LOS GATOS — A man was seriously injured in a mountain bike crash at a Los Gatos preserve on Tuesday, authorities said.

Firefighters were dispatched to St. Joseph’s Hill Open Space Preserve at 1 p.m. for a report of a mountain biker down on a trail, according to the Santa Clara County Fire Department.

The man crashed while riding downhill on the Manzanita Trail, said Leigh Ann Gessner, a spokeswoman for the Midpeninsula Regional Open Space District that manages the 270-acre preserve. Two other mountain bikers found him and called 911.

Gessner said the man was wearing a helmet.

Firefighters called for a medical helicopter after learning that the man’s injuries were serious, according to the fire department. He was airlifted from the preserve to a trauma center just after 2 p.m.

Open to all uses, St. Joseph’s Hill Open Space Preserve is popular with mountain bikers, said Gessner, adding that rangers use radar guns to enforce the 15 mph speed limit on trails.

Gessner said mountain bikers are urged to wear helmets, stay on trails and ride with a partner.

“A lot of people use and enjoy the preserve every day,” she said, “and we want them to do so safely.”