Drivers of plant and rhizosphere community assembly in Hong Kong's regenerating secondary tropical forests

Abstract

Secondary forests represent a growing proportion of global forest cover, and dominate the East Asian tropical region of Hong Kong, China. Hong Kong’s secondary forests are regenerating through passive secondary succession and active reforestation following historical clearances. The influence of secondary succession and restoration upon i) aboveground plant species beta- diversity and phylogenetic structure and ii) the below-ground soil biome including bacterial, mycorrhizal and invertebrate taxa and the impacts of this on ecosystem functions, have not been investigated and these knowledge gaps were explored during this thesis.

Chapter 2 investigated plant community assembly along a successional gradient (7-70 years). Plant survey data were analysed, generating Simpsons turnover and nestedness subcomponents of Sorenson Beta Diversity. Dissimilarity matrices were generated and modelled as functions of transformed environmental matrices of forest age (years), inter-community distance (metres), and soil moisture saturation (%) across elevational bands via generalized dissimilarity models (GDMs). Nonmetric Multidimensional Scaling (NMDS) of plant communities was conducted with Bray-Curtis dissimilarity matrices. Inter-community distance and age drove species Turnover in Lowland but not Montane forest. ANOSIM found significant differentiation between plant communities at different successional stages. Functional groups transitioned from early successional communities with a high percentage of shrub species, to later successional communities with a higher percentage of tree species. Chapter 3 investigated plant community phylogenetic structure and phylogenetic beta-diversity during secondary succession. Mean-pairwise-distance (MPD) and Mean-Nearest-Taxon-Distance (MNTD) metrics were calculated in plant communities, in addition to phylogenetic beta diversity metrics relative to null models of random phylogenetic assembly. MPD and MNTD were compared between elevational and successional classes and modelled as products of intercommunity distance, elevation and age, and Phylogenetic NMDS explored the phylogenetic structure of lowland and montane forest. Plant communities in secondary forests in Hong Kong exhibited basal phylogenetic clustering with increasing elevation, and spatial phylogenetic clustering. Chapter 2 and 3 suggest differential community assembly in lowland and montane forests, and plant compositional changes during succession, and potential barriers to plant dispersal in lowland forests.

Chapter 4 used environmental DNA to compare i) soil bacterial, fungal and micro-faunal communities, and ii) soil carbon respiration between actively and passively regenerating forests. Significant differentiation occurred between soil communities, with a shift from r-strategist bacterial and fungal taxa during early stages of active restoration, towards communities dominated by K-strategist bacterial and fungal taxa in later stages of restoration. Significant differences were found between carbon flux rates, with lower rates in older actively restored and secondary forests, and higher rates in recently restored forests. Findings suggest actively replanting forests may form soil bacterial and fungal communities and carbon flux patterns convergent with older secondary forest.

Findings from this thesis described plant species beta-diversity and phylogenetic structural characteristics of tropical lowland and montane forests in Hong Kong and conducted the first critical comparison of active and passive reforestation impacts upon the soil biome and soil carbon fluxes in the region. Findings advance the field of applied conservation by indicating outcomes of secondary succession and active replanting upon temporal above- and below- ground community dynamics and ecosystem functions.