Abstract
Many interconnected factors influence plant composition in forest ecosystems and ultimately the services that forests provide. Understanding native plant diversity in exotic forests is a globally important issue because exotic forestry plantations are expanding, and they may provide lower habitat value than native forests. Additionally, disturbance regimes such as forest management can influence plant composition because of their effect on environmental conditions and niche availability. Transitional forestry is an approach where exotic forests are transitioned to native forest and may help improved ecosystem health in degraded areas. This thesis aims to assess factors that influence plant diversity in native and exotic forests in eastern Otago, Aotearoa New Zealand. It was predicted that native forest would support higher levels of understorey diversity.
To quantify understorey plant diversity, surveys of 18 forest plots with either native or exotic canopy species were undertaken at seven sites. In order to control for proximity to seed sources, all exotic plots were within 300 m of native forest. The abundance of and diversity of non-vascular and vascular plant species were sampled using quadrats within each 125 m x 60 m plot. From 2308 individual identifications, 254 unique species across 540 quadrats, within 18 plots were found. Species consisted of 132 vascular species, and 120 non-vascular species, which were found to be positively correlated in species richness, Shannon diversity and Pielou’s evenness using a Pearson’s correlation analysis. Surprisingly, there was no difference in total species richness between the native and exotic canopy types, however there was consistently higher native regeneration under both canopy types, and at one site no exotic understorey species were found. Additionally, there was higher native plant diversity under native canopies than under exotic canopies, with no difference in exotic understorey species between canopy types. When assessing the impact of management approaches in exotic forests, there was significantly higher diversity of both non-vascular and vascular plant communities, in plots with higher intensity intermittent disturbance compared to plots with low intensity but more continuous disturbance. Additionally, when assessing substrate specialisation in non-vascular species, there was higher diversity of lignicolous species (occurring on dead wood) in intermittently managed forest compared to continuously managed forest because of higher substrate availability of coarse woody debris. A range of environmental and soil variables were measured in each plot and the only clear effect was that increasing soil sodium was negatively associated with non-vascular species diversity.
Overall, this thesis provides valuable information on plant diversity and composition in native and exotic forests in Otago, Aotearoa New Zealand. It provides insights into non-vascular and vascular plant correlation and their responses in forest systems. Furthermore, this study offers insights into how forestry management can influence native plant regeneration when there is a native seed source within 300 m, and the potential for transitional forestry approaches to improve biodiversity outcomes in the region.