Abstract
The close relationship between plants and mycorrhizal fungi has major effects on ecosystem structure and function. Nothofagus forests and the ectomycorrhizal symbioses that support them are particularly widespread and important in the southern hemisphere, yet little is known about the diversity, distribution, and specificity of the fungal species involved. Historical deforestation and the threat of further habitat reduction under future climate regimes makes understanding the dynamics of ectomycorrhizal fungal communities, and the role of ectomycorrhizal fungal species in forest establishment, crucial for maintaining diverse, healthy, and resilient ecosystems. This thesis aims to determine the critical factors required for undertaking successful Nothofagus restoration, test the relative effects of ectomycorrhizal fungal availability and other factors limiting Nothofagus seedling establishment in grasslands, and investigate ectomycorrhizal fungal community assembly processes and Nothofagus host-symbiont specificity on a regional scale.
I used a meta-analysis approach to determine which restoration techniques are most critical for successfully establishing trees, by combining the results of 91 Nothofagus restoration trials from 22 publications that tested the effects of different techniques on seedling survival and growth (Chapter 2). Providing shelter was the most commonly implemented technique, and this improved both seedling survival and growth in almost all situations. Controlling weeds also improved survival, and protecting from herbivores improved growth. Other widely applied techniques, such as fertilising, had no consistent benefits. Surprisingly, no trials tested the use of ectomycorrhizal fungal inoculum as a restoration technique. Given the obligate relationship between Nothofagus and ectomycorrhizal fungi, this highlights a major research gap. This review provides a valuable source of information for practitioners to improve future Nothofagus restoration outcomes, and forms a base for directing future research efforts.
To better understand the relative importance of different factors influencing seedling establishment outside forest boundaries, I conducted a large multi-factorial experiment to test the effects of ectomycorrhizal fungal availability, availability of shelter, competition from grass and pasture species, fertiliser addition, and herbivory on Nothofagus seedling establishment in grasslands (Chapter 3). The availability of shelter had the strongest effect on Nothofagus establishment and survival; no seedlings survived in unsheltered plots. Pasture species initially facilitated establishment, but negatively affected survival at later stages. Seedling survival and health were positively related to ectomycorrhizal colonisation, but the formation of ectomycorrhizae did not depend upon supplied inoculum, indicating that grasslands are not devoid of natural inoculum. As well as informing restoration practices, these results provide insights into the constraints acting on the natural spread of forests into neighbouring grasslands, and therefore the ability of Nothofagus forests to cross barriers and migrate in response to climate change.
Ectomycorrhizal fungal communities have strong effects on ecosystem function, and fungal species partition niche space along numerous key environmental gradients. I investigated ectomycorrhizal community assembly processes by undertaking a regional-scale environmental DNA survey that characterised ectomycorrhizal communities in 81 forest patches spanning the length and breadth of New Zealand’s South Island (Chapters 4 and 5). Multi-site generalised dissimilarity modelling (MS-GDM) revealed that ectomycorrhizal community assembly processes are dominated by deterministic effects of environmental filtering, and that these filters differ between rare and common species (Chapter 4). Soil pH and C:N ratio had particularly strong effects on turnover of the whole community. Ground cover, organic soil depth, forest patch size and precipitation drove turnover patterns of rare fungal taxa, whereas temperature variables and host tree size had stronger effects on the turnover of more common taxa. Host species identities had moderate effects on turnover that were weaker than some environmental variables, but host effects explained additional variation in turnover independently of the many environmental variables tested. The MS-GDM method for separating the environmental effects acting on different components of communities (i.e., rare versus common species) has not previously been applied to fungal communities, so these results provide novel insights into ectomycorrhizal community ecology.
Further examination of host effects on ectomycorrhizal communities (Chapter 5) revealed distinct differences in the fungal communities hosted by different Nothofagus species, with the strongest differences between hosts from different subgenera (Lophozonia and Fuscospora). Twenty-four percent of fungal sequence variants tested using indicator species analysis showed preferences for particular host taxa. Fungal phylogenetic relationships also showed congruence with host taxa relationships, i.e., cophylogeny, where more closely related hosts harboured more closely related fungal taxa. Additionally, there was a trend that fungi associated with the older Lophozonia subgenus had longer phylogenetic branch lengths than those associated with the younger Fuscospora hosts. There has been little previous evidence that ectomycorrhizal species show preferences for different Nothofagus host species, and evidence of host specificity occurring in congeneric host systems is lacking in general. These results also provide the first evidence of cophylogeny occurring in ectomycorrhizal systems at the regional scale.
Overall, this thesis provides valuable information for improving Nothofagus restoration efforts, and a more nuanced understanding of the factors controlling seedling establishment in non-woody ecosystems in general. Additionally, this thesis provides novel insights into how the environment and host trees affect ectomycorrhizal community ecology and evolution. As the first regional-scale ectomycorrhizal data of its kind in the southern hemisphere, the ectomycorrhizal dataset produced here provides significant future potential for expanding knowledge of ectomycorrhizal biogeographical and ecological processes more broadly. Understanding the drivers of below-ground community organization, and the effect of ectomycorrhizal communities on above-ground vegetation establishment, is essential for understanding how forest ecosystems will likely change in the future and how we can better mitigate the detrimental effects of deforestation and climate change.