SANTA ROSA PLATEAU : History Lives 500 Years Later : Conservancy reserve has 15 miles of trails for hikers, bikers and other nature lovers.

As is most open acreage in Southern California, the Santa Rosa Plateau is surrounded by mile after mile of fencing. Here, however, the fences are not designed to keep valuable cattle from wandering off the property. Rather, their purpose is to ensure that cattle never get in.

From dawn to dusk every day of the year, the reserve’s 15 miles of trails and roads are open to hikers, bird watchers and other nature lovers. Cows, however, had damaged the area for nearly 200 years with their grazing habits and were ousted forever when the national Nature Conservancy bought the property in 1984.

A few miles west of Temecula in the Santa Ana Mountains, the 3,100-acre Santa Rosa Plateau Ecological Reserve offers visitors the rare chance to see what California looked like 500 years ago. Docent-led walks, horseback riding and mountain bike tours are some of the ways to experience this unique reserve.

Because it is a reserve and not a park, strict rules apply to use. All visitors must stay on trails and dirt roads at all times (bicycles and horses are permitted on roads only). Picnicking and camping are not allowed, although a picnic area is in the planning stages.

Last year the conservancy acquired an additional 3,825 acres in a cooperative arrangement with Riverside County and state wildlife agencies. The new area, which will include a visitors’ center and parking lot, will be open for public use in April, 1993, said Robin Wills, director of reserve.

The reserve’s diversity of natural communities proved an irresistible purchase for the Arlington, Va.-based Nature Conservancy, whose nationwide mission is the protection of endangered species and communities through buying and managing land.

“The Santa Rosa Plateau was a priority purchase because it was identified as having the largest number of endangered natural communities in one place in the entire state,” said Gary Bell, a Southern California area ecologist for the conservancy.

These communities are oak woodland, native grasslands, sycamore-alder stream-bed woodland, coastal sage-scrub and chaparral. About half the oaks on the property are the rare and endangered Englemann, Bell said. The reserve, in fact, contains the world’s last protected reproducing stand of Englemann oaks.

Docent-led nature walks begin at 9:30 a.m. on the first Saturday of each month. These walks, by reservation only, start from the reserve’s front gate. Bird walks begin at 7:30 a.m. on the third Sunday of each month at the front gate.

On one Saturday a month during winter and spring, mountain bikers take an 8.5-mile loop tour of the reserve that includes a visit to an 1840s adobe. The rides, also by reservation only, begin at the Waterline Road entrance.

First-time visitors to the reserve should not miss the Oak Tree Trail, a self-guided, 45-minute walk that introduces many of the reserve’s features. These include mortero holes, in which Indians ground acorns; and tenajas, small ponds that provide year-round water for wildlife. Oak and stream-bed woodland as well as the finest examples of bunchgrass prairie in the entire state also can be seen, Bell said.

Some of the last remaining vernal pools in Southern California are in the southwestern portion of the reserve. These small ponds, which once were scattered throughout central Orange County, hold rainwater during winter and spring and support migrating waterfowl, fairy shrimp and a variety of endangered plants.

Quiet visitors can catch glimpses of abundant wildlife at the reserve. According to Bell, the same species that roamed the area 1,000 years ago are still there, with the exception of the grizzly bear, pronghorn antelope and California condor.

In the tenajas, look for southwestern pond turtles, which take sun baths on exposed rocks. In the fall, Hammond’s two-striped garter snake, an aquatic snake, can sometimes also be spotted. But be careful not to step on the dozens of tiny Pacific tree frogs, which hop about on the moist ground near these ponds.

Returning the reserve to its pre-grazed state, in part through prescribed burnings of non-native plants, is a primary goal of the conservancy, Bell said. For 200 years before conservationists bought it, grazing cattle trampled oak seedlings, eroded stream beds, eliminated native grasses and reduced plant diversity.

About a dozen volunteers in a “Team Stream” project are working to repair eroded stream beds and increase the number of tenajas. Twice a month they build check dams and sediment dams to collect silt upstream, Bell said.

To reach the Santa Rosa Plateau Ecological Reserve, take the Clinton Keith Road exit from Interstate 15 and drive five miles south to the front gate. For docent-led nature walks, call the main number at (714) 677-6951. For bicycle tours, call Guido Schulte at (714) 677-0555. To book a group tour, call (714) 677-7341; for bird walk information, call (714) 676-5195.