Impact of Climate Warming and Plant Invasion on the Soil Microbial Community in Alpine Zones

Human activities have rapidly increased atmospheric concentrations of greenhouse gasses such as carbon dioxide, which drives global climate warming. Alpine ecosystems, like other cold biomes, are particularly sensitive to warming. Alpine ecosystems form isolated pockets of both plant and animal diversity, creating unique biodiversity hotspots globally. Climate warming causes shifts in alpine plant community composition and facilitates the establishment of numerous invasive plant species. Most of the Earth’s biodiversity is found belowground, and while there is increasing knowledge on this large amount of biodiversity, there is still limited understanding on how it will be affected by global change drivers or the subsequent effects on ecosystem functions. Plant-mycorrhiza associations are particularly sensitive to changes in both temperature and the plant community. In Chapter 2, I investigate the effects of an invasive species (Common Heather, Calluna vulgaris) on the mycorrhiza of a native tussock grass (Red tussock, Chionochloa rubra) and the associated soil microbial community in Tongariro National Park, New Zealand. C. rubra and C. vulgaris have a mycorrhizal mismatch, associating with arbuscular mycorrhizal fungi (AMF) and ericoid mycorrhizal fungi (ErM), respectively. Fifteen sites were established in locations that varied in C. vulgaris density. At each site, five circular plots were established. Each plot was 2 meters in diameter and centred on an individual C. rubra tussock. I recorded percent cover of vegetation in each plot and collected soil cores for the quantification of mycorrhiza in roots and soils (75 plots total). I separated C. rubra roots and soils from both cores per plot. I cleared and stained C. rubra roots and quantified AMF structures using a magnified intersection method. The biomass of the soil microbial community was assessed by fatty acid methyl ester analysis (FAME), using the phospholipid fatty acid PLFA fraction to assess microbial biomass and the proportions of different microbial groups (including ericoid mycorrhizal fungi), and neutral lipid fatty acid (NLFA) fraction to assess AMF biomass. I found that higher densities of C. vulgaris resulted in lower AM colonisation in the native C. rubra roots, reduced the diameter of C. rubra plants and reduced overall plant cover of neighbouring plant species. However, the invasion of C. vulgaris did not cause significant shifts in the general microbial groups. In Chapter 3, I investigate the effects of experimental warming and changes in plant community composition, through dominant species loss, on alpine soil microbial communities, globally. I utilize the Warming and Removal in Mountains (WaRM) network, which applies a 2 × 2 factorial warming × plant species removal experiment at both low and high elevations in 9 countries. I received soil samples from each site and used PLFA analysis to characterise the microbial response to treatments. Both warming and removal treatments had no significant effect on the overall microbial biomass. Elevation had a marginally significant effect, suggesting high elevational sites had greater microbial biomass in comparison to the low elevation sites. I found that the removal of dominant plant species in alpine regions had a significant effect on the abundance of both fungi and bacteria in opposing directions, while warming had no significant effects. Elevation had a significant effect on AMF and gram-negative bacteria, showing that incorporating and understanding elevational gradients is important when studying the alpine soil microbial community. In summary, my thesis demonstrates that changes to plant community composition, elicited by the addition or removal of dominant plant species, have significant effects on alpine soil microbial communities, which can exceed the direct effects of warming. Therefore, understanding the effects of shifting alpine plant communities, including an increase in invasive plant species, is crucial in determining the impacts of global change for alpine ecosystems.