What did the lorax warn would happen if you cut down the trees?

pollution will rise and all living beings that are dependent on plant in some or the other way will get affected.

Explanation:

As per Lorax, on cutting down the trees the following would happen -

a) Pollution from industries which was earlier absorbed by the plant thereby providing oxygen and fresh air will no longer will occur.

b) All species of animals and birds that depend on plant and trees for their habitat and fooding will leave the forest area to find a second/new habitat as the tress were cut down tremendously.

c) The air in the environment will become toxic and polluted and hence not breathable.

d) Stability of ecosystem will get disturbed.

this is a good question! i'll do some research and come back when i have the answer!