bark is a protective covering around the trunk of / branches of a tree & as the thickness of something increases , the resistance  of that something to damage will increase ->  thick bark protects a tree from damage  fire causes burning & burning a living thing usually causes harm to that living thing ->  fire causes harm to living things  thick bark protects a tree from damage & fire causes harm to living things ->  thick bark protects a tree from damage caused by fire  thick bark protects a tree from damage caused by fire & protecting a living thing has a positive impact on that living thing 's survival -> thick bark has a positive impact on a tree's survival in a fire