According to https://confluence.atlassian.com/display/CONF35/Pagetree+Macro#PagetreeMacro-AllParameters you can do the following:
Specifying the Parent Page by Name
Use the following code to specify a page name for the parent or root of the tree. The tree will include all children and grand-children of the specified root. The tree will not include the specified root page itself.
=> Try to just enter the pagename at "Root Page" in the pagetree admin area.
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