The State of the Canopy: Trees in Philadelphia

Trees are universally loved – for their timeless beauty, shade, healing powers, health benefits, and ability to manage stormwater – but trees have also been touted as one of the most effective strategies for capturing atmospheric carbon and mitigating the imminent dangers of climate change. A recent study published in July of this year in Science, by Thomas Crowther, a climate change ecologist and founder of the Crowther Lab at ETH in Zurich, Switzerland, shows that trees hold the potential to save the planet. Here in Philadelphia, the Pennsylvania Horticultural Society is educating those who work and reside in the city and the region about the importance of planting and preserving trees, as part of this critical and timely, global effort. 

According to Crowther and team’s report, “The global tree restoration potential,” trees will be the most powerful tool in the fight against climate change because trees absorb carbon dioxide emissions. Crowther and his team used satellite images and soil and climate data to determine existing tree cover and available land that could support forests. After mapping tree populations across the planet at the square-kilometer level, they found that a worldwide tree-planting effort covering 11 percent of the Earth’s land could remove two-thirds of all the emissions that have been released into the atmosphere.

Crowther’s study showed that “Excluding existing trees and agriculture and urban areas, there is room for an extra 0.9 billion hectares of canopy cover, which could store 205 gigatons of carbon in areas that would naturally support woodlands and forests.” Crowther estimates that planting the needed 1 trillion trees would come with a $300 billion cost. 

Too Hot to Handle

Heat islands are one of the most dangerous conditions that exist in more built-up parts of cities – areas with more pavement and buildings capturing the sun’s heat, combined with a sparse tree canopy. According to the Philadelphia Office of Sustainability, average surface temperatures in some neighborhoods of the city can be as much as 22 degrees hotter than others. Identifying the areas of Philadelphia that have the highest need for canopy, including those most at risk for elevated temperatures, and planting trees in these areas are priorities of PHS’s volunteer Tree Tenders groups. “Some neighborhoods can reach life-threatening temperatures in the summer, and those who are vulnerable or have health issues will suffer” says Dana Dentice, PHS Urban Forestry Program Manager. 

Hunting Park and other sections of North Philadelphia fall into this category. “We’re excited that we’ve been working with Esperanza to increase tree canopy for residents in neighborhoods at most risk for elevated temperatures. We’ve collaborated with them to plant about 75 trees over the last year and a half,” says Dentice. 

Diversity is Key

“When it comes to planting forests, diversity is the key,” says Chad Rigsby, PhD, Host-Insect Ecologist at Bartlett Tree Research Laboratories. “Diversity in species, size classes, and age is important because it creates a community that is able to readily bounce back from disturbances. If a homeowner in Philadelphia had twelve ash on their property, ask them how they bounced back from emerald ash borer,” says Rigsby. “Alternatively, if a homeowner had twelve trees, only one of which was an ash, they might have lost that ash, but their ‘forest’ is still there.”

One of our latest insect threats is the spotted lanternfly, which loves the tree of heaven and other common tree species. “We are seeing it in great numbers in the greater Philadelphia region,” says Dentice.

A recent study from Purdue and the U.S. Forest Service estimates that invasive forest insects have killed enough trees to equal the emissions of approximately 4.4 million cars. “We can talk about reforestation and planting trees to combat climate change, but let’s also talk about forest health and management, individual tree health, invasive insects and diseases,” Rigsby says.

Education is Vital

“The Pennsylvania Horticultural Society’s Tree Tenders Program is one of the leading volunteer programs in the country focused on educating urban and suburban residents on tree biology, care, pruning, planting and monitoring. The program really is a fantastic model for community tree care programs throughout the country,” says Rigsby. “Educating and empowering people to care for trees in their community – like the Tree Tenders Program -- can make a huge difference in planting and maintaining a healthy urban canopy for all to enjoy.” 

Mindy Maslin, PHS Tree Tenders Program Manager, and founder, got the idea for a citizen stewardship program while she was attending a tree conference in Chicago in 1991. “I fell in love with the idea, and immediately knew that this was what Philadelphia needed,” she says. “We needed a program that empowered citizens to take the lead in organizing their own neighborhoods. Twenty-six years later, it seems to have worked,” says Maslin. Every planting season, about 60 Tree Tenders groups throughout the region participate by planting and caring for trees. 

“The PHS Tree Tenders program promotes the varied benefits of city trees by endorsing a culture of stewardship with neighborhood groups all over the city,” says Lara Roman, PhD, Research Ecologist, Forest Service, Philadelphia Field Station, Northern Research Station. 

“Through decades of planting events and thousands of trained volunteers, the Tree Tenders program gets free trees into Philly communities. Volunteers become empowered to network with their neighbors to plant trees, care for them after planting, and ultimately build a constituency to support our city’s urban forest.” 

Since 1993, more than 5,000 volunteers in the region have become certified PHS Tree Tenders, receiving hands-on training on the biology, identification, planting and proper care of trees. Organized according to community and neighborhood, the program’s volunteers are responsible for planting more than 1,300 trees annually in southeastern Pennsylvania and caring for new and existing trees. As one of the oldest and most established volunteer tree planting and stewardship programs in this country, PHS Tree Tenders is at the forefront of the urban tree movement across the country. PHS encourages individuals to take the Tree Tenders training course, and to start, or join, a Tree Tenders group in their neighborhood.

Right Tree, Right Place

Jason Henning PhD, Research Urban Forester at The Davey Institute and USDA Forest Service, Philadelphia Field Station, agrees on the importance of planting trees, but doing so wisely. “Trees need to be hand-picked for specific locations and individual needs,” says Henning. “That’s why you have PHS, or similar organizations, with that knowledge who can help make that decision. London planetrees are really good, but if you have a disease that impacts London planes, you can get into trouble.” 

