There are plenty of other little details on these arboreal monuments if you take the time to look closely at them.
I had read that they were used as grave markers for members of the Woodmen of the World, a fraternal organization along the lines of the Elks and so forth. Founded in 1890, they did indeed put up distinctive tree themed gravestones for their members up until the 1920s when the costs became prohibitive. So the dating of the tombstones I found fits, most were late 1890s up to about 1910. But there are some features missing....
Several of the specimens I photographed actually had Masonic emblems on them, which would also seem to make an association with a rival fraternal organization unlikely.
Hence the mystery. If there was no link to the Woodmen's organization, why are there significant numbers of these tree tombstones? I suppose they are nice enough stylistically to be popular, but it would seem to be an expensive purchase.
Update November 2014. The Tree Shaped Tombstone topic has expanded into dozens of postings. If the subject interests you click on the relevant topic header on the right side of the page!