An individual oak tree pulls thousands of gallons of water out of the ground and their roots grow where there is the most moisture in the soil. Ranchers and real estate developers and vineyards and highway engineers have removed thousands of these trees over the years and the end result is an increase in landslides. A fellow working for the then California Division of Highways, now "Caltrans", did a study of hundreds of "cuts" and "fills" on highway projects in 1973. He showed that fills where soil was brought into and area along with seed material was far more stable than "cuts" where the soil was removed. Half a century later the civil engineers still ignore trees and vegetation and will stick pipes into a slope think that this will help drain out the water. One cannot fix stupid with organizations that repeat the same mistakes year after year after year and expecting a different outcome.
On the plus side, when roads are closed to motor vehicles they become ideal for bicyclists.