There’s no such thing as a tree phylogenetically (2021)
Humans care about wood, so they made tree (thing which produces wood) a category, as it can be used to build bridges, ships and other structures.  Wood being dense also made it a better feedstock for charcoal (one can use grass, however).<p>As far as why trees don't seem to evolve into grasses, (or grasses into trees), it might be all about C3 vs C4 carbon metabolism, the latter being a complex structural system that pre-concentrates atmospheric CO2 before feeding it into the photosynthetic biochemistry (which is the right way to do artificial photosynthesis as well).  This is apparently difficult to do in trees?<p><a href="https://pubmed.ncbi.nlm.nih.gov/32409834/" rel="nofollow">https://pubmed.ncbi.nlm.nih.gov/32409834/</a><p>> "Since C4 photosynthesis was first discovered >50 years ago, researchers have sought to understand how this complex trait evolved from the ancestral C3 photosynthetic machinery on >60 occasions. Despite its repeated emergence across the plant kingdom, C4 photosynthesis is notably rare in trees, with true C4 trees only existing in Euphorbia."<p>And here we have the world's largest Euphorbia:<p><a href="https://www.wikidata.org/wiki/Q5851367" rel="nofollow">https://www.wikidata.org/wiki/Q5851367</a>