Urban Runoff Killing Off Centuries-Old Live Oaks

At first glance, the oak trees appear gracefully bowed in the midday sun, their long, gnarled branches gently touching the forest floor.

On closer inspection, however, their trunks are cracked, their bark is sloughing off and their leaves are brown.

Groves of coast live oaks--some two centuries old--in Gen. Thomas F. Riley Wilderness Park east of Mission Viejo are drowning in runoff from nearby housing developments.

They used to thrive along Wagon Wheel Creek, but now the lower part of their trunks are constantly submerged in water, which is killing their roots.

Trees that once stood 85 feet tall now are curled over, their branches hitting the ground and supporting their weight. Black spheres of fungus crawl up some of the trunks.

“The oaks that live right close to the creek bed are in standing water,” said senior park ranger Ginny McVickar, who first noticed the oaks were dying last year. “The roots are drowning. . . . It’s an issue we can’t ignore anymore.”

More than 14,000 acres of the state’s 18 species of oaks are lost annually because of development and other human activities, according to the California Resources Agency.

Besides urbanization, a threat from humans is air pollution. Increased ozone levels have been found to stunt oak growth.

The coast live oaks lining Wagon Wheel Creek are the victims of urban runoff. Though these oaks are doomed, building standards enacted in recent years should help protect other groves.

Stricter permits have mandated that developers plan new subdivisions with built-in mechanisms to cleanse and slow runoff, and state water officials are expected to toughen these rules in coming months.

Homeowners are also being educated to decrease their water use, and McVickar hopes that these conservation measures will become as ingrained as recycling.

Standing in Water Suffocates Tree Roots

Coast live oaks--evergreen trees that can live 250 years--are found within 50 miles of the coast from Mendocino County south into Baja California, according to the California Oaks Foundation.

Though coast live oaks can tolerate some flooding, they cannot withstand as much as other creek-side trees such as sycamores. Standing in a pool of water effectively suffocates a tree’s roots.

“There’s not sufficient oxygen and the roots essentially die, and without roots the trees will die,” said Rick Standiford, associate dean of forestry at UC Berkeley and an expert on oak woodlands.

“If it’s very wet late into the season, you can get certain fungi that throw everything out of balance and can invade the trees and kill them.”

Wagon Wheel Creek once was a seasonal stream, flowing usually only during winter storms. Now, water runs through it year-round, fed by lawn watering in subdivisions.

“Since the development that’s contiguous to the park, we’ve got an awful lot more velocity and volume of water coming through the creek,” McVickar said. “Now, there is no dry period.”

The runoff is coming from developments at Coto de Caza, Wagon Wheel Canyon and beyond. Much of the building occurred in the 1980s and into the mid-1990s, before stringent restrictions were imposed to require developers to stem or slow runoff.

Tests show that the waters in Wagon Wheel Creek contain high levels of nitrates and phosphorous--earmarks that it’s laden with fertilizers.

But experts say the nutrients aren’t occurring at levels high enough to cause the trees’ death.

Near the park’s entrance, where a storm drain from a Coto de Caza development has created a new tributary, 11 trees are dying or dead.

Nearby Urbanization Accelerating Problem

Ecosystems change constantly in nature. But Wagon Wheel Creek’s transformation has been accelerated by upstream developments.

As a result, thick canopies of oak leaves that once shaded much of the area are gone, letting in sunlight that has allowed native cactuses to grow.

McVickar said it is the cumulative effect of various developments, not individual subdivisions or homeowners, that is to blame.

“It’s not like people are maliciously doing it--it’s just the nature of civilization.”

However, McVickar does urge people to water their lawns less frequently and to landscape with native drought-resistant plants that use less water.

One way she tries to get that word out is to invite new residents to the park or to arrange picnics with homeowners associations. Another message McVickar tries to get across is that beach pollution begins upstream. Wagon Wheel Creek, she notes, flows into San Juan Creek, which empties into the Pacific at polluted Doheny State Beach.

“The solution is up here,” she said.