Concise term for Native Americans of California

Solution 1:

The word you are looking for may be indigenous, which is better known than the equivalent autochthonous. Both are used, as the following pair of OED citations prove:

• 1646 Sir T. Browne Pseud. Ep. ᴠɪ. x. 325 ― Although··there bee··swarmes of Negroes serving under the Spaniard, yet were they all transported from Africa··and are not indigenous or proper natives of America.
• 1860 Sat. Rev. X. 149/1 ― Most of them [the Red Indians] believe themselves to be autochthonous.

So you could speak of the indigenous or autochthonous inhabitants of the present-day American state of California. There are related nouns for each, respectively indigene and autochthon. You might have Californian indigenes, or even Californian autochthons for maximal opacity.

But all these are just fancy words for native, so you might want to try to find a way to use the normal native.

BTW, the indigenous peoples of California Alta and of California Baja appear to differ substantially, so you should probably also make sure people know which people, and place, you’re talking about. They were all (reasonably) native to the Americas, and to North America in particular, so could rightly be called Native Americans — even the ones in the Mexican half of that equation.

Solution 2:

Among anthropologists, I think the term "Precontact indigenous peoples of California" covers those natives at least as far back as 17,000 BC but prior to contact with people from outside North America.

You might also refer to the tribal populations of California. This would include current residents known as native Americans. There are reportedly over 100 "native" tribes in California (and nearly every one of them operates a casino!).