Private trees contribute uniquely to urban forest diversity, structure and service-based traits

Abstract

The urban forest provides our communities with a host of benefits through the delivery of ecosystem services. To properly quantify and sustain these benefits, we require a strong baseline understanding of forest structure and diversity. To date, fine-scale work considering urban forest diversity and ecosystem services has often been limited to trees on public land, considering only one or two green space types. However, the governance of urban green spaces means tree species composition is influenced by management decisions at various levels, including by institutions, municipalities, and individual landowners responsible for their care. Using a mixed-method approach combining a traditional field-inventory and community science project, we inventoried urban trees in the residential neighbourhood of Notre-Dame-de-Grȃce, Montreal. We assessed how tree diversity, composition and structure varies across multiple green space types in the public and private domain (parks, institutions, street rights of way and private yards) at multiple scales. We assessed how service-based traits – traits capturing aspects of plant form and functions that urban residents find beneficial – differed across green space types, with implications for the distribution of ecosystem services across the urban landscape. Green space types displayed meaningful differences in tree diversity, structure, and service-based traits. For example, the inclusion of private trees contributed an additional 52 species (>30% of total species) not found in the local public tree inventory. Trees on private land also tended to be smaller than those in the public domain. Beyond patterns of tree richness, size, and abundance we also observed differences in the composition of tree species and service-based traits at site-scales, particularly between street rights-of way and private yards. While species composition varied considerably across street blocks, blocks were very similar to one another in terms of mean service-based traits. Contrastingly, while species composition was similar from yard to yard, yards differed significantly in mean service-based trait values. Our work emphasises that public tree inventories are unlikely to be fully representative of urban forest composition, structure, and benefits, with implications for urban forest management at larger spatial scales.