He explains that this is what happened in Philadelphia with ash trees. “We’re losing seven percent of our trees in the city because of ash borer. This is an example of why you don’t put all your eggs in one basket when you’re talking about species diversity.”

Henning’s work as part of the Philadelphia Field Station team is to help evaluate how existing trees and tree canopies are performing, including those in more natural areas, like the Wissahickon Park watershed within Fairmount Park. “These natural areas cover about 10 percent of the city but provide about 40 percent of tree benefits to the city,” says Henning. “Trees planted through the TreeVitalize Watersheds program, managed by Bob Adams at PHS, are important because the trees are protecting those areas as naturally functioning forests.”

According to Henning, “These riparian plantings can be where we get the most bang for the buck in creating new canopy, as opposed to street trees, that total approximately three percent of the city’s trees and require more care and maintenance.” He agrees that this topic is complicated because street trees are the trees that people see and enjoy every day on their walk to work. Street trees are also helping to remediate heat islands and improve air and water quality -- critically important needs in the city of Philadelphia.

“Trees are multi-taskers. If you focus only on stormwater management, you might do better with a retention pond, but if you consider all the benefits of trees -- reducing heat islands, carbon storage and sequestration, reducing heating and cooling costs, improving mental health, increasing property value, and sustaining wildlife – the list gets long,” says Henning. “These are the things you won’t get from a retention pond.” 

The Bigger, The Better

Preserving large stature trees is vital. “Large trees -- trees that will get over 50 feet at maturity – are the most important. If you cut down a big tree, it will take five replacement trees more than 20 to 30 years to get back the benefits we had before,” says Henning. 

He cites research done in 2015 by Corinne Bassett, studying trees on the University of Pennsylvania campus. The study collected data from 4,086 trees on campus. One extraordinary finding was discovered about the six historic London planetrees that were planted at the end of World War II and preserved during construction. These six trees, standing about 80 feet tall and 80 years old, were together storing 14,291 lbs. of carbon each year. In comparison, the 1,316 smaller trees in the study store a combined total of only 16,567 lbs. of carbon. The fact that six large trees can almost encompass the benefits of 1,316 small trees supports the premise that growing trees to large sizes should be a priority.

“People want flowering crabapples, but they will never get big enough to provide this kind of benefit,” says Henning. “Protecting the larger, older trees is a top priority, as is planting the larger canopy trees that will get that big. Planting them on streets with no utility wires, or in private yards, through TreePhilly’s giveaway program, are two solutions.” 

Dentice asserts that we find that many people want a small stature tree for their property, whether their space is large or small. However, small trees, with fewer leaves and less biomass for carbon storage and water uptake and interception, won’t have as much of an impact in mitigating climate change. It’s the larger canopy trees we need to turn to for the most impact. “Unfortunately, there are many places in urban areas, like under powerlines and along narrow sidewalks, that are (or should be) limited to a smaller tree. Thus, wherever there is a large enough space to support a canopy tree, it’s ever more important that a larger stature species be selected -- Always think right tree, right place,” says Dentice. “People should consider shade trees like oak, elm, hackberry (seen to the right), horsechestnut, linden and Kentucky coffeetree if they have the space.”

“Trees help to address climate change by sequestering carbon, and in cities, trees also shade homes which lowers demand for air conditioning, reducing energy usage,” says Roman. “For trees to accomplish this they need to be large-stature and long-lived, so it’s important to not only plant trees, but to pick species, whenever possible, that get large at maturity and to maintain them over time, “she explains.

For help selecting a large canopy tree for your location, use the Arbor Day Foundation’s Tree Guide or the Missouri Botanical Garden Plant Finder as a starting point. Then, if you want to dive in further to determine the best tree placement for energy savings or calculate the benefits of a new (or existing) tree, check out the Forest Service’s i-Tree Design tool. 

Off to a Good Start

Most importantly, you can’t just plant a tree and walk away. “The first two years that the tree is in the ground are the most critical to allow that tree to establish in its new environment and thrive. After a tree is dug in the nursery, it’s in shock. A large percentage of its roots get left back in the field where it was growing (80 to 90 percent of roots for a balled and burlapped tree). Watering and basic tree care are so important when you plant a tree. It’s like a baby, depending on its parents to provide the vital resources it needs to grow and become self-supporting,” says Dentice. 

Planning Ahead in Philadelphia

The city of Philadelphia is also thinking hard about strategic tree planting and maintenance and where and how to focus its resources to meet its goal of 30% tree canopy in every neighborhood. Some neighborhoods have less than 10% canopy cover. The city is embarking upon the creation of the Philadelphia Urban Forest Strategic Plan, which will set forth a new approach for achieving its canopy goals, prioritizing equity, and public health. PHS has been an advisor to guide the scope of work and planning process. “The plan creation process will take place in 2020 and will include a strong public engagement component,” says Lori Hayes, Director of Urban Forestry for Philadelphia Parks & Recreation.

“Our partnership with PHS enables a science-to-practitioner-to-community feedback loop where the Forest Service can inform important decisions and make recommendations on which trees to plant, why and where. We gather feedback from PHS and community members on allergens, susceptibility to storm or pest damage, and climate. We can be the most informed as to where we put these trees in the landscape to do the most good,” states Henning.

“Trees can be part of the solution to many of the environmental and public health problems our neighborhoods face -- crime, mental health, access to healthy food, heat, air quality, and flooding,” says Dentice. “We encourage everyone to take part in planting and tending trees in their own neighborhood to collectively make the greatest impact possible.”