Carbon Sequestration Potential of the MillionTreesNYC Initiative

By Jesse Moore

In April of 2007 Michael Bloomberg, Mayor of New York City, announced PlaNYC. PlaNYC is a comprehensive strategy to make New York City a more sustainable city. PlaNYC is comprised of 127 separate initiatives that focus on different aspects of sustainable development. Public transportation, sustainable housing development, energy efficiency, and air and water quality are a few of the issues being considered in PlaNYC (MillionTreesNYC 2009).

MillionTreesNYC is just one of the initiatives outlined in PlaNYC. The overall goal of the MillionTreesNYC program is to plant one million new trees in the five boroughs of New York City within ten years (by 2017). Planting one million new trees in New York City would increase the forest canopy of the city by 20%. In addition to providing benefits to the citizens of New York City, such as increasing property values, reducing heating and cooling costs, and helping to mitigate storm water runoff; the million trees will also sequester carbon dioxide from the atmosphere, thus helping to mitigate the effects of global climate change. It is estimated that the trees in New York City currently store 1.35 million tons of carbon and sequester an additional 42,000 tons annually from the atmosphere (MillionTreesNYC 2009). An important question arises from this, how much carbon will the one million new trees sequester over a given period of time? One must examine the plans of the MillionTreesNYC program more thoroughly to arrive at an answer.

The current plan for MillionTreesNYC calls for the one million trees to be split into three categories based on who is doing the planting and where the trees will be planted. A total of 220,000 trees will be planted by the New York City Department of Parks and Recreation (NYC Parks) as street trees. Another 380,000 trees will be planted by NYC Parks as what they call reforestation. The remaining 400,000 will be planted by “private partners” on land not owned by New York City. In order for a tree to be counted as one of the million trees it must be a minimum of two feet tall and be grown in at least a two-gallon sized container prior to planting. The trees must also be planted in the ground (as opposed to a planter, or some other structure). The 380,000 trees previously described as “reforestation” trees will be planted primarily on 2,000 acres of New York City parkland (MillionTreesNYC 2009).

While it is easy to imagine the conditions under which the 200,000 street trees will grow in New York City (evenly distributed, limited canopy overlap at a mature age, etc.), it is more difficult to intuitively know the planting regime of the 380,000 “reforestation” trees. Further investigation reveals that the “reforestation” would be better named “afforestation” trees because they will generally be planted in areas that have not been forested within the past 50 years. Many of the afforestation plantings will extend from already forested areas into lawns or other areas lacking trees. In fact, trees will not be counted toward the one million trees goal if they are planted in the understory of an already existing canopy. The afforestation trees are being planted three to four feet apart at densities of roughly 1,000 trees per acre. At these densities it is expected that a large number of the trees will not survive to maturity but that forest canopy will remain closed over the lifespan of the trees (Bounds 2009).

The growing conditions of the remaining 400,000 trees that are to be planted by private partners are even more ambiguous. These 400,000 private trees are not to be planted on city owned property, but rather private, state or federal owned property because of space limitations (Bounds 2009). It is unclear what mechanisms are in place to ensure that these private trees will be planted within the ten-year timeframe of the MillionTreesNYC program. It is assumed that the 400,000 private trees will be planted in a setting more similar to street trees than afforestation trees. It will be interesting to see how private involvement in the program will be affected by the current economic conditions.

A study by Nowak and Crane looked at carbon sequestration by urban trees in a number of cities across the United States. The authors noted that urban forests generally store less carbon than forest stands on a per-area basis, primarily due to the lower planting density in urban forests. In spite of this fact, urban forests typically surpass forest stands in terms of both carbon storage and sequestration when normalized for canopy cover. It is estimated that individual forest stand trees contain only one quarter of the carbon in an individual urban tree. The differences between carbon storage and sequestration are believed to be due, principally to the higher light availability to urban trees as well as the fact that many urban trees are much larger than forest stand trees (Nowak and Crane 2002).

