It is an "agricultural checkpoint" while entering the state of california. We live in phoenix and go to the dunes just west of yuma several times a year. Sometimes there is someone standing there, other times nobody there. I have been through there 50 times and never been stopped once.
If they do stop you, they ask if you have any fruit. My answer is always no. They never checked me. One time I was eating a banana while going through, they asked me if I had any fruit on board, I just laughed and said no, and was waved through instantly.
I have heard the firewood thing somewhere before too. However, I pull an open trailer with all the sand toys on it, a huge pile of firewood and all kinds of other things, and have never been stopped for bringing firewood in.
Hope this helps.