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• All age classes of regenerating secondary forest are affected by typhoon damage. • Exotic monoculture plantations are most affected, with little standing vegetation remaining. • Typhoon damage arrests the succession due lack of a viable species pool and dispersal agents. • Competition among large numbers of weak stems regarded as the 'new normal'. Typhoons of varying intensities severely impact ecosystem functioning in tropical regions and their increasing frequencies and intensities due to global warming pose new challenges for effective forest restoration. This study examines the impact of a super-typhoon (Mangkhut) on the regenerating native secondary forest and exotic monocultural plantations in the degraded tropical landscape of Hong Kong. The super typhoon, which hit Hong Kong on 16 September 2018 lasted for 10 h (09:40–19:40) and was the most severe storm affecting Hong Kong over the past 100 years. Hong Kong's secondary forest is a mosaic of forest patches recovering through natural succession since 1945, and plantation stands of exotic monocultural species. We determine the loss in biomass by performing NDVI (Normalized Difference Vegetation Index) difference analysis using two Landsat-8 multispectral images acquired before and after the typhoon. This the assessment of typhoon impacts according to successional age group, structural stages of vegetation, landscape topography, and on stands of exotic plantations. Results indicate that hilltops, open shrubland and grassland were hard hit, especially on southwest and southeast facing slopes, and almost 90% of the landscape showed abnormal change. Patches of exotic monoculture plantation (Lophostemon confertus, Melaleuca quinquenervia, and Acacia confusa) were the most severely damaged by the typhoon, showing more than 25% decrease in NDVI, followed by young secondary forest. Field observations confirmed that in exotic plantations, almost the entire canopy was destroyed and there is no generation of young under story trees to replace those lost. The affected young forests and shrublands are mainly dominated by fast growing, soft wooded early successional species such as Mallotus paniculatus or Machilus chekiangensis as well as weak, multi-trunked, fungus infected, or other structurally deficient trees, which were uprooted or seriously damaged by typhoon gusts. The net effect of typhoons in Hong Kong's degraded landscape, appears to reinforce the arrested succession of dense, less diverse stands of weaker early successional species due to the absence of late and middle successional species and native dispersal agents. In order to obtain a stronger, more resilient forest, it would be necessary to enhance biodiversity by artificially planting a species mix, which resembles primary forests in the region. This could be achieved by thinning of young secondary forest followed by enhancement planting of pockets of high diversity forest. [ABSTRACT FROM AUTHOR]

Estimation of atmospheric visibility (VR) using ground and satellite sensors is ineffective under Hong Kong's complex atmosphere and climate. Therefore, the relationship between columnar Aerosol Optical Depth (AOD) from four space-borne sensors (OMI, MODIS, MISR and CALIOP) and Bext from two visibility-recording stations was evaluated, to recommend an effective satellite-based method and spatial resolution, for estimation of VR over Hong Kong. Since most column-integrated aerosol particle extinction occurs within a mixing layer height (MLH) of 1–3 km, column-based AOD from satellites is expected to give a good indication of surface-level conditions, especially when MLH is a known input. The AOD from both MODIS and MISR showed high correlations with Bext; therefore, both were subjected to rigorous statistical analysis along with climatic data to simulate visibility. The best estimate of ground visibility was obtained from MODIS AOD combined with surface-level climatic data, and this explained 84% of the variance in VR, with a low distance error of 0.27 km. Results suggest that the water vapor mixing ratio (Q) alone can explain the combined effect of Atmospheric Pressure (P), Temperature (T) and Relative Humidity (RH) on VR, and that the advection term (VT) alone is sufficient to explain the effects of T, WS and WD on dispersion of aerosols, and hence on VR. [ABSTRACT FROM AUTHOR]