There are a tall tree and a short flower. Could I say “there are respectively a tall and a short tree and flower”
“Respectively” is an adverb meaning “in the order mentioned.” We use it when we want to emphasize the relationships between two pairs or items in two parallel lists (i.e., two lists that contain the same number of items).
When building a sentence with respectively, it can be helpful to think of two or more parallel sentences, and then combine them.
The tree is tall.
The flower is short.
The tree and the flower are tall and short, respectively.
Alice ate two slices of pizza.
John ate five slices of pizza.
Alice and John ate two and five slices of pizza, respectively.
I was born in December.
My brother was born in June.
My sister was born in October.
My brother, my sister and I were born in June, October and December, respectively.
The trouble with your example phrase ("a tall and a short tree and flower") is that the parts are in the wrong order. To use "respectively" correctly, the elements in the second list should describe, modify, or give more information about the elements in the first list. You have put the list of adjectives before the list of nouns that the adjectives describe, which is backwards.