>>2650451USDA search shows 412 tree species in the lower 48. When you repeat this search under the heading "characteristics" search, you recieve a result of 1495, however if you scroll through, you will see that they have included subspecies in this result. For example they list four subspecies of rocky mountain maple in this characteristics search list- but only one rocky mountain maple in the broad search. Rocky Mountain Maple does not constitute four species.
Also about the article you sited:
>a woody plant with usually a single stem growing to a height of at least two meters, or if multi-stemmed, then at least one vertical stem five centimeters in diameter at breast height. Additionally, we use these parameters only as they apply to a species' habit in its contiguous US distribution, and therefore, we excluded species that grow above 2 m outside of the United States but under 2 m within the contiguous United States. The definition of a tree used for this checklist includes treelike monocots that technically lack true wood (secondary xylem) but form arborescent structures through other types of secondary growth, as seen in palms and yuccas.>2m, palms and yucca etc. So basically the global tree assessment expands the definition of tree to include (admittedly) non-trees to assess as many at risk plants as possible. The inclusion of shrubs, and yuccas is obviously going to skew the number of species in the western states because literally most of their vegetation is shrubs and have numerous species of yucca.
