Aboveground-belowground Ecological Linkages in Exotic Plant Invasion: Thymus vulgaris L. in Central Otago, New Zealand

Abstract

The objective of this thesis was to improve our understanding of the processes and mechanisms underlying the successful invasion of Thymus vulgaris L. in Central Otago, New Zealand by carrying out aboveground and belowground studies across multiple spatial scales. I examined the effects of invasion on ecosystem nutrient cycling, soil properties, soil bacterial community structure and function, and plant species diversity among sites, at the edge of invaded plant communities and between north- and south-facing slopes. I also evaluated the role of thyme’s foliar chemical diversity in the invasion process.

The effects of thyme on ecosystem nutrient cycling processes were investigated using nitrogen and carbon stable isotope analysis of thyme leaves and soil. Lower soil δ15N inside thyme stands may indicate that thyme impacts nitrogen cycling or it could reflect the effects of increased animal grazing on the adjacent uninvaded communities. Thyme leaves were significantly 15N depleted compared to soil. This finding requires further studies to evaluate how thyme is accessing nitrogen and the role this may play in thyme’s invasive success in the nitrogen-poor soils of Central Otago.

There were significant changes in soil properties associated with thyme. Soil moisture, ammonium and organic phosphorus increased from inside to outside of thyme stands whilst sand content decreased from inside to outside thyme stands. There were no significant invasion impacts on soil bacterial community structure or function which may be due to the limitation of the methods employed. In general, site differences were the major determinant of variability in soil properties and soil bacterial diversity.

There was no evidence of a relationship between aspect or invasion edge and patterns of variation of thyme’s essential oil chemistry. However, higher p-cymene levels in thyme leaves in thyme’s introduced range here in Central Otago compared to its native habitat suggest a possible role for thyme’s essential oil chemistry variation in invasion.

Univariate and multivariate analyses revealed greater plant species diversity at the edge and outside of thyme stands than inside of thyme stands in terms of plant community composition, abundance and patterns of spatial variability. Abundances of exotic forbs, exotic grasses, exotic shrubs and native shrubs showed the greatest increases from inside to outside of invaded thyme stands. Species richness was lower inside of thyme stands than at the edge, but there were no significant increases from the edge to outside thyme.

I also examined the effects of soil from under thyme on the germination rate and seedling growth of thyme-associated exotic and native grass species. Native grasses had lower germination rates than exotic grasses on soil from under thyme as well as on the control soil. Competition reduced seedling biomass of exotic and native grasses grown in soil from under thyme as well as control soil. Species differences were the major determinants of variation in germination rates and seedling growth.

Using an integrated aboveground-belowground approach this study has shown the direction of ecological change associated with the invasion of Thymus vulgaris is context-dependent. Although the invasion of thyme significantly affects plant species diversity and is associated with changes in ecosystem nutrient cycling and soil properties, site differences and the interaction of site and aspect explain the most variability in plant assemblages, ecosystem nutrient cycling, soil properties and soil bacterial community structure and function inside, at the edge and outside of thyme stands.