Native and exotic woody vegetation communities in domestic gardens in relation to social and environmental factors

Vegetation in private gardens contributes significantly to plant species richness and vegetation volume across urban areas. Drivers of garden diversity and structure are complex, reflecting the diversity of social, cultural, and socioeconomic characteristics of the householders who manage their gardens, as well as their predecessors. Here we investigate the woodiness of gardens, and focus on (1) the prevalence of native versus exotic woody plants and (2) the influence of characteristics of garden owners, the gardens, and their proximity to neighborhood green spaces to identify the degree to which these factors explain patterns in native and exotic woody species communities in entire (back and front) gardens in southern temperate New Zealand. We found few consistent patterns in structure in woody species community composition. Outlying gardens were characterized by low species richness and abundance. Thirty-seven species commonly occurred across most gardens: most of these were exotic. Twelve native species were common throughout most gardens. There was significant but weak matching to social and environmental variables: vegetated area, species knowledge, and education explained pattern in native communities, whereas vegetated area, species knowledge, and householder age explained variation in exotic communities. Native trees >5 m tall occurred in only 58% of gardens. Tall tree density was 10/ha, and 29% of gardens lacked any trees >5 m. Tree presence was weakly (positively) associated with extent and proximity of neighborhood green space. We suggest that the legacy of previous owners' gardening practices is important to consider when identifying drivers of garden plant community structure.