Urban forest fragments are highly multifunctional and provide many ecosystem services to people. However, the expansion and densification of cities is threatening this particular type of urban forest, resulting in further fragmentation, degradation and a change in forest function. Understanding how urbanization affects urban forests fragments and the multiple ecosystem services they provide is important for the creation of livable cities. We used a 7 km long urban-to-rural gradient in Maple Ridge, Canada, to explore relationships between urban forest structure, urbanization, surrounding landscape structure, and the supply of eight different ecosystem services, including merchantable timber, carbon storage, flood control, food (berries), invasive species control, cultural use, and habitat provision. Along this transect we identified all forest fragments greater than 0.2 ha in size on public land and randomly chose 21 plots to sample, stratified by distance from the city center. Adapting protocols from i-Tree Eco, we collected data on live and dead trees, understory shrubs, cedar bark quality, and fine/coarse woody debris, and used these indicators to estimate the supply of our eight ecosystem services. We then investigated relationships between ecosystem service provision, distance along our gradient and landscape structure using generalized additive models and model averaging.
Results/Conclusions
We found that the supply of multiple ecosystem services was higher at the rural end of our gradient and that forest fragments spaced closer together showed strong negative associations with most services. In particular, our models predict a five times increase in merchantable timber volume (p = 0.005) along our urban-rural transect and a tripling of stored carbon (p = 0.014). Conversely, as forest fragments became increasingly clumped across the landscape, we saw negative impacts on timber (p = 0.004), carbon (p = 0.013), and cedar bark (p = 0.010). Crucially, forest fragment size had very weak effects on most services (p > 0.05), emphasizing the importance of conserving small urban forest fragments with large trees. We also identified key tree species, including Piceaglauca and Thuja plicata, that could be retained or planted in urban forests to maximize individual services and ecosystem service multifunctionality. Our results highlight the potential for strategically managing urban landscapes to maintain forest fragments and enhance their ability to provide ecosystem services and multifunctionality. Our findings are relevant to inform urban conservation and greening strategies in cities around the world that are currently expanding into surrounding forests or losing greenspace through densification.
