Sentence:  Hybrid 800 was an <WSD>arbuscle</WSD> only 1.5 m in height at the age of six years because of suppression by adjoining poplars and honeylocusts. ... Further observations have shown that plants subjected to summer pruning when young suffered little from frosts. They continued to grow as <WSD>arbuscles</WSD> and no longer needed summer pruning. Some are now no less winterhardy than the most winterhardy specimens. 
Meaning of the word marked with <WSD> tags:  A plant midway in height between a shrub and a tree; a dwarf tree.