Original: This means that for every tree that meets the 6D ’ s criteria and needs to be removed, it is replaced with a species of tree that is better suited to a street environment.
Simplified: If a tree meets the set of conditions known as 6D, it is [allegedly] necessary to cut it down.  If this happens, a new tree will be planted.  The new tree will [allegedly] be chosen to suit the location better than the original tree.