Let me first address the elephant in the room: I know that saguaro aren't technically trees. However, they are described as being tree-like cactus and they happen to be my favorite large cacti, so they get to have a Fantastic Trees feature!
These gorgeous and old saguaros were found in the small "city" of Black Canyon City, just off the interstate. If you drive down the interstate from Flagstaff towards Phoenix the scenery changes pretty quickly at one point, going from forests filled with pines and cedar trees to desert dotted with giant saguaro. The elevation changes, and with it the flora with it. These are some of the northernmost saguaro, as they only grow in the Sonora Desert!
Fun fact: Not all saguaro cacti grow arms, but when they do grow their first arm they are usually around 100 years old!
P.S. Saguaro look almost exactly like the cactus emoji 🌵🌵🌵!