Is the collective noun of trees a herbarium?

Answer 1

The noun herbarium is a word for a collection of dried plant forms or a work of classification of plant forms, and a room where such collections are kept.

The collective nouns for trees are a forest, a grove, an orchard, or a stand of trees.

Answer 2

There is no standard use of the noun herbarium as a collective noun, however, collective nouns are an informal part of language. Any noun that suits the situation can function as a collective noun, for example, a herbarium of specimens, a herbarium of flora, a herbarium of spices, etc.