Soil legacy and fungal community responses to Cytisus scoparius invasion. (2020)
Type of ContentTheses / Dissertations
Thesis DisciplineBiological Sciences
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
AuthorsWainer, Ralphshow all
The goal of my thesis was to study the effects of soil under various levels of invasive Cytisus scoparius (Scotch broom) and then examine whether the unique soil legacy of C. scoparius was contingent on how C. scoparius shaped soil fungal communities.
I began my research by studying the effect of the soil legacy of C. scoparius in a controlled environment (via a greenhouse experiment; Chapter 2). Knowing the effect of the soil legacy of C. scoparius under regulated conditions, I then undertook a field survey (via a natural experiment; Chapter 3), in which I systematically recorded changes in fungal community composition across a natural density gradient of C. scoparius invasion. I subsequently investigated whether the environmental DNA (eDNA) metabarcoding techniques I applied throughout my natural survey could be optimised for future researchers (via a methodological experiment; Chapter 4). Lastly, I analysed how different fungal communities found near C. scoparius may underlie the results of my greenhouse experiment (via mixed-effect modelling; Chapter 5).
In Chapter 2, I found contrary to my hypothesis that the effects of soil extracted under various levels of C. scoparius invasion favoured the growth of native New Zealand plants over its own taxonomic family in a controlled greenhouse environment. Given that the predominantly positive soil legacy of C. scoparius could only be partly attributed to soil chemical traits, microbial effects likely played an underlying role in the invasion success of C. scoparius. In Chapter 3, I found that fungal diversity in soil under C. scoparius was unexpectedly higher than in grassland uninvaded by C. scoparius, and that C. scoparius invasion resulted in increased homogenisation of certain fungal groups within the overall soil fungal community. My results suggested that coalescence between previously separated fungal communities may have occurred due to C. scoparius invasion. Apart from C. scoparius having a definite effect on soil fungal communities, it is possible that the soil fungal communities themselves might contribute to the shrub’s invasiveness, which I further tested in a field-experiment (Appendix E). In Chapter 4, I present the pitfalls and benefits of eDNA pooling, identifying a fungal taxon-wide bias in the proportional abundance of fungi in pooled eDNA samples. I demonstrate how rarer fungi remain increasingly unaccounted for with increased degrees of pooling, yet also show how pooling may benefit researchers who wish to study the larger-scale effect of environmental drivers (e.g., anthropogenic effects, invasive species impacts). In Chapter 5, I show how increased arbuscular mycorrhizal richness found in more homogenised soil communities (studied in Chapter 1) were partly responsible for the generally positive soil legacy of C. scoparius, especially for exotic Fabaceae which can probably benefit more from arbuscular mycorrhizal fungi-facilitated P enrichment due to their ability to fix N.
By demonstrating how changes in fungal communities caused by an invasive N-fixing plant may impact plant growth and nutrient acquisition, the results of my thesis highlight the importance of incorporating fungal community composition in soil legacy studies. Although biodiversity losses of plants and other organisms following invasion are common, I show how soil fungal communities may be considered an exception to the rule. I highlight the importance of systematic sample processing and encourage the use of eDNA metabarcoding techniques to better understand how changes in soil fungal communities may possibly benefit native plants in ecological restoration projects or adversely underlie an exotic shrub’s invasiveness.