So the tree data itself mainly comes from municipal open data, just like yours does. Street Trees datasets are pretty common across cities. I just added SF yesterday after replying here :)
Otherwise the map tiles are coming from OpenFreeMap [1] which are indeed based on OSM.
Next steps I'm interested in are including economic + ecological benefits of the trees, highlighting potential pests / invasive species, maybe some other basic info about the species sourced from Wikipedia.
I like how you've got different icons for different types of trees; I've been thinking about how to encode DBH data as well but haven't settled on anything yet.
[1] https://openfreemap.org