Units that are in trees are considered to be "in cover". Any units that are in cover only have a 70% chance of being hit. Keeping more vulnerable units, such as Marines, under the cover of trees greatly increases their life expectancy and allows them to hold their own against more powerful units.
Units attacking from lower ground to higher ground only have a 70% chance to hit. Also, units on lower ground cannot see higher ground but will counterattack any units that are attacking from higher terrain (even if the higher terrain is not visible.) Using units with longer ranges, such as Siege Tanks, on high ground areas gives them a better field of vision and makes them much less vulnerable to attack.