For greater urban tree cover, it’s time to respect our elders

Article: Hilbert, D.R., A.K. Koeser, L.A. Roman, K. Hamilton, S.M. Landry, R.J. Hauer, H. Campanella, D. McLean, M. Adreu and H. Perez. 2019. Development practices and ordinances predict inter-city variation in Florida urban tree canopy coverage. Landscape and Urban Planning 190: 103603. DOI: https://doi.org/10.1016/j.landurbplan.2019.103603

Featured Image: Saving old urban trees may be the most effective means of keeping a healthy urban tree canopy. (Photo: Matthew Schwartz, Wikimedia Commons)

Every city has its distinctive features. New York has its skyscrapers; Los Angeles, its freeways; Venice, its iconic canals. People might not also include the urban trees in the picture of what makes up a city’s unique identity, though tree cover too can vary a great deal from place to place (Nowak et al., 2013). But up to now, it has not been obvious if municipal policies might be behind some of the differences in tree cover between cities. A recent study by Deborah R. Hilbert and colleagues explores how different tree policies might drive differences in tree cover across several Florida cities – and found that for cities seeking a denser urban forest, it might be a matter of trying to hold on to the oldest trees.

Leafy streets

As many cities in the U.S. and elsewhere develop local policies for dealing with the effects of climate change, urban trees and green spaces are emerging as potential allies. By providing a covering of leaves, a healthy urban forest can help to reduce excessive temperatures related to the urban heat island. Tree canopy could also help reduce some kinds of urban air pollution, and can potentially offer a host of other social and physical benefits to urban residents. But recent large-scale programs to expand tree cover in some cities have seen significant shortfalls in meeting their goals, and it is not yet clear which policies available to decision-makers work best to encourage greater tree cover.

Cities like Los Angeles (left) and Boston (right) can differ a great deal in tree canopy cover. (Photo: Andrew Trlica)

It is known that trees in the city respond to both their physical environment, as well as to social forces in the communities that host them. Often higher population density is related to lower canopy cover, but not always. Other effects come into play too. Canopy cover tends to be higher in areas with wealthier and more educated residents, and where other metrics also indicate greater access to local avenues of power (including differences in neighborhood racial distribution). Because trees take so long to grow large, land use history can play a role as well. Older and single-family neighborhoods often host more tree cover, and recent redevelopment is usually associated with less. However it has not, up till now, been clear whether specific municipal policies dealing in tree planting and preservation could have similar effects on tree cover. Without knowing what policies have worked before, it is difficult for decision-makers to know how to move forward with efforts to improve local urban forest cover.

Asking around

To explore the association between municipal tree policy and urban tree cover, the study authors paired two unique sources of information. They first gathered survey data from 43 urban municipalities in the U.S. state of Florida, from towns as small as Hypoluxo to the metropolis of Miami. The survey gave the researchers in-depth information on the policies and resources each city dedicates to managing urban trees, which they supplemented with information from the U.S. Census on several metrics of socioeconomics and development history. The policies examined included measures like ordinances regulating the removal of trees, and resources like whether or not the city had a certified arborist on staff. The authors also prepared a detailed assessment of tree canopy cover in each city using aerial photos taken by the U.S. Department of Agriculture. They then analyzed these socioeconomic and policy factors against the total amount of tree cover in each municipality to tease out the policy effects.

Their analysis revealed a striking finding: Cities that had ordinances to preserve heritage or significant trees showed on average about 6.7% more canopy cover – a statistically significant result, on top of an expected decline seen with greater housing density. These heritage tree ordinances specifically dealt with preventing the removal of older or socially significant trees. The effects of other policies were too small to be distinctive enough for true statistical certainty. These less important factors included, for instance, establishing a community tree board, requiring permitting for removal or replacement of existing trees, calling for local tree inventories, or establishing tree canopy cover goals.

Respect for the elders
Old urban trees are at risk from disease or redevelopment; policies to preserve them may result in greater canopy cover. (Photo: Andrew Trlica)

This research bolsters support for urban municipalities preserving heritage trees with the goal of maintaining and expanding their canopy cover. These results are in line with other recent work showing that preserving older larger trees may produce greater tree canopy cover over time compared to adding new smaller trees to the landscape. The researchers hope to expand their research to include more municipalities in Florida as well as cities outside the state, to get a fuller picture of how these factors vary geographically and with greater variety of data. In the tricky business of putting urban forests to work helping cities adapt to climate change, their work underlines an important realization: We may need to start devoting to the venerable old trees in our communities the same love and concern we bring to issues like traffic zoning around schools and historic building preservation.

References:

Nowak, D.J., E.J. Greenfield, R.E. Hoehn and E. Lapoint. 2013. Carbon storage and sequestration by trees in urban and community areas of the United States. Environmental Pollution 178: 229–236. https://doi.org/10.1016/j.envpol.2013.03.019

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Andrew Trlica

Andrew Trlica

Recent PhD out of Boston U's Dept. of Earth & Environment. Interested in landscapes defined by the human presence, and how humanity’s choices connect our landscapes to the causes and consequences of climate change. My own research has dealt with the urban carbon cycle and urban forest, the urban heat island, agricultural practices and soil quality, and disturbed land reclamation. Twitter/Insta: @places_we_made

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