, and _T. orientalis_ is a common native shrub found in the study sites.