Above and below-ground disturbance responses in Mountain Ash forests

Abstract

Above and below-ground biological communities and abiotic properties form interactions and feedbacks that underpin critical ecological functions across all terrestrial ecosystems. Despite this, disturbance ecology has traditionally focused on understanding disturbance responses above-ground, with little consideration of those below-ground, or of the interactions between them. New methods such as DNA and RNA meta-barcoding provide an exciting opportunity to quantify below-ground biodiversity and broaden our understanding of the ecological impacts of disturbances. This thesis takes advantage of these new approaches to describe how soil properties, microbial and plant communities, and their respective multi-trophic interactions are influenced by wildfires, and clearcut and salvage logging. My research is based on 81 sites across a multi-decadal chronosequence (differing in disturbance histories) in the Mountain Ash forests of the Victorian Central Highlands in south-eastern Australian. My findings in Chapter 1 revealed differences in the responses of plant communities to clearcut logging and multiples fires. Specifically, I found that resprouting plant species are more sensitive to mechanical disturbances such as clearcut and salvage logging than the impacts of wildfires alone. Further, multiple wildfires resulted in declines in the abundance of some onsite-seeder plant species, which may indicate they are maladapted to short-fire-return intervals. Chapter 2 details my research findings of multi-decadal disturbance impacts on forest soils. Specifically, it describes declines in key soil nutrients including nitrate and available phosphorus for up to at least eight decades post-wildfire and three decades post-clearcut logging, relative to long-undisturbed forest. These adverse impacts on soil measures were more pronounced in the topsoil and were exacerbated with compounding disturbance. Chapter 3 highlights how highly-functional, yet poorly described, fungal communities, and their respective functional groups are impacted by disturbances. This chapter provides evidence of changes in the composition of fungal communities for up to 34 years post-disturbance. It also shows that clearcut logging reduces symbiotrophic fungi and salvage logging increases pathotrophic fungi. Chapter 4 describes the highly-diverse, yet highly-cryptic soil microbiome of the Mountain Ash forests for the first time. It also provides evidence that disturbances can reduce the diversity of individual microbial communities (bacteria, archaea, fungi) and total microbial diversity, especially in the topsoil. This chapter also identifies key edaphic and environmental drivers of the composition of individual microbial communities post-disturbance. Finally, Chapter 5 integrates all previous chapters by disentangling the complex relationships between disturbances and plant-soil-microbial interactions. Specifically, this chapter provides evidence wildfires, logging and post-fire (salvage) logging can simultaneously impact plant and microbial communities, and abiotic soil properties both directly and indirectly through plant-soil-microbial interactions. Further, it also describes over 68 direct and indirect disturbance impacts, including those mediated by plant-soil-microbial interactions. These findings highlight the extent of which disturbance impacts can penetrate whole-forest ecosystems. My research concludes that multiple fires and logging can have extensive adverse impacts below-ground, which may have widespread functional implications in forests. This suggests that traditional above-ground-focused approaches to land management may be underestimating disturbance impacts in forests. Further, my research highlights the need for a multifaceted approach to management that considers whole forest ecosystems including below-ground ecological components, and their interactions above-ground. Show more