Andy Lipkis [founder of Tree People] has been thinking about the plight of trees and the humans who depend upon them for more than thirty-five years.

For some people, urban forestry is about managing trees, Andy says. To me, it's both a real thing and a metaphor. A city like Los Angeles is two-thirds paved. That paving, the concrete, covers a living, functioning ecosystem. Because we paved over that ecosystem, we now have to pay a lot to recreate the benefits that would be free if the ground weren't encased in concrete. I'm talking about free things like water, like natural cooling, like oxygen.

LA could meet half of its water needs if it could somehow capture and retain its rainfall each year. Instead, eighty-five percent of that water slips out of the city's grasp, washed out to sea and never seen again. Because we don't save that water, Andy says, we have to buy water that falls in Salt Lake City. It's the height of absurdity.

As we saw earlier, each time you try to solve a problem in LA, you are inevitably forced to grapple with another. Many had tried and given up. This Gordian knot seemed unbreakable, until Andy started to think about how elegantly nature solved many of these problems. There was never too much water in woods, because the water always had someplace to go. There was never any polluted water because plants and dirt filtered it before it reached underground reservoirs. Wasn't there some way to mimic nature's example, Andy wondered.

When he heard that the county was planning to build a $50 million storm drain in the south Los Angeles neighborhood of Sun Valley, he knew he had to do something. Sun Valley was lower middle-class to middle class Latino area renowned for some of the worst flooding in the city. Every time there's a huge storm in LA, news crews dash out to Sun Valley because they know they'll get dramatic footage. Zev Yaroslavsky, LA County Supervisor of the Third District, says the flooding issue is a vortex taking the community down.

Andy started phoning Yaroslavsky's office and showing up at community meetings, pleading with them not to install a new storm drain. He was fighting an uphill battle here. For years residents had demanded relief form the city. Now they seemed on the cusp of getting it. And here was Andy this dorky tree-loving guy who wanted to try something new and exotic that had never been done before. Asked to explain his idea, Andy laid it all out: He dreamed of retrofitting many of the homes in Sun Valley with cisterns to capture rainwater. (Later, that same water could be pumped out to irrigate lawns, plants, etc.) He wanted to landscape their front and back yards in such a way that the ground itself would act like giant basins to catch water. He wanted to design driveways to strain out toxic gunk before it seeped into the soil.

The vision didn't end there. Andy had grander plans for major public structures around town. We wanted giant cisterns under the athletic fields at the local schools. He wanted to rip out asphalt parking lots and replace them with porous pavement. Wherever there was concrete and blacktop, he wanted trees and grass to cool the school campuses down in warmer months and reduce the need for air conditioning. He wanted the schools to implement a policy of mulching all green waste on-site and using it to feed their landscaping beds.

People listened to Andy's dream and asked him what it would cost. He told them and they nearly choked: $200 million, four times the price of the storm drain they wanted in the first place. Of course arguments and lawsuits ensued. In the end, Tree People demonstrated through research that the $200 million retrofit would be a less expensive option in the long run. The concrete storm drain would simply funnel that excess storm water out of the neighborhood. It would do nothing to produce or save water. It would do nothing to recharge groundwater. Weeks after a storm, residents would still end up having to buy water to water their yards, and the vicious, unsustainable cycle would continue again.

When the engineers and city planners plotted out Andy's plan, they could see what a thing of beauty it was. Each component addressed every one of the major air and water quality problems facing LA. Instead of one problem leading to another, each solution led to another. Each link in the chain was integrated with the next, a perfect model of something called integrated resource management.

Consider:

• The problem of green waste? Solved: Residents and schools would mulch their waste on site and feed their planting beds.
• The problem of too much or little water? Solved: Hundreds of thousands of gallons would be captured and stored during storms or else fed into the ground to recharge reservoirs or aquifers.
• The problem of polluted water? Solved: Driveways and parking lots would be designed to filter out toxins.
• The problem of high-energy demands? Solved: Trees and grass would shade homes and schools, reducing the demand for air conditioning.
• The problem of poor air quality? Solved: The new urban forest would filter out particulates and CO2.

Even the perennial problem of creating jobs in urban areas had been addressed. This was the truly masterful part of Andy's plan. A retrofit of this size 2,700 acres encompassing 8,000 homes would require the employment of tons of people of different skill levels, from trained engineers and technicians to competent landscapers and day laborers. In the years to come, trees would need to be tended, cisterns maintained, filtering systems installed or repaired, grass mown, green waste mulched. There was an endless supply of good, honest work awaiting hundreds of people in Sun Valley alone.

To demonstrate his point, Andy arranged for an army of workers to descend one day on the eighty-year-old home of Rozella Hall, who lives in Sun Valley. Her place is a cute California bungalow with a postage-stamp-sized front yard and an equally small back yard. The team installed what is called a drywell at the end of Ms. Hall's driveway. Rainwater rolls down the driveway to a small grate and falls into a chamber to be filtered and cleaned, then released into the ground. Drain spouts, which normally expel rain into the street, were repositioned so they send water into Ms. Hall's newly landscaped garden. The edges of her front and back yard have been raised to form the shape of a bowl. Water collects here and gently seeps into the earth. In her backyard, Tree People installed not one, but two 1,700-gallon cisterns. The two green rectangular boxes huddle against the side fence, collecting water through a drain spout that runs down from the roof. The yard looks pretty much the way it did before Tree People arrived, except that the flower and shrub beds have been more amply beefed up with mulch. The cost of all this was about $10,000, paid by the county and a bond initiative.

The first day Tree People tested the site, workers lugged what looked like fire hoses to the top of Ms. Hall's roof. Officials in suits stood by, watching from the safety of their umbrellas. Someone gave the word, and the water gushed from the hoses, soaking the guys on the roof and spewing a gentle, simulated rainfall over the entire house. Two tons of water came down that day, and in the end, none of it ended up in the street. Mission accomplished.

Andy Lipkis, the grown-up Tree Boy who is neither a scientist nor an engineer and refers to himself as a college dropout, stood in the backyard. Thirty years ago, he got his start inspiring people to care enough to plant a single tree. The idea was so simple that it could not be ignored. His ideas are still just as powerful: Plant a tree. Take ownership of it. Be bold enough to remake the place where you live into a little piece of nature. A little watershed.

Now, outside the Hall residence in Sun Valley, the smile on his face grew wider and wider.

All our cities were built without understanding nature, he said, this is about working in partnership with nature.

Seven thousand, nine hundred and ninety-nine more houses to go.

Today, thanks to Andy's urging, LA city and county water supply and flood control departments are doing a better job of talking to each other and integrating their policies. Watershed is one of their new buzzwords. Right now LA's watershed extends as far as Utah and as north as Montana. Rain or snow that falls within that huge swath of land can and does end up in LA's water system. But imagine if you scaled up Andy's idea and brought it to the rest of the 9.5 million people living in Los Angeles County? Or shared the idea with the greater LA region, with Orange, San Bernardino, Riverside or Ventura counties, too? You could effectively address the massive green space problem that plagues Los Angeles. You could worry a little less about finding vacant land for parks because the city itself would become a vast garden, protecting and replenishing the watershed upon which it sat. And the current multi-state watershed could shrink to the size of LA’s own footprint.

Imagine what an example that would set for other cities that face similar issues of sprawl, such as Las Vegas, Denver, San Diego, and host of others.