Ecological Responses to Fire in Open Ecosystems: Plant Composition, Traits and Carbon

Abstract

Fire is a fundamental regulator of structure and function in open ecosystems, yet the mechanisms by which recurrent burning shapes plant communities, functional traits, and soil carbon remain poorly resolved, particularly in temperate regions. This thesis investigates how fire regimes act as an ecological filter across two temperate open systems, North American oak savannas and British coastal heathlands, using a combination of long‐term field experiments, a global meta-analysis, and post-fire successional gradients. In a 60-year fire frequency experiment at Cedar Creek Ecosystem Science Reserve (Minnesota, USA), increasing fire frequency maintained open, herbaceous-dominated savanna with higher local species richness and Shannon diversity, contradicting intermediate disturbance predictions. Frequent burning also drove strong directional species turnover and increased $\beta$-diversity, while filtering leaf economic traits towards more conservative, stress-tolerant strategies (higher LMA, C:N; lower SLA), with shifts driven largely by species replacement rather than intraspecific variation. Investigating the impacts of fire on soil carbon, a global meta-analysis of 31 open-ecosystem sites showed that fire reduced soil carbon stocks by $\approx$ 20–26% on average, with losses amplified in coarse-textured soils and in systems with large pre-fire carbon pools, whereas climate variables explained little additional variation. Returning to Cedar Creek, detailed measurements of soil carbon and root traits across a fire frequency gradient revealed a hump-shaped relationship between soil carbon and fire, with maximum stocks at intermediate burning. Frequent fire reduced deep, coarse roots, shifted biomass towards shallow fine roots, and altered root morphology, while variance partitioning identified root traits and mycorrhizal colonisation as key predictors of deep soil carbon, highlighting root-mediated rather than purely combustion-driven controls on carbon storage. In Cornish coastal heathland, a time-since-fire chronosequence showed predictable post-fire trajectories: early-stage, species-rich communities with acquisitive root strategies transitioned to Calluna-dominated stands with denser, more conservative roots and accumulating soil organic carbon. These dynamics expose a management trade-off between maintaining high biodiversity through more frequent burning and maximising carbon storage under longer fire-free intervals. Collectively, this thesis demonstrates that fire regimes structure temperate open ecosystems through coupled effects on composition, traits, and carbon. It shows that frequent fire can sustain high diversity yet deplete deep soil C, that soil and root traits mediate belowground resilience, and that context-specific, mosaic fire management is needed to balance biodiversity conservation with carbon-climate objectives.