American university's short tree or shrub (6)

'shrub' is the definition.
The answer and definition can be both plants as well as being singular nouns.
Maybe there's an association between them I don't understand?

'american university's short tree' is the wordplay.
'american university's' becomes 'corn' (I can't justify this - if you can you should believe this answer much more).
'short' means to remove the last letter.
'tree' becomes 'elm' (elm is a kind of tree).
'elm' with its final letter taken away is 'el'.
'corn'+'el'='CORNEL'

'or' acts as a link.

Can you help me to learn more?