Yes it is a verb. The sentence is written in a style typical of American news headlines, using a comma in place of 'and'. Written out in full, it would read as: "Elephants plant trees and play a big role in forest structure". The sentence has two verbs. Following Perkeo's suggestion, a compound noun meaning 'trees planted by elephants' might be 'elephant-planted trees', definitely not 'elephant(s) plant trees'.

You need to read a little more to understand that sentence properly.

QUOTE

Elephants play a disproportionately large role in spreading the seeds of a particular tree in Thailand, according to a recent study. Its findings indicate that a forest’s composition is likely to change as certain kinds of animals disappear.

UNQUOTE

As you can see, they are talking about trees whose seeds are dispersed by certain kind of animals. In particular they are talking about large animals, among which elephants and their big role in the spreading of seeds.

yes it is a verb, like place (a seed,bulb, or plant) in the ground so that it can grow.