>> reporter: much about giant sequoia trees is on a grand scale. with that distinctive red/brown bark covering their thick trunks, sequoia trees can only be found in california's sierra nevada mountains. >> this is a resilient tree. they are tough. almost nothing can kill them. >> reporter: but climate change is changing that. shrinking the giant sequoia's footprint. >> a giant sequoia that was first weakened by drought was then subject to impacts by the bark beetle, which then further weakened the tree and potentially made it more susceptible to mortality from fire. >> reporter: the stag tree is said to be the fifth largest tree in the fire world. it's lived more than 3,000 years. and yet we're seeing that wildfire is threatening these giant sequoias more than ever before. >> the castle fire was a wake-up call, an estimated 7,500 to 10,600 trees were destroyed in