Temporal and spatial variation in garden and street trees in six eastern Australian cities

This Australian study analyzed spatial and temporal variation in the presence and density of street and garden trees. The number of trees in gardens and in adjacent areas (street trees) in 53 suburbs of six cities was determined from historical (1961) and contemporary aerial imagery (2006). The sampled suburbs were stratified into three median income classes and two education levels (with or without tertiary education) for the two time periods in question. Imagery was also used to estimate plantable area (land not covered by buildings or large impervious surfaces such as tennis courts or swimming pools) and, for 2006, to classify trees into four taxonomic groups: eucalypts, pines and pine relatives, evergreen angiosperms, and deciduous angiosperms. Between 1961 and 2006, there was a strong growth in the number and density of garden and street trees across all cities. In 2006, more than 75% of all sampled properties had at least one garden tree. High frequencies and densities of trees were correlated with (1) low levels of unemployment, (2) high proportions of rental dwellings, (3) high levels of tertiary education, and (4) low proportions of Australia-born residents. The most common taxonomic group differed among the cities and could be explained in terms of climate (water scarcity and the need for temperature moderation), native vegetation, and garden size, among others. The authors conclude that doubling the number of street trees is feasible under current circumstances, but “significant increases in garden tree numbers would depend on increasing the income and higher education attainment of lower socioeconomic groups.”