Most “octopus” trees that I encountered, are Sitka Spruce trees that are growing atop a stump of a logged tree. Here in the coastal environs of northern California and southern Oregon the stumps are usually redwood or spruce. In my opinion, the best examples are of trees that rooted atop an old spruce stump. The spruce stumps are less resistant to rot than the redwood, and so they can decompose under the growing tree that is on it. Eventually the stump will decompose into the forest floor, leaving the spider tree as its memorial. If the tree that grows atop a redwood stump is a spruce, it may not last as long as the stump, and so are less common to my eyes*.
*I am not an expert, but these are my observations.