More than green: Tree structure and biodiversity patterns differ across canopy change regimes in Baltimore’s urban forest

Abstract

Urban tree canopy is a vital component of modern cities, and many cities aspire to increase canopy cover to bolster ecosystem services. However, canopy-based ecosystem services are disproportionately provided by old, large-statured trees. Legacies of where trees have been maintained are evident in urban landscapes and loss of these older trees is not easily negated by current planting efforts. Furthermore, tree species composition varies considerably across the landscape and may determine present canopy longevity, structure, and contribution to ecosystem amenities or disamenities. In this study, we leverage long-term data from 188 i-Tree Eco plots in Baltimore, Maryland, USA, to examine how canopy changes across 10 years are related to tree structure and biodiversity patterns. We delineated five distinct clusters based on starting canopy conditions (high or low) and the trajectory of canopy change (gain or loss) and compared how tree structure, community composition, and turnover varied among clusters and across time. We found distinct patterns of tree structure and biodiversity across clusters that can help clarify links between land use, community composition, and ecosystem function in the urban forest. Residential land use typically had fewer and larger trees than forests or parks and residential clusters supported high biodiversity. Regardless of land use, areas with increasing canopy were dominated by native species while areas experiencing canopy declines were dominated by non-native species. In all, these findings help untangle the complexities of urban forest change and clarify different canopy vulnerabilities and assets across an urban landscape.