Los Angeles has an estimated 10 million trees. They reduce excess CO2 in the air, act as wildlife habitat, improve water quality, reduce energy use, and increase surrounding property values.

But that number is far less than two decades ago.

That's according to a new study from the University of California, which says the amount of tree cover in LA County has declined rapidly since 2000.

The study — led by USC spatial scientists Su Jin Lee, John P. Wilson, Travis Longcore, and Catherine Rich — suggests that large single-family homes, often known as McMansions, are to blame for the loss of LA's trees.

Americans' growing preference for large single-family houses, along with the increase in driveways and swimming pools that come with home expansion, is the largest driver of tree cover loss in the US, according to the study.

Looking at satellite imagery and data from the LA County assessor's office, the researchers found about one-third of the city's trees in single-family housing neighborhoods was eliminated from 2000 to 2009. During that period, tree cover may have decreased up to 55%.

In a press release, Longcore said sacrificing trees for redevelopment cuts across all Southern California neighborhoods, regardless of socioeconomic status.

It's perhaps no coincidence that more homeowners gravitated toward McMansions (large, luxury, mass-produced, single-family homes) starting in the late 1990s. McMansion developments often lead to suburban sprawl that sacrifice flora for pavement.

Surprisingly, the researchers also found that 1950s suburban development may have been good for trees, at least in LA. Private land owners planted trees on their land during that decade, contributing to a richer urban forest in the city.

"These ecologically beneficial consequences occurred organically — not as the result of conscious environmental policy, but rather as an outgrowth of the cultural aesthetic and economics of the times," the researchers write.

They say homeowners at the time valued greenery and shade as key neighborhood selling points, perhaps more than McMansion owners do today.

Today, nearly a decade after the 2008 housing bust, property development is as ambitious as ever. In Los Angeles, the average new home spans 2,687 square feet, and nearly one-third measuring over 3,000 square feet, according to the 2016 US Census Bureau annual survey.