The aim of my thesis was to investigate the connection between invasive plants and the resident plant community via soil modification. Pines were chosen as the primary model invasive species for my project due to the dramatic transformations associated with pine invasions and local significance in Aotearoa | New Zealand.

I began my research by leveraging soils from a previous experiment which involved various plant communities grown in the same steam sterilised soil. This provided known soil legacies to test various legacy factors associated with the previous plant communities and how those would affect the growth of future plants. For chapter two, I grew locally-relevant invasive species from three different functional groups, Pinus contorta (tree), Cytisus scoparius (shrub), and Holcus lanatus (grass) in a greenhouse experiment with these legacy soils. I found that legacies with a high proportion of exotic plants or with the presence of pine resulted in plants with the largest biomass. A potential biological mechanism was investigated by scoring nodules on Cytisus and mycorrhization on Pinus, which did not show a measurable effect across treatments. There were significant trends on fungal DNA sequences from pine seedling root samples, including showing that a pine legacy decreased the fungal community diversity while increasing pine seedling growth.

Although the soil legacies for chapter two included invasive pine species in the legacy community, at most it was two seedlings in each growth phase, which might not have the same impact as soil from established adult trees or from various degrees of invasion. To address this issue, I sampled soil from along a pine invasional gradient, as measured by pine dominance. Also, I tested whether plant responses to the legacy soils differed if plants were grown individually or in communities; a closer representation of the natural system. This greenhouse study involved a community pot per sampling plot as well as an individual pot for each species represented in the community assay. An increasing pine legacy was found to be beneficial to most plants, but in a community context any benefit to native plants was obscured by the strong competitive fitness of many exotic species.

Chapters two and three indicated that there was a biological effect in pine legacy soils, and that there was a reciprocal positive interaction between grasses and pines. Fungal endophytes were chosen as a biological indicator as they are associated with their host plant’s health and also will be affected by changes in resident soil. I collected grass roots (both exotic and native species) from paired plots (a pine invasion and a nearby uninvaded grassland). The endophytic fungal communities from exotic grasses and pine invasion sites had a greater abundance of potential pathogens, while sharing many generalists. This indicates a potential for spill over from exotic to native grasses, and that pine invasion soils might be a reservoir for pathogens. Exotic grass species might be better equipped to deal with these pathogens due to previous experience, compared to a lack of past interactions with native grass species, particularly if natives are under competitive stress.

By demonstrating the effects of invasive pine legacy and various community legacy effects, my research could be helpful to land managers looking to control invasive pine spread. My thesis showed a pine soil legacy can be particularly beneficial to other exotic invasive species, which, in turn, can facilitate future invasions.