I am surprised that you see hackberry in Northern California. Not exactly a tree that would be planted and the only species that I know of that naturally grows that far west is pretty rare out there, not that I know that much about other hackberry species.
There is a native species of Ash that grows out there. In fact, it is sometimes called California Ash. It could also be a non-native species that was planted or was from one nearby that someone besides the city planted. The seeds spread and sprout fairly easily. Before the emerald ash borer started becoming a problem, Ash tree were widely planted by cities.
Below is a picture of the seeds. Most Ash trees have seeds very similar to this. The leaves are also fairly distinctive as well. The buds on the end of that stem in your picture above are a little atypical for ash in my experience but that could be normal on whatever species that is.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
