OOS 49-4 - Urban forest of tomorrow: functional perspective on large-scale woodland regeneration dynamics in Toronto, Canada

Background/Question/Methods
In cities, naturally regenerating forest fragments represent important micro-climate oases for both citizens and wildlife, and we rely on them to provide ecosystem services like carbon capture. The capacity of woodlands to perform these functions far into the future will depend both on the traits that are successful in urban conditions and, given the balance of winners and losers, on the diversity that will be maintained. It is currently an open question whether urban woodlands are regenerating following expected successional trends.
We investigated the impact of urban conditions, including urban climate, on the regeneration dynamics of urban woodlands across the Greater Toronto Area (Canada). Focusing on canopy-forming species, we ask (1) whether the composition of regeneration layers departs from the canopies that they will replace. We then ask (2) whether functional change is associated with the urban climate and (3), whether the combined effects of environmental filters and modified colonization processes is leading to changes in the functional diversity of woodlands that could affect their resilience to future environmental change. We leverage a large-scale database of vegetation inventories collected by local management authorities for over 10 000 urban woodlands, and investigate resulting large-scale functional trends using linear mixed models.

Results/Conclusions
We found strong but trait-specific directional shift in the regeneration layer of urban woodlands. While some traits demonstrated no net change (Leaf N content), other traits closely related to specific ecosystem services like wood density, specific leaf area (SLA) and canopy height showed marked differences between canopy and regeneration layers. These results suggest that future urban woodlands may provide a different mix of ecosystem services. Because these shifts were partly mediated by the local urban climate, our results raise the question whether these changes might signal a greater adaptation of woodlands to city life. We found no evidence of an overall loss or increase in functional diversity at the local scale. While these results suggest that urban woodlands might not experience any overall loss of resilience, we also found evidence of functional homogenisation along some functional dimensions. As forests adapt to the urban environment, there is reason to expect a shift in the composition, structure, and diversity of forest fragments. With implications for resilience, biodiversity and the provision of ecosystem services far into the future, our result provide important context for the development of sustainable urban forestry practices in an uncertain future.