In order to estimate the carbon sequestration potential of the MillionTreesNYC program the carbon sequestration of the 220,000 street trees and 380,000 afforestation trees must be calculated separately because of their inherently different carbon sequestration rates related to their associated growth conditions. The 400,000 private trees will be excluded all together from the calculations because too many assumptions would need to be made and thus the final estimation would be nearly meaningless.

The United States Department of Energy has developed a method for calculating the carbon sequestration of street trees based on tree age, species, and number planted. This method was applied to the 220,000 street trees that will be planted (US Department of Energy 1998). A number of assumptions were made in order to calculate the carbon sequestration of the street trees. First, it unrealistically assumed that all trees would be planted in the first year of the project. Second, all trees were assumed to be planted at the minimum size requirement (two feet in height). Lastly, two different scenarios were selected to calculate carbon sequestration; it was assumed that the types of trees planted were either hardwoods with moderate growth rates or hardwoods with fast growth rates. The moderate growth rate calculation represents the average value, while the fast growth rate represents the optimal value of carbon sequestration. Potential carbon sequestration was calculated over the first 15 years after planting.

table-1-jesse-moore1
The carbon sequestration potential over the first 15 years of growth for the 380,000 afforestation trees was calculated based on values from the United States Department of Agriculture’s methods for calculating forest carbon. This calculation required a different set of assumptions. It was assumed that the tree species planted would most closely match the northeast oak-hickory stands of the USDA’s methods (Smith et al. 2006). Next, calculations were made using two different assumptions of the total acreage planted. First, an even planting density of 1,000 trees per acre was assumed based on information from Katerli Bounds. Second, a total planting area of 2,000 acres was assumed based on information from the MillionTreesNYC website.

table-2-jesse-moore
Based on these analyses the total carbon sequestered by the 220,000 street trees and 380,000 afforestation trees over the first 15 years ranges from 21,776 to 88,912 metric tons. These values converted to CO2 equivalents are 79,918 to 326,307 metric tons. This amount of carbon dioxide sequestration, traded at the current price of $1.65 per metric ton on the Chicago Climate Exchange, would approximately be worth between $132,000 and $538,000 (Chicago Climate Exchange 2009).

It is important to recognize the uncertainty of these calculations as a result of the assumptions made and other factors. For example, currently NYC Parks is having difficulty acquiring enough trees of the minimum size and as a result have been planting one-foot tall trees. These undersized trees are not currently being counted toward the million tree goal, and it is unclear if they will be counted in the future after they reach the minimum size. The tree supply problem is being eliminated as new contracts are being signed by nurseries for the long-term supply of 55 different tree species. Currently, there are also many trees being planted by contractors hired by NYC Parks that are much larger than the minimum size (Bounds 2009).

There are many other aspects of the MillionTreesNYC program that require further investigation to determine the net effect on greenhouse gas emissions. What are the effects on soil carbon? How long will the carbon be stored? What are the emissions reductions associated with reduced heating and cooling costs? Despite these unanswered questions, it is clear that the MillionTreesNYC program will positively impact the lives of New Yorkers and people all over the globe.

References

Bounds, Katerli. New York City Parks. Personal communication on April 15, 2009.

Chicago Climate Exchange website. Available at: http://www.chicagoclimatex.com/index.jsf. Accessed  April 16, 2009.

MillionTreesNYC website. Available at: http://www.milliontreesnyc.org/html/planyc/about.shtml.   Accessed April 12, 2009.

Nowak, David J. and Daniel E. Crane. Carbon Storage and Sequestration by Urban Trees in the USA. Environmental Pollution 116 (2002) pp. 381-389.

Smith, James E., Linda S. Heath, Kenneth E. Skog and Richard A. Birdsey. Methods for Calculating Forest Ecosystem and Harvested Carbon with Standard Estimates for Forest Types of the United States. US Department of Agriculture : Forest Service. General Technical Report NE-343. April 2006. Delaware, OH.

U.S. Department of Energy: Energy Information Administration. Method for Calculating Carbon Sequestration by Trees in Urban and Suburban Settings. April 1998. Washington, DC

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