2007-08-22 / Features

Census Shows Street Trees Add Value To City Life

BY LINDA J. WILSON

The London plane tree The London plane tree The city Department of Parks and Recreation conducted a census of street trees throughout the city in 1995 and 1996 and repeated the process 10 years later, in 2005-06. More than 1,000 volunteers logged a total of 30,000 hours, the largest participatory urban forestry project in any city in the United States. Equipped with maps, clipboards, tape measures, and tree identification keys, surveyors enumerated trees by species, size, location and condition in neighborhoods across New York City. They found that almost 20 percent more trees line the city's sidewalks today than was the case 10 years ago.

In Queens, the number of street trees- enumerated as 217,111 during the 1995- 96 tree count- increased by 22,771 to 239,882, or 10 percent more for 2005-06. Queens has the most trees, just over 40 percent of the total population. Manhattan has the leafiest streets, with an average frequency of 49.4 trees per mile of sidewalk, but Queens comes in at a close second at 49.1 trees per sidewalk mile.

In New York City as a whole, the London plane tree comprises 15.3 percent of the top 10 street tree species in New York City, followed by little leaf linden, 4.7 percent; Norway maple, 14.1 percent; green ash, 3.5 percent; Callery pear, 10.9 percent; red maple, 3.5 percent; honey locust, 8.9 percent, silver maple, 3.2 percent; pin oak, 7.5 percent, and Ginkgo, 2.8 percent. The more species that comprise a tree population, the less impact pests and diseases can have on the health and vitality of the whole population. Ideally, a tree population should be comprised of no greater than 10 percent of any species. Another measure of diversity is the extent to which a single species dominates a population. In general, no one species should exceed 25 percent of a population. Queens is relatively close to the ideal. Norway maples account for the greatest part of Queens' tree population, 18 percent, followed by London plane tree, 14 percent and pin oak, 8 percent. Callery pear and honey locust tie at 7 percent. Ninety percent of those trees are in good and excellent condition. Some 10 percent of Queens' street trees are in poor condition or dead.

A healthy street tree population does more than the obvious of providing shade and a cooling environment in summer. The results of the census were fed into a model created by the U.S. Forest Service that quantifies the annual benefits of street trees in each borough in terms of the amount of air pollution removed, emissions avoided, storm water runoff intercepted and energy saved. The Forest Service figures translate into the fact that for every $1 invested in planting a tree, there's a more than $5 return, according to city Parks Commissioner Adrian Benepe.

Trees improve air quality through their leaves, which absorb gaseous pollutants (carbon dioxide, nitrogen dioxide, and sulfur dioxide), and capture air-borne particles, including dirt, dust and soot. Trees also prevent the release of many airborne pollutants by reducing energy generation. Ground level ozone, a contributor to greenhouse gas formation, is reduced through trees' ability to lower air temperatures.

By reducing building energy use, trees indirectly reduce emissions of carbon dioxide (CO2) from power plants. Also as trees grow, they remove CO2 from the atmosphere and store it in woody plant tissue. At the same time, trees release CO2 as they decompose. These releases are subtracted from the total amount of CO2 avoided from power generation and absorbed by tree growth to calculate the net CO2 benefit. The annual benefit value to New York City as a whole is $754,947; to Queens, $342,000.

Trees provide shade, reducing the demand for electricity for cooling in the summer. Trees also reduce wind speeds, slowing the loss of heat from interior spaces during the winter. Trees cool the air through the process of transpiration, where moisture is converted to water vapor. An estimate for energy usage for every building in New York City was derived from data on building age, tree shading effects, and local climate. This estimate was drawn with two scenarios- with and without street trees- in order to show the difference in the resulting energy use. Local energy prices were then used to calculate the value of the impact of trees on building energy use. Street trees had a benefit value to the city of $27.8 million a year; In one year, Queens' street trees saved the borough's residents and business owners $12,308,000. New York City's street trees provide an annual benefit to the city of as a whole of $5.3 million in air quality improvement; Queens reaped the benefit of improved air quality worth $2,375,000 in one year.

Trees help reduce flooding and improve water quality, as runoff flowing over impervious surfaces picks up contaminants, including oil and metals. Trees intercept rain on their leaf, branch and stem surfaces and by absorbing water through their roots. New York City garnered $36 million in annual benefit value of reduced storm water runoff; Queens' share was $16,238,000.

Research has shown that homes with a tree in front sell for almost 1 percent more than similar homes without trees, a difference in sale price that indirectly reflects the value buyers place on trees and their more intangible benefits, such as aesthetics. This difference was applied to the median New York City home resale price of $537,300 to calculate the total annual benefit value to New York City of $52 million. Property values throughout the borough increased by $21,567,000 annually. The total monetary value returned to the borough by street trees' very existence exceeded $52 million.

More than 35 percent of the city's street trees are growing under wires, and with the exception of Manhattan, overhead wires are the predominant urban infrastructure that conflicts with trees in all neighborhoods in New York City. The next biggest problem either caused by or affecting trees is damaged sidewalks, and almost 50,000 trees in Queens present this problem. Some 2,000 of the borough's street trees present problems with canopy debris; slightly more than 6,000 choke wires close to them, 18,258 have conflicts with close paving, 813 are choked by the grates that sometimes surround them; 702 are damaged by or cause damage to tree lights and 324 were found to have some sort of conflict with electric outlets.

Despite the conflicts, Benepe's goal is to plant another 1 million trees in the next decade. New York City streets are about 73 percent stocked, with space for approximately 220,000 additional street trees across the five boroughs. Queens should be getting 55,000 new trees by the time the next tree census is due to begin in 2015.

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