Not all of the landmarked trees were alive when encountered and photographed. A dead tree can stand in place for many years, losing branches and mass as it disintegrates back into the biosphere. It may be horizontal on the ground as a dead log.
Perhaps it was killed by human activity, or natural disease, or old age. Careful study of the tree may give clues as to how it died.
You can tell how recently these trees died by how much the tree has disintegrated. Does it still have fine branches? Is there bark still on the trunk? It will eventually, like every other living thing, be reincorporated into the cycle of biomass.