Trees improve communities. They cool neighborhoods, provide shade and green space, and can help mitigate climate change. But what happens when trees get in the way of construction? You could cut them down. Many developers would, says Jon Hillis of Houston-based Environmental Design, which provides tree services all over the world.
Or, you can move them.
AMLI Development Company faced this predicament earlier this year as it planned a 13-acre community along Midway Road in Addison. The project will include a five-story apartment building, 34 townhouses, a separate three-story complex with attached garages geared toward seniors, and retail space. Construction is set to begin in early 2024.
Before that, AMLI had to figure out what to do about the trees. For years, the location had been occupied by an old event center and a collection of defunct office buildings, along with dozens of trees. Unfortunately, most of those trees were too close to the buildings to be saved, says Taylor Bowen, AMLI’s president. Four heritage live oaks, though, were far enough away that they didn’t have to go.
Each of those live oaks are 50 to 70 years old, Bowen says. They are 35 to 40 feet tall, with spreads of 45 to 60 feet. “These are the best that we found out here,” he says.
Trees are vital for reducing heat in cities, says Rachel McGregor, the Texas Trees Foundation’s urban forestry manager. They are especially important as populations become denser and infrastructure proliferates. They shade, of course, preventing sunlight from hitting concrete, which absorbs sun that then radiates heat. Trees also act like a swamp cooler in a process called transpiration, McGregor says. Moisture from trees’ leaves evaporate, “and that results in a decrease in temperature in the immediate area around the tree.”
Additionally, trees can cool temperatures and provide cleaner air by storing carbon, in a process called sequestration. Different species have varying levels of storage. Live oaks, like the ones on the AMLI property, are some of the most efficient in storing carbon. Plus, the larger and more mature the tree, the bigger the impact it can have, McGregor says, “just the sheer size of the tree and the capacity of the tree.”
Because of the impact trees have on their surroundings, some cities have reforestation funds that developers pay into when they cut down trees. Then, cities can use that money to acquire or improve green space elsewhere. The city of Addison has a tree mitigation fund. So, to sweeten the deal with Addison, AMLI said it would move those live oaks to the back of the property, where the city is building out a new park and trail extension. But, considering the trees weigh about 160,000 pounds each—the same weight as an empty Endeavor space shuttle—AMLI needed help. So, it turned Hillis and Environmental Design.
Founded in 1977 as a “a general green industry-type business,” Environmental Design has grown into the tree farming and tree supply biz, says Hillis, a company vice president and certified arborist. It’s made its name, however, in transporting large trees by moving tree trunks and their root systems, some as heavy as a million-plus pounds, from one location to another.
To do this, they’ll box up the trees or move them with cranes. But for the Addison project, Environmental Design used its ArborLift technology, which it patented in 2014, and dragged each live oak across the property on inflated air bags.
How Do You Move a Tree?
Prep work for moving day starts at least three months ahead of time, Hillis says. His team heads to the site to do hand work and root pruning. They’ll feed, water, fertilize, and provide insect preventative treatments to the tree before it’s uprooted. This pre-shocks the tree, he says, “or basically giving the tree information in advance of what is to come, and it starts to adjust and self-regulate with its diminished root system.”
During prep, the most important step is determining the size of the root ball, he says, which is the mound of dirt and roots under the tree that gets dug up and moved. You don’t have to go all the way out to the edges of the root before you start shoveling soil. The critical root zone of the tree extends out to the drip line, or edge of the canopy cover, McGregor says. While roots can run two-to-three-times the length of the drip line, that critical zone is “really what you’re trying to preserve.”
Generally, Environmental Design uses a 1-to-10 ratio, Hillis says, so a 10-inch diameter tree trunk would have a 100-inch diameter root ball. The team will walk out 50 inches from the base of the trunk on all sides, then dig a moat down about four feet.
The resulting slice of earth looks like a cake, Hillis says, which they’ll then wrap in heavy gauge, wire, burlap, and plastic wrap. After that, they’ll insert pipes under the root ball, cross hatch them with beams to create a lifting platform, and the tree’s ready to move.
To do so, a crew of six to eight workers will stick deflated airbags underneath the platform and then fill them with air through a compressor or air hose. With chains, they’ll rig the platform to a large construction machine, like a bulldozer. Then, they’ll drag the tree across the airbags, continually putting new bags in front of the platform, until they reach the final location.
“The process is something that probably the Egyptians and Aztecs came up with before we did to move large stones on logs or something like that,” Hillis says. “But they didn’t have a patent office at the time, I guess, so we perfected the technique.”
Moving the trees usually only takes a few hours. It’s fascinating to watch, says Bowen. “It looks like a forest is moving,” says Hillis. For the AMLI development, Environmental Design moved the four trees last month.
En route, the biggest risk to the tree is moisture management and making sure it doesn’t dry out. The airbags prevent contortions, and the rigid steel platform doesn’t flex “so it keeps the integrity of the root ball intact,” Hillis says. Once it’s re-planted, the tree might have to deal with years of nearby construction, which could cause damage, and irrigation access issues.
Live oaks are hardy, but Environmental Design will spend another three-to-five years monitoring the tree, ensuring it’s watered and not stressed, he says. Signs of shock include wilting and discoloration of leaves, because of the diminished root system, as well as burnt leaf tips, McGregor says. Monitoring, watering, and mulch correcting (covering the roots but not touching the trunk), can help save it. Environmental Design wants to catch any problems “before there’s a disaster and the tree really goes downhill,” Hillis says. After this period, the tree has established itself enough to make it on its own.
What’s It All Worth?
The whole process of moving a tree is “not a cheap exercise,” Hillis says. The base cost comes from the tree’s diameter, and then other factors, like soil composition, construction schedule, species, and seasonality, all of which can drive the price up. Bowen says the project cost more than $500,000, but it was worth it. (The optimal time to move is October to February.)
Trees like these are irreplaceable, Hillis says. “We can’t supply a tree of this size.”
Deforestation, construction, and development heat cities up in a phenomenon called the heat island effect. Because the concrete and impervious surfaces absorb and radiate the sun’s heat, it keeps things hot, even after dark when technically temperatures are decreasing, McGregor says. “It takes a lot longer for that area to cool down.”
But tree cover can combat the negative effects, McGregor and Hillis say. They improve the air quality, thanks to all that carbon storage. And they can improve water quality by slowing and filtering rainwater through their leaves, absorbing more water into the soil, and decreasing polluted runoff.
On a smaller scale, tree canopy can provide respite for folks outside on Addison’s collection of trails and parks. “The first thing that people clamor for when they’re out in the middle of the sun is to get into some shade,” Hillis says.
AMLI is invested in improving the Addison trail system with its current development, called the AMLI Tree House, Bowen says. In addition to the four live oaks, the company also plans to plant 350 new trees along its new Redding Trail Park and Extension. Doing so means the company won’t have to pay into Addison’s tree mitigation fund for cutting down all the original trees on the property. But, they want to improve walkability in the neighborhood and get residents out and about. Doing so will only add to Addison’s retail economy along Midway Road, he says. And their first step is moving four, heavy trees.
“Sure, we could have just paid into a fund or maybe just planted more smaller trees,” Bowen says, “but we just felt like this was a really a good thing to do.”
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