What does this mean?
“If less vulnerable large herbivores such as deer are unable to replicate the seed dispersal role of threatened mega-herbivores such as elephants,” McConkey said, “megafaunal fruit could suffer range contractions, affecting forest community composition and potentially forest carbon stocks.”

What does "forest community composition" mean?
And what does "potentially" mean here?

Thanks!

Question

What does this mean?
“If less vulnerable large herbivores such as deer are unable to replicate the seed dispersal role of threatened mega-herbivores such as elephants,” McConkey said, “megafaunal fruit could suffer range contractions, affecting forest community composition and potentially forest carbon stocks.”

What does "forest community composition" mean? 
And what does "potentially" mean here?

Thanks!

Answer

The forest community composition is the types of plants that make up the forest.

If other animals are not able to take over the role of the threatened species that normally spread the seeds, there will be less variety of plants in the forest. Potentially means "possibly" in this sentence. Forest carbon stocks could possibly be affected.

Answer

Forest community composition = different types of trees that grow in the forest

potentially = possibly