It is hard for me to accept that there can be a mature forest that does not provide shade, but the saguaros qualify as forest and let most of the sun get to the ground. Saguaros are remarkable organisms. They grow slowly and can reach an age of 200. They grow only in the Sonoran Desert and are well adapted to the limited & seasonal rainfall. They soak up water when they can and save it for the dry times which always follow.
The saguaros take a long time to get established. They often grow under the shelter of mesquites or palo verde trees. Later they return the favor by soaking up much of the water and out competing the nursery trees.