Are you talking about forest cover - the green shading that indicates forested areas? If so, you will find very few maps on this site with that feature. Data is not readily available for this in most places, and where it is, it makes the map much larger.
I have been including forest cover on my own maps, here in NJ the state offers a really good landcover/land use dataset. DE and MD also have pretty good datasets. But note that these are three of the smallest states in the US, so it is a less formidable task to map this data.
There is something called the "National Land Cover Dataset" which covers the entire US however. It was created by the USGS from LandSat imagery, but is rather low resolution. So it isn't nearly as good as datasets that the states have created by actually tracing the outlines of vegetated areas. It's also raster imagery (a picture, basically) and not immediately usable in a Garmin format map.
So, my map of New Jersey has forest shading for the whole state and I have been working on a map of the Middle Atlantic region using a technique I developed to generate it from the LandSat imagery. I sort of doubt I'll ever finish that map though, it's very time consuming and I have too many other interests.
Sorry, none of this is going to help you with California though.

Garmin does not include forest cover on their own commercial maps either. I always thought this was a big omission, but it is what it is. As I said, it makes a map MUCH bigger.
About the only option I can suggest is Garmin's Birdseye Topo product. These are scans of the "Classic" USGS 1:24000 Topo Maps. They are available by download for an annual subscription of $30 (but you can keep what you download after it expires). Of course, the USGS stopped making these maps a long time ago, so they are going to date back 20 years or more.
https://buy.garmin.com/en-US/US/prod98816.html