'arborsculptuor' is the term generally used to decribe branch topiary
although their are other words that are used to describe this art:
pleaching, permaculture, tree trunk topiary, botanical art,
botanical architecture, biotechture, living art, visionary art,
and... really weird.

seen here are works by richard reames (top 2 images)
dr. christopher cattle (bottom 2 images).
find out more about this unique art form in the
designboom article: ' my tree grew an inch this year '

via designboom