Urban trees as nature-based solutions: tree-related microhabitat diversity and management effects in Colletta Park (Turin, Italy)

Urban biodiversity is increasingly threatened by land-use change, habitat fragmentation, and intensive management practices. Within this context, urban trees represent a key nature-based solution (NbS) that simultaneously supports biodiversity, improves ecosystem functioning, and contributes to human well-being. Among the structural features provided by trees, tree-related microhabitats (e.g., cavities, deadwood, bark features, and epiphytic substrates), also called TreMs, are crucial for hosting a wide range of organisms, yet they remain underinvestigated in urban environments. This study examines the distribution and drivers of TreMs in an urban park ecosystem, focusing on Parco Colletta in Turin (NW Italy). A total of 423 trees were surveyed from a population of approximately 1,700 individuals. For each tree, we collected information on species identity, functional group (conifer versus broadleaf), origin (native versus non-native), diameter, height, planting configuration (groups, rows, or isolated trees), management intensity, and presence and type of TreMs. Overall, 97% of the surveyed trees hosted at least one TreM, with a total of 1,194 structures identified and an average of three TreM types per tree (range: 0–9). The most common types were dead branches, bark microsoil, and fork split at the intersection. Broadleaf species, particularly Fagus sylvatica L., Acer saccharinum L., and Quercus rubra L., exhibited the highest abundance of TreMs. Trees with low management intensity and standing dead individuals showed substantially higher TreM richness, highlighting the influence of management practices on habitat availability in urban environments. While several variables impacted TreM presence in univariate analyses, diameter and management intensity stood out as the primary explanatory factors. These findings highlight the value of TreMs as effective structural indicators of urban biodiversity and NbS performance. Incorporating biodiversity-focused management into urban green infrastructure planning can enhance the ecological value and resilience of urban ecosystems under ongoing environmental change.