The remodeling of single-family homes, including the controversial practice known as mansionization, has resulted in the destruction of the urban forest in Los Angeles County cities by as much as 55 percent, according to a study released Monday.

USC researchers calculated at least a 10 percent average decline in green cover from 2000 to 2009 across the 20 largest cities studied, as homeowners squeezed bigger homes with additional bathrooms and bedrooms on same-sized lots.

“What we saw was lot-line development, sometimes referred to as mansionization. And when there is a larger footprint for a building, there’s more hard-scape, less trees, less grass and shrubs,” said Travis Longcore, a professor of architecture at USC and a co-author of the report.

Longcore worked with professor Su Jin Lee with the Spatial Sciences Institute at USC on the study, published in the online journal Urban Forestry & Urban Greening.

The study found that for each home expansion, one-third of the existing green cover on each single-family lot is lost.

Local loss

Locally, San Pedro, Wilmington, Harbor City, Harbor Gateway and Watts — which make up Los Angeles’ 15th Council District — saw the greatest tree loss.

According to the study, in 2000, those neighborhoods were in the 51-60 percentile for green cover on single-family lots. By 2009, the canopy diminished to 22-30 percent, putting the area in the top four for tree loss after Baldwin Park, Compton and the northeast San Fernando Valley.

Building and hard-scape went up by more than 25 percent on single-family lots in the district, according to the data.

Other parts of the South Bay, including Carson, Torrance, Inglewood, Hawthorne and L.A.’s 11th Council District — which takes in Westchester, Playa del Rey and Playa Vista — also lost trees, but not as significantly as the 15th District.

There have been efforts to boost the Harbor Area’s urban forest in recent years.

In 2014, the L.A. Conservation Corps worked with San Pedro residents to plan a “waterfront-to-hills” urban greening project, including new trees, using funding from a $250,000 state grant.

That same year, Councilman Joe Buscaino’s office led efforts with community groups to plant more than 100 trees in Wilmington.

And a $111 million cleanup of Machado Lake at Ken Malloy Harbor Regional Park in Harbor City, which began three years ago and is nearly complete, includes 600 additional trees.

The cost of losing cover

The consequences of losing the urban forest can include a loss of carbon sequestration, since trees produce oxygen and intake carbon dioxide, a greenhouse gas that causes global climate change, the study said.

As temperatures rise, the replacement of urban trees with concrete, driveways and bigger homes on smaller setbacks could amplify the heat island effect, in which air temperatures multiply as the sun radiates off the hard-scape.

“Trees work against that,” Longcore explained. “They ameliorate summer-time temperatures and the loss of life that we can see during heat waves.”

Fewer trees and shrubs can destabilize a hillside in the event of a heavy rainstorm, increasing the chance of erosion and landslides, said professor Tracey Takeuchi, professor of plant sciences in the Department of Agriculture at Cal Poly Pomona.

Takeuchi, who was not part of the study, said the benefits of trees and greenery around houses are not well understood by homeowners, who see them purely as ornamental. But trees can have a direct link to public health, she noted.

“The rate of asthma is increased in areas where trees are not present because trees are very effective at filtering dust particles,” Takeuchi said.

Bigger homes resulting in a declining green-scape is occurring in all the cities studied, including Los Angeles, Long Beach, Torrance, Burbank, Inglewood, Whittier, Alhambra and Baldwin Park.

Green cover in Baldwin Park, a city of 81,604 people in the San Gabriel Valley, dropped from 70 percent in 2000 to 31 percent in 2009, a loss of 55 percent of green cover on single-family lots, the highest decrease of any city in the study.

The city of Compton lost 41 percent of its green cover. Downey, San Pedro, Sylmar and Pomona experienced 20 percent losses, according to the study.

Smaller cities, such as Burbank, Glendale and Pasadena, showed the least amount of loss, only about 14 percent, the study said.

The de-greening took place during the city of L.A.’s “Million Trees” initiative, a tree-planting campaign that began in 2007. Longcore said planting new trees takes decades for the trees to mature and that many of them die. The better goal is to protect current trees, both private and public trees.

“You have to take more steps to protect trees that exist, in addition to putting more trees in the ground,” Longcore said.

Staff writer Megan Barnes contributed to this report.