Alnus rhombifolia - White Alder

Common CA native riparian tree, not commonly planted but found here and there. The alders are medium large trees growing to 50' with an upright pyramidal or spreading habit. Usually found growing in groves so they don't form a wide spreading shape. Below is a nice pair at the Polo Grounds in Aptos. 

Leaves are deciduous, alternate, simple, rhomboid usually but broadly ovate to elliptical as well, 4" long, serrated to doubly serrated, dark green upper surface, lighter green below. Nice venation pattern.

Flowers are monoecious. Males overwintering as catkins elongating into 4-6" long inflorescences, generally from lateral branches. Females small, 1/4" long, with a reddish tinge.

Fruit is a cluster of nuts resembling a cone. Smaller than the other alder found here, only 1/2 -3/4" long.

Bark is smooth gray with interesting branch collars. 

Misidentification:
Alnus cordata perhaps. However the leaves are different and the size of the "cones" is different.

Location:
Aptos
Soquel Drive but the address is likely Aptos Hills Lane.
Polo Grounds County Park - at the Polo Dr. end

Soquel
Soquel Creek - almost anywhere along the creek but easy to see on the walking bridge from Main St to the elementary school.