The effects of urban forest restoration and environmental heterogeneity on microbial diversity and ecosystem functioning

Abstract

Increasing global urbanisation poses extensive environmental threats, including the loss of biodiversity and associated ecosystem functions. Ecosystem restoration interventions like native plantings can assist in halting and reversing the degradation of forest patches in urban settings, which is particularly relevant for the United Nations’ decade of ecosystem restoration. While the benefits of restoration plantings for aboveground communities have been well established, belowground community responses remain understudied. It is assumed that the soil microbiome will self-assemble to a pre-disturbance state following restoration. However, as microorganisms are rarely the target of restoration interventions themselves, various questions remain regarding the extent to which microbial communities will respond and recover with aboveground plantings, and how environmental variability might influence trajectories of microbial community reassembly. In this thesis, I contribute to this knowledge gap by investigating the impacts of forest age since restoration alongside a suite of environmental covariates on microbial community responses. My fieldwork took place in a New Zealand-wide chronosequence of restored urban forest sites. I analysed the diversity and taxonomic composition of bacterial, archaeal, fungal, and protist communities from these soil samples using metabarcoding data in chapter 2. Furthermore, I investigated changes in total microbial biomass, bacterial and fungal taxonomic group biomass via PLFA, total basal respiration, the spatial variability of microbial biomass and respiration, and carbon metabolism on substrates of varying recalcitrance using MicroResp™ in chapter 3. My synthesised findings show that environmental variability is typically more explanatory of microbial responses than forest age alone. Plant community factors in particular—such as native woody seedling density and tree species richness—were most often linked with enhanced microbial diversity and respiration, as well as driving increased local spatial variability. Specific phylum-level shifts from metabarcoding data indicated microbiome recovery on a trajectory towards mature forest soil communities, yet PLFA data showed that community composition does not yet reflect fungal dominance as would be expected in a mature or fully recovered forest soil. This thesis provides a holistic assessment of microbial community responses to urban forest restoration interventions, by assessing both diversity and microbial functions at multiple taxonomic resolutions. In conclusion, results from my thesis highlight the importance of taking into account the recovery of the soil microbiome and its associated ecosystem functioning to improve both aboveground and belowground success in future forest restoration projects.