Vegetation of roadside cuttings and its effect on erosion in the Port Hills of Christchurch, New Zealand : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

Soil erosion is a concerning issue in New Zealand that threatens the environment, people, and the economy. Sediment runoff caused by erosion enters waterways and presents numerous ecological and health issues. Roadside cuttings are a visible source of erosion in urban and rural landscapes and, with little space available in the road corridor for treatment, sediment runoff often flows directly to waterways and harbours, where it accumulates. The revegetation of bare soils on roadsides does not only have the potential to reduce erosion and sediment runoff but also offers an opportunity to increase the native vegetative cover and provides other ecosystem services, such as nutrient cycling, which can improve soil health in the long run. This study examines the vegetation of roadside cuttings in the Port Hills of Christchurch to understand what influences it, and assess how it affects the ability of the soil to withstand erosion. Furthermore, an assessment of erosion control products was also carried out to identify the best product to achieve the greatest amount of plant cover in loess roadside cuttings. Roadside cuttings are dominated by naturalised grasses and a higher proportion of native cover is found mostly in well-vegetated areas. This shows that there is a need to optimise the vegetation cover in roadside cuttings to further increase and preserve the native plant cover. The results of this study show that plant cover in roadside cuttings is important in resisting erosion in sedimentary soils. The soil aggregate stability is high when the corresponding levels of vegetation cover are also high (specifically, a 70–100% cover). Soil aggregate stability can determine the ability of soil to retain its structure or resist different environmental stresses, like erosion. Soil aggregate stability is also more significant in grasslands than in shrubland since grasslands tend to have higher organic matter, cation exchange, and plant available nitrogen, based on the soil analysis. Species diversity and evenness are also higher in grasslands, and this could be driven by the higher level of nutrients found in grasslands. However, there are many challenges to consider when conducting revegetation in roadside cuttings. Roadside cuttings in the Port Hills have a low level of phosphorus (P), organic matter, and cation exchange capacity. These low level of nutrients, which are essential to plants, could limit their growth. Furthermore, soil moisture varies in different areas of the slope and this can also affect the growth performance of plants. Elevation, longitude, and the interaction between the slope and aggregate stability strongly influence plant communities in roadside cuttings. Other environmental variables, such as slope angle, aspect, and surrounding vegetation, should also be considered in the restoration of roadside cuttings. Erosion control products, such as coir fibre blankets (jute), wool blankets, and hydromulches, are effective in reducing surface runoff on bare slopes. In successfully establishing higher vegetation cover and higher species abundance, hydromulch on an organic base with a seed mixture of Festuca rubra, Agrostis capillaris, Poa annua, and Trifolium pratense, which is a product of PGG Wrightson Seeds, was most effective. The cover and survival of native plug plants planted on roadside cuttings were higher on rolled erosion control products such as jute, wool, and the combination of jute and hydromulch (PGG), since these products had less plant competition from naturalised herbaceous, grasses, and woody vegetation.