Streets and Highways Code § 990
CHAPTER 2. Powers and Duties of Boards of Supervisors
Where practical or desirable, the board of supervisors shall, along any highway under its jurisdiction, possession, or control, replace trees that have been destroyed or removed because of projects undertaken to widen the highway.
Money received by county from the Highway Users Tax Fund available for the widening of highways is also available for the planting of trees pursuant to this section.