Researchers answer 'how green are urban trees?' question

Researchers answer 'how green are urban trees?' question

The study suggests that reducing mechanized planting, pruning and removal processes can significantly reduce carbon emissions on a single tree.

June 8, 2017

Washington, D.C. and Columbus, Ohio — June 7, 2017 — How green are urban trees? A group of researchers from the University of Illinois, University of Florida, and University of Kentucky, posed that very question.

Research shows how trees benefit urban environments, such as filtering particulate matter, lowering urban heat island effects and storm water mitigation. One more attribute to consider is a tree’s ability to sequester carbon over its lifespan.

Trees are natural contenders to sequester carbon in efforts to mitigate climate change. In this process, trees and other plants absorb carbon dioxide from the atmosphere through photosynthesis and then store it as carbon in biomass, either in plant material or soils. 

However, carbon is released into the atmosphere during production and maintenance practices, such as planting, pruning, irrigation and even removal upon tree death. Pruning styles and frequency, for example, have a great impact on carbon emissions. Common sense says manual pruning reduces carbon emissions (over using a chainsaw), but even the accessories used affect the amount of carbon released. A bucket truck emits more carbon than an aerial lift or a tree climbing rope system. 

Researchers led by Dr. Dewayne Ingram, University of Kentucky, set out to determine at what point urban trees sequester as much carbon as is emitted during maintenance practices over their lifespan. Another way to phrase this is: at what point do urban trees become carbon neutral?

“Our research shows the importance of selecting good quality trees that are suited for the planting site and doing what we can to enhance their longevity," Dr. Andrew Koeser, University of Florida, says. "Failed plantings and premature tree death can end up causing environmental disservice.“  

After accounting for many possible maintenance factors, the answer is not so simple. Certain practices, especially pruning, during the tree’s lifespan have a great impact on how quickly a tree becomes carbon neutral. Dr. Ingram’s team conducted a series of surveys of urban forestry programs in the Chicago metro area to gauge the scope of urban tree maintenance. Based on the 48 respondents, the information gained was factored into a model to estimate at what point an average urban tree reaches carbon neutrality. 

Before the models were run, researchers standardized a few key variables. First of all, red maple, Acer rubrum, was chosen as the model tree, and the target tree size was set at 2 – 2.9” caliper at the time of purchase. Information on the tree’s carbon balance when it leaves the nursery was generated from previous research, and this value was the starting point for calculations. 

Additional carbon emissions were based on four possible scenarios: highly mechanized, moderately mechanized, slightly mechanized and the ideal scenario, which relies on manual labor whenever possible. Two urban site types were defined as likely to impact tree growth differently, residential and commercial-industrial. Residential trees live longer, on average, than trees in commercial-industrial settings. The model estimated the carbon balance for red maple trees over their lifespan maintained under each scenario in both the residential and commercial-industrial sites.

Average lifespan of red maple:
Residential = 48 years
Commercial-Industrial = 30 years

Emissions associated with the manufacture of planting and maintenance equipment, such as chainsaws, trucks and chippers, were not included in this study. However, emissions associated with tree stabilization, such as T-posts and wire, and tree bags for watering were included. 

This study illustrates that using less mechanized planting, pruning and removal processes, carbon emissions can be reduced significantly on a single tree. 

• Highly mechanized = 2919 kg
• Moderately mechanized = 1725 kg (reduces emissions by 1194)
• Slightly mechanized = 1693 kg (reduces emissions by 1226)
• Ideal = 1340 kg (reduces emissions by 1579)

• Highly mechanized = 2127 kg
• Moderately mechanized = 1296 kg (reduces emissions by 831)
• Slightly mechanized = 1264 kg (reduces emissions by 863)
• Ideal = 1034 kg (reduces emissions by 1093)

Urban red maple trees reach carbon neutrality in commercial-industrial sites faster than in residential sites, due to their shorter lifespan and therefore, less pruning requirements. 

• Highly mechanized = 33 years old
• Moderately mechanized = 26 years
• Slightly mechanized = 26 years old
• Ideal = 26 years old

• Highly mechanized = 30 years old
• Moderately mechanized = 24 years
• Slightly mechanized = 24 years old
• Ideal = 24 years old

All surviving urban red maples are expected to achieve carbon neutrality over the functional life, regardless if they are located in a residential or commercial-industrial site. However, additional carbon sequestration benefits can be realized if modifications are made to planting and maintenance practices.

The Horticultural Research Institute, the foundation of AmericanHort, provided funding for this research.