Assessing the ecological impact of urban greening: A case study of roadside planting in Wellington, New Zealand

Roadside reserves in Wellington, New Zealand have been the target of a government-led, community-implemented urban greening initiative for the past 25 years. Prior studies of urban greening have shown numerous benefits to neighbourhoods and communities through increased engagement and stewardship, yet there remains a need for research into the ecological effects these programmes have on individual urban landscapes. This research conducted site surveys to determine the variation in ecological functioning and biodiversity within 36 reserves involved in the Wellington Free Plants Programme (FPP). These measures were compared to historical planting data for each site retrieved from council records. Candidate models were constructed based on novel and classical ecological theory, which sought to explain observed variation between physical and ecological measures across study sites and the relationship between these variables and biodiversity. Sites were small with an area ranging from 5.9m² to 246.5m² (mean = 37.8 ±49.5m²), and biodiversity levels (assessed using a Shannon-Weiner Index) ranged from 0.1 to 2.9 (mean = 2.1 ±0.7). The top performing candidate models to predict biodiversity included area, shape, and seedbank density. An examination of the effect of varying urban greening efforts across these sites utilised a multivariate analysis which included measures of ecological functioning, biodiversity, the number of years a site had been planted, and the number of individual plants provided over those years. A significant negative relationship was found between site disturbance and the number of planting years (F33.1 = 4.092, p = .051) while a somewhat significant positive relationship was found between biodiversity and the number of individual plants provided (F33,1 =3.536 , p = .069). These results indicate that current urban greening efforts contribute to the ecological health of roadside reserves and that the patterns and processes governing the biological composition of these reserves may be partially explained with traditional ecological theory.