Characterising the response of plant-mycorrhizal networks to a changing alpine environment
Anthropogenic climate change effects are particularly acute in alpine ecosystems. New Zealand’s alpine regions are experiencing climatic changes at higher than global mean rates, particularly warming and drying. These communities are also facing increasing rates of invasion by exotic plant species. Notably, multiple drivers of change, such as warming and invasion, have been evidenced to interact and facilitate greater ecosystem change. This is of particular concern as New Zealand's alpine plant communities are unique globally and represent national hotspots of biodiversity. Therefore there is a pressing need to understand how they may be affected by the independent and interactive drivers of global environmental change. Alpine plant species form ubiquitous and obligate symbiotic associations with mutualistic mycorrhizal fungi. Plant-mycorrhiza networks are foundational interactions that underpin the diversity and function of terrestrial communities. Plant-mycorrhiza networks are also particularly sensitive to temperature shifts and plant invasions. In this thesis, I investigate the independent and interactive effects of warming and the presence of an invasive species (Common Heather, Calluna vulgaris) on the fungal community composition and the network of mycorrhiza interactions of alpine plants in Tongariro National Park, New Zealand. I sampled the roots of plant species within the Warming and species Removal in Mountains (WaRM) experiment, a factorial combination of warming and Calluna vulgaris removals (n = 8 per treatment) established in TNP in 2015. The plant community at the site consists of plant species that form either arbuscular mycorrhizas or ericoid mycorrhizas. I selected the three most abundant plant species of each mycorrhizal type at the site scale for sampling in each of the 32 plots. DNA was extracted from plant roots, and the internal transcribed spacer of the fungal rRNA gene was amplified by PCR and sequenced on the Illumina Mi-seq platform. Sequence data was demultiplexed and fungal OTUs were identified using the PIPITS pipeline, referencing the UNITE fungal database. In my second chapter, I consider plant species and treatment effects on the diversity and community composition of mycorrhizal fungi. I found WaRM treatments were significant determinants of mycorrhizal compositions in host plant species. Warming simultaneously increased the mycorrhizal fungal diversity and richness of invasive Calluna vulgaris and reduced that of the native host plant species. In chapter 3, using network analyses from the bipartite package of R, I constructed 32 plant mycorrhizal networks of the plot sampled and calculated metrics pertaining to properties of network structure and robustness at the whole network, trophic-level and species/mycorrhizal fungal OTU scales. I then examined the responses of these metrics to the WaRM treatments. I found that warming significantly reduced the robustness of native plant-mycorrhizal networks and increased the strength of the interaction network associated with invasive C. vulgaris. The removal of C. vulgaris had a secondary effect on how mycorrhizal fungal compositions and interaction networks responded to warming. As a generalist C. vulgaris was critical for the ongoing diversity of ericoid mycorrhizal fungi, particularly under warming. However, C. vulgaris simultaneously suppressed the mycorrhizal interaction- networks of native plant species, which further fragmented under warming. I conclude that warming and the presence of invasive C. vulgaris synergistically reduced and decentralised the native plant-mycorrhizal interactions within the network. In summary, my thesis demonstrates the below-ground interactions of alpine plant communities are destabilising under multiple interacting drivers of global environmental change.