A city‐scale assessment reveals that native forest types and overstory species dominate New York City forests

Abstract

Cities are increasingly focused on expanding tree canopy cover as a means to improve the urban environment by, for example, reducing heat island effects, promoting better air quality, and protecting local habitat. The majority of efforts to expand canopy cover focus on planting street trees or on planting native tree species and removing nonnatives in natural areas through reforestation. Yet many urban canopy assessments conducted at the city‐scale reveal co‐dominance by nonnative trees, fueling debates about the value of urban forests and native‐specific management targets. In contrast, assessments within cities at site or park scales find that some urban forest stands harbor predominantly native biodiversity. To resolve this apparent dichotomy in findings, about the extent to which urban forests are native dominated, between the city‐scale canopy and site‐level assessments, we measure forest structure and composition in 1,124 plots across 53 parks in New York City's 2,497 ha of natural area forest. That is, we assess urban forests at the city‐scale and deliberately omit sampling trees existing outside of forest stands but which are enumerated in citywide canopy assessments. We find that on average forest stand canopy is comprised of 82% native species in New York City forests, suggesting that conclusions that the urban canopy is co‐dominated by nonnatives likely results from predominantly sampling street trees in prior city‐scale assessments. However, native tree species’ proportion declines to 75% and 53% in the midstory and understory, respectively, suggesting potential threats to the future native dominance of urban forest canopies. Furthermore, we find that out of 57 unique forest types in New York City, the majority of stands (81%) are a native type. We find that stand structure in urban forest stands is more similar to rural forests in New York State than to stand structure reported for prior assessments of the urban canopy at the city scale. Our results suggest the need to measure urban forest stands apart from the entire urban canopy. Doing so will ensure that city‐scale assessments return data that align with conservation policy and management strategies that focus on maintaining and growing native urban forests rather than individual trees.

Emerging Urban Forests: Opportunities for Promoting the Wild Side of the Urban Green Infrastructure

“…Comparing results from Berlin with other studies on successional forests suggests that cities in different biogeographic regions share some dominant tree species, including A. platanoides, R. pseudoacacia, and A. altissima, as shown for a Mediterranean city [45] and for temperate North American cities [13,46,64,83]. Whether the shared dominance of the same tree species in different cities will lead to a biotic homogenization of urban successional forests deserves further comparative studies on a global scale.…”

“…Communities of these species have been described since the 1960s for Berlin and beyond [94,95], starting with studies on early successional stages on post-war rubble soils [137] and later covering a broader range of sites, e.g., within transportation corridors (see early synthesis in [40,82]. While many previous studies on urban successional forests largely relied on measures of abundance, e.g., [45,79,138], but see [46,64], our analysis adds insights into the relative importance of different tree species as dominants of successional forests in terms of covered area at the city scale.…”

“…Although rewilding is likely a common process in many urban greenspaces, resulting wild woods have been mostly reported for old cemeteries in Europe, e.g., [60][61][62][63]. As an exception, Pregitzer et al [64] recently reported successional forests in urban parks in New York City. In cities, emerging forests can result from different development paths.…”

Defining and assessing urban forests to inform management and policy

“…Urban forest assessments are the basis upon which their benefits are quantified, policy is determined, and management is implemented (McPherson 1992, Brack 2002, Cowett and Bassuk 2017. However, methods for urban forest assessment vary markedly in scale and focus (Wilson et al 2004, Corona 2016, yielding contradictory conclusions about the status of forest biodiversity and invasive species (Pregitzer et al 2019). For cities to sustain and enhance urban forests, assessments that accurately characterize the urban forest to inform policy and management are needed.…”

“…For example, some of the larger urban forested natural areas include Seward Park (121 ha) in Seattle (US), Richmond Park (955 ha) in London (UK), Metropolitan Natural Park (232 ha) in Panama City (Panama), and Stanley Park (405 ha) in Vancouver (Canada). In New York City (US) forested natural areas are made up of mostly native tree species (82%) and are similar in structure to forest types found in rural areas of New York State (Pregitzer et al 2019). Although forest patches in cities vary in size, stand age, and species composition, trees in these stands experience the same ecological processes, and are often subject to the same management objectives as non-urban forests.…”