they are stuff. almost nothing can kill them. >> but climate change is killing that. shrinking the giant sequoias' 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 fire. >> reporter: said to be the fifth largest tree in the entire world, it's lived more than 3,000 years. and yet, we are seeing 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 that one fire alone. >> reporter: started by lightning in august 2020, the castle fire was part of the sequoia complex that burned more than 174,000 acres, scorching several sequoia groves.