IMO the author is referring, in an abstract sense, to the triangular gussets that are often used to reinforce perpendicular joints. This will probably need some adaptation for English-speaking readers, as I doubt that most of them will immediately recognise gussets at the junction of a tree trunk and its roots (as in the given example).
see e.g.
Build the gusset plates with the ...
512 x 336 - 16k - gif
www.strand7.comGusset - Wikipedia, the free encyclopedia
1 Dec 2008 ... A gusset is a device, often triangular, used to reinforce a connection between two components. Gussets are commonly used in engineering, ...
en.wikipedia.org/wiki/Gusset