What is the main subject of this conversation?

M: Have you ever visited a redwood forest? I recently had chance to go to Muirwoods National Monument, north of a Francisco.
W: I’ve never seen a redwood tree. I really can’t imagine how big they are.
M: The coastal redwoods are the tallest living things. Some are more than three hundred and fifty feet high. But none of the trees at Muirwoods is that high. You have to go further north in California to see the tallest trees.
W: You said that Muirwoods is near San Francisco? I guess it must be quite a tourist attraction.
M: Yes. It’s less than an hour’s drive away, so it’s easy to get to.
W: I’ve heard that many redwood trees are thousands of year old. Are the ones in Muirwoods that old?
M: The oldest documented age for a coastal redwood is more than two thousand years. The trees at Muirwoods are 400 to 800 years old.
W: Why have they survived so long?
M: They have remarkable resistance to forest fires. Their tough, thick bark protects the trees during a fire. The coastal redwoods also like a damp, foggy climate.
W: Then since Muirwoods is near foggy San Francisco, it must be ideal for the trees survival. I can’t wait to go there and see them.