Ecological Applications: “A Cityscale Assessment Reveals that Native Forest Types and Overstory Species Dominate New York City Forests.” December 6, 2018

Cities are increasingly focused on expanding tree canopy cover as a means to reduce heat island effects, promote better air quality, protect local habitat, and otherwise improve the urban environment. The majority of these tree canopy expansion efforts focus on planting street trees, or changing the species composition of natural areas by planting native tree species and removing nonnatives. Many urban canopy assessments conducted at the city-scale–including both landscaped trees and those in natural areas–reveal codominance by nonnative trees, fueling debates about the ecological value of urban forests and native-specific management targets. Yet some assessments focused specifically on urban forested natural areas within cities find that some of these areas harbor predominantly native species.

To resolve this difference in findings, we measured forest structure and species composition across 2,497 hectares of urban forested natural areas in New York City by collecting data from 1,124 locations distributed across 53 parks. Results suggest that, on average, urban natural area forest canopy in NYC is 82% native. 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 found that out of 57 unique forest types in New York City, the majority of stands (81%) are a native type, and that forest structure in urban areas 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. These results suggest that other results showing urban canopy as co-dominated by nonnatives may result from those assessments including predominantly street trees and other landscaped canopy. Our results suggest that urban natural area forests have different characteristics than the rest of the urban forest and should be assessed separately. Doing so will ensure that city-scale assessments provide information that aligns with conservation policy and management strategies that focus on maintaining and growing native urban forests rather than individual trees.

Journal: Ecological Applications

DOI: https://doi.org/10.1002/eap.1819

• Author(s): Clara C. Pregitzer, Sarah Charlop-Powers, Silvia Bibbo, Helen M. Forgione, Bram Gunther, Richard A. Hallett, Mark A. Bradford
• Date Published: December 6, 2018