Species clustering, climate effects, and introduced species in 5 million city trees across 63 US cities

Abstract

Sustainable cities depend on urban forests. City trees-pillars of urban forests - improve our health, clean the air, store CO2, and cool local temperatures. Comparatively less is known about city tree communities as ecosystems, particularly regarding spatial composition, species diversity, tree health, and the abundance of introduced species. Here, we assembled and standardized a new dataset of N=5,660,237 trees from 63 of the largest US cities with detailed information on location, health, species, and whether a species is introduced or naturally occurring (i.e., 'native'). We further designed new tools to analyze spatial clustering and the abundance of introduced species. We show that trees significantly cluster by species in 98% of cities, potentially increasing pest vulnerability (even in species-diverse cities). Further, introduced species significantly homogenize tree communities across cities, while naturally occurring trees (i.e., 'native' trees) comprise 0.51%-87.3% (median=45.6%) of city tree populations. Introduced species are more common in drier cities, and climate also shapes tree species diversity across urban forests. Parks have greater tree species diversity than urban settings. Compared to past work which focused on canopy cover and species richness, we show the importance of analyzing spatial composition and introduced species in urban ecosystems (and we develop new tools and datasets to do so). Future work could analyze city trees and socio-demographic variables or bird, insect, and plant diversity (e.g., from citizen-science initiatives). With these tools, we may evaluate existing city trees in new, nuanced ways and design future plantings to maximize resistance to pests and climate change. We depend on city trees.

Determinants of spatial distribution of trees outside forests along urban-rural gradients: A review

“…However, these trees exhibit higher diversity and better conditions (Chambers-Ostler et al, 2023). Hence, further research on trees outside forests and their relationship with socioeconomic and demographic factors is necessary to understand dense urban morphologies and enable inter-city comparisons (Dobbs et al, 2013;McCoy et al, 2022;Sikuzani et al, 2020).…”

Responses and Post-Recovery of Physiological Traits after Drought–Heatwave Combined Event in 12 Urban Woody Species

“…With the rapid development of urbanization, urban environmental conditions, such as impervious surfaces and the urban heat island effect, can locally exacerbate destructive extreme climate events [18][19][20][21][22]. Woody species growing in urban areas are the main way to maintain urban ecological services [23,24], which form green spaces in cities, increase the elasticity of the urban ecosystem, mitigate flooding and improve the quality of the urban environment [25][26][27][28]. In the context of climate change, urban woody plants are facing increasingly severe pressure and harm [11,29,30].…”