Urban nature is often portrayed as degraded, lacking in native biodiversity and consisting mostly of weedy, invasive species. But a new study shows that even the most populous city in the United States contains significant stands of forest dominated by native tree species.
The findings, which appear in the journal Ecological Applications, underscore the importance of studying urban forest stands separately from street trees, which are often non-native species. Lumping the two together can result in an underestimate of the biodiversity and ecosystem services associated with a city’s trees.
Many cities contain significant forested area in parks or natural areas. In New York City, these urban forests cover 2,947 hectares. In the new study, researchers surveyed vegetation in 1,124 plots across all 53 New York City parks that contain forest stands.
They found that 84% of all canopy trees in these plots are native species. More than half of all plots have 100% native canopy. The ten most common canopy species are all native, with sweetgum and northern red oak at the top of the list.
Those results are in sharp contrast to a previous survey that assessed trees across the city as a whole. It estimated the urban canopy to be 55% native and 45% non-native, and suggested that the invasive tree-of-heaven is the most abundant tree in New York City, accounting for 9% of the urban canopy. But tree-of-heaven composes only 1.1% of the canopy in the forest plots surveyed in the new study.
Previous studies that assessed trees across land-use types have also failed to capture just how many trees there are in New York City. One such study suggested that the city contains about 5 million trees overall. But based on stand density measurements from the new study, the researchers estimate that there are 5 million trees in the city’s urban forest stands alone – which cover just 5.4% of the city’s area.
While New York City’s forest stands may be strong now, their future is a little dicey. “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,” the researchers write.
Moreover, nearly three-fifths of the forest plots contain non-native vines climbing in canopy trees. Such vines can suppress trees’ growth and shorten their lifespans. And non-native plants in the understory of these plots could decrease the survival of native tree seedlings.
But now that these threats are clear, managers can intervene, the researchers say. They found that stand structure and forest types found in New York City are similar to those of natural forests in rural areas of New York State. So foresters could bring well-known silvicultural techniques to the city – removing invasive species in the midstory and understory, and perhaps planting some native seedlings – to maintain the health of the native tree canopy for the future.
Source: Pregitzer C.C. et al. “A city-scale assessment reveals that native forest types and overstory species dominate New York City’s forests.” Ecological Applications 2018