Supporting data for "Drivers of Plant and Rhizosphere Community Assembly in Hong Kong’s Regenerating Secondary Subtropical Forests"

Secondary forests represent an increasingly larger proportion of global forest cover, and in the East Asian subtropics these forest types dominate regions such as Hong Kong, China. Forests in the region have been subject to extensive historical disturbances and are now regenerating through secondary succession, in addition to governmental and NGO initiated active reforestation and afforestation regimes. Trajectories of secondary succession are well documented in Hong Kong’s secondary forests, while explorations of species turnover and forest phylogenetic structure have been overlooked. While records do exist for aboveground plant communities during secondary succession, the soil rhizosphere including microbial, mycorrhizal and micro-invertebrate taxa have been neglected within the study of secondary succession. Additionally, a critical comparison of active reforestation with passive spontaneous secondary succession has not yet been made in Hong Kong which evaluates the impacts of these conservation approaches upon soil community structure and ecosystem function.

This thesis addressed research gaps related to aboveground plant communities by assessing plant species and phylogenetic structural change over a secondary successional chrono-sequence in data chapters 1 and 2. In data chapter 3, research gaps related to the soil rhizosphere were explored through assessment of the soil biome in actively and passively regenerating forests in Hong Kong.

Data chapter 1 investigated plant community assembly along a successional gradient from 7 to 70 years following the onset of secondary succession. Plant survey data for 28 plots were analysed, generating additive Simpsons turnover and nestedness beta diversity metrics as subcomponents of Sorenson Beta Diversity. Dissimilarity matrices were generated and modelled as a function of transformed environmental matrices of forest plant community age (years following onset of secondary succession), inter-community distance (metres), and soil moisture saturation (%) across three elevational bands. Generalized dissimilarity models were generated for plant species turnover and nestedness. Nonmetric Multidimensional Scaling of plant communities was conducted with Bray-Curtis dissimilarity matrices. Findings indicated that inter-community distance was the primary driver of plant species Turnover, while age and played a prominent but secondary role. Models of nestedness found that plot age and soil moisture saturation were significant drivers of nestedness patterns in plant communities across elevational classes. Turnover represented a higher proportion of Sorensen beta diversity than nestedness, while ANOSIM found significant differentiation between plant communities at different successional stages. Turnover patterns suggest a deterministic model of community assembly, with clear compositional shifts between lowland and montane forest types. NMDS analysis and functional compositional assessments suggested a transition from early successional communities with a high proportion of shrub species, to later successional communities with a higher proportion of tree species, with an increase in species turnover with greater age dissimilarity.

Data chapter 2 investigated patterns of plant community phylogenetic structure and phylogenetic dissimilarity. Mean pairwise distance (MPD) and Mean Nearest Taxon Distance (MNTD) metrics were determined between co-occurring species within 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. Phylogenetic non-metric multidimensional scaling tested phylogenetic associations of species throughout the landscape of Hong Kong secondary forests. Plant communities in secondary forests in Hong Kong exhibited patterns of basal phylogenetic clustering with increasing elevation, and spatial phylogenetic clustering. Findings suggest clear breaks between the selective regimes of lowland compared to montane forests in Hong Kong, as well as evidence for barriers to plant dispersal in lowland plant communities.

Data Chater 3 used environmental DNA to characterise the biodiversity of soil microbial, fungal and micro-faunal communities, and infra-red gas analysis (IRGA) to measure the carbon respiration of soils in actively and passively regenerating forests. Soil microbial, fungal and nematode communities were compared between secondary forest, young and old actively replanted forests and grasslands. PERMANOVA found significant differentiation between soil microbial, fungal and nematode communities, with a shift from copiotrophic microbial and fungal taxa during early stages of active restoration, towards communities dominated by oligotrophic microbial and fungal taxa in later stages of restoration . ANOVA found significant differences 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 after sustained periods of regeneration may form soil microbial and fungal communities convergent with old secondary forest.