Effects of flow regime on the distribution, richness and abundance of alien plants in braided rivers of New Zealand

Abstract

Braided river floodplains in the Canterbury Plains of New Zealand are classified as threatened ecosystems currently undergoing dramatic changes, including adjacent land-use intensification and increased water abstraction. In addition, both the regional environmental management authority and conservation NGOs are concerned by the invasion of these ecosystems by alien plants that can reduce biodiversity and alter ecosystem processes. However, whether these plant invasions are driven by flow regime has yet to be elucidated. In this thesis I address how we study the effects of hydrological flows on the ecology of ecosystems by developing a framework for setting up flow-gradient analyses. I then evaluate the three aspects of invasion organised from most general to most specific. First, I address whether flow regime, land-cover, climate and floodplain factors are associated with alien versus native cover and richness. I then ask whether groups of alien species with the same regenerative and growth traits respond to floods and hydrologic drought similarly. Finally, I take a targeted approach to determine whether flow and other drivers are associated with three woody legume invaders in the system. Using flow data from 19 river reaches, a regional multi-scale plant survey and data from the seed bank, I found that alien and native floras were driven by completely different drivers with aliens associated with flow variability and floodplain variation while natives were associated with climate and land-cover. The most prevalent species were woody and perennial herbaceous legumes, while annual and bienniel weeds had the greatest combined frequency across all the sampled rivers. Annual and biennial weeds increased in prevalence as winter flow variability increased, whereas perennial herbaceous species were more prevalent in rivers with a greater number of low flow days. Finally, flow regime played only a minor role in predicting the distribution and cover of the three most frequent woody legumes in the system. However, local patterns in the distribution of these three species suggested that decreases in mean flows coupled with flow stabilisation could allow them to establish across more of the floodplain than they already occupy. These results highlight a few key lessons. Flow-gradient studies can be used to study flow-ecology relationships if applied rigorously. Alien and native floras can be driven by completely different variables in the same ecosystem, thus management against aliens cannot presume management for natives. Finally, changes in river flows have the potential to influence aspects of alien invasion in the ecosystems and should be considered in the context of changing climate and resource use patterns in Canterbury. Future work on understanding invasion in the ecosystem could benefit from monitoring alien species responses to flow events through time and addressing whether invasive legumes drive invasional meltdown of the ecosystem.