BERKELEY — Plant expert Glen Schneider say he feels a sense of natural wonder hiking past the many varieties of wildflowers, trees and shrubs on a ridge trail cooled by San Francisco Bay fog at times and baked by inland valley sunlight at others.

He says the diverse Skyline Gardens trail is magical but fears the magic is in danger if the East Bay’s largest water district proceeds to open the 1.25-mile long trail segment to mountain biking.

“It should not be trampled, eroded by noisy mountain bikers,” said Schneider of the California Native Plant Society and coordinator of the Skyline Gardens project. “You also have to worry about the safety of putting mountain bikers on the same narrow trails as hikers and equestrians.”

His group, the Sierra Club and other environmental and equestrian groups are rallying to derail a proposal by the East Bay Municipal Utility District to open some of its backcountry watershed trails to mountain biking for the first time.

Hikers and horse riders are allowed on a network of mostly dirt trails on 28,000 acres around water reservoirs in Contra Costa and Alameda counties with some of the most scenic and secluded territory in the East Bay.

Mountain bikes are not allowed on the trails — with the exception of some highly developed recreation areas such as the Lafayette Reservoir.

“Their policy is outdated and unfair,” said Mike Udkow, a retired Oakland doctor who heads the Bicycle Trails Council of the East Bay, an advocacy group for mountain bikers.

In response to cyclists’ requests, the water supplier for 1.4 million East Bay residents is considering a two-year pilot project to allow mountain biking on four watershed trail segments covering about 10 miles along the Bay Area Ridge Trail near Berkeley, Oakland and Pinole.

“We are looking at pilot project options that allow mountain biking without compromising our highest priority — protecting drinking water quality for our customers,” said Andrea Pook, a district spokeswoman.

None of the four trail segments is debated as sharply as the 1.25-mile portion that conservationists call the Skyline Gardens trail, which parallels Grizzly Peak Boulevard between Tilden Regional Park and Fish Ranch Road.

Many of the 100 people who attended a public meeting on the proposal in Oakland last Monday had vocal opinions on both sides of the issue.

“It would be a mistake to allow bicyclists on this narrow, single-track trail where fast-moving cyclists could collide with people on foot and horse,” said Helen Burke, a former EBMUD board member opposing the mountain bike access.

Coalition members said they have measured 11 sharp, blind turns on the trail where a descending cyclist would have little room to spot hikers or horse riders around the bend.

In response to the safety concern, EBMUD officials say it may be possible to rebuild portions of trail to eliminate blind or steep segments.

Native plant experts say they have identified 233 species along or near the trail, including the rare leatherwood, with bright and delicate yellow blooms in February that are likened to tinsel.

Leatherwood grows nowhere else but a few places in Bay Area woodlands, Schneider said.

On a recent hike there, he pointed out many plants, ferns, bay and oak trees as well as striking views of Mount Diablo to the east.

The Skyline Gardens area, he said, is a botanical mixing pot of diversity because it straddles the line between climates near wet San Francisco Bay and the arid Lafayette and Diablo valleys.

“We call it the garden trail because you can enter many different areas that are like different natural gardens, with a shade canopy in one place and then sunny areas with rocks around a curve,” Schneider said. “Hikers can be shivering in the fog, and then baking in the sun a few minutes later.”

Udkow, the bicycle coalition leader, said that while he wants to end the ban on bikes, he has become sympathetic to arguments for keeping mountain bikers out of the Skyline Gardens trail.

“It is a steep and environmentally sensitive area,” Udkow said.

He suggested the water district look for another, less-sensitive nearby route as an alternative to close the gap in the Bay Area Ridge Trail, which will encircle the bay when complete.

Meanwhile, Udkow said he believes EBMUD should let mountain bicyclists use the other three proposed trail areas.

“It’s a major change on EBMUD’s part to relax their trail ban on mountain biking,” Udkow said. “You start with baby steps.”

Water district officials say they don’t have a specific timetable to decide whether and where to allow mountain bikes for a trial period.

Before a pilot project, the water board will consider a watershed-management plan that allows exceptions to the bike ban, said Pook of EBMUD.

The watershed plan could come before the water board by the end of the year, but the decision will be postponed to well into 2017 if the district decides to do an environmental impact report on plan alternatives, she added.

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff or facebook.com/denis.cuff.