NARRATOR：

Now listen to part of a lecture on the topic you just read about.

MALE PROFESSOR：

Unfortunately, we still don’t know what’s killing the yellow cedar.

None of the explanations discussed in the reading is adequate.

First, the cedar bark beetle. Well, the problem with this explanation is that healthy yellow cedars are generally much more resistant to insect infestation than other tree species.

For example, the bark and leaves of the yellow cedar are saturated with powerful chemicals that are poisonous to insects.

So, healthy cedars are unlikely to suffer from insect damage.

So how can we explain those dead cedars that were infested with beetles?

In those cases, the beetles attacked trees that were already damaged or sick and would have probably died anyway.

So the beetles are not the fundamental cause responsible for the decline of the yellow cedars.

Second, although bears damage some trees, they’re not the cause of the overall population decline.

Yellow cedar populations have been declining all across the northwestern coast of North America, both on the mainland and on islands just off the coast.

There are no bears on the islands, yet the island cedars are still in decline.

Since the decline occurs with and without bears, the bears cannot be responsible.

And finally, the theory about the roots suffering from frost damage.

Well, the reading passage forgot to take one fact into account—many more trees are dying at lower elevations, where it is warm, than at higher elevations, where it is cold.

If freezing damage were responsible for the decline, we would expect to see more trees dying in the cold weather of high elevations.

Instead, more trees are dying in the relative warmth of the low elevations.

So, although the climate change may have made the cedar roots more sensitive than they used to be, this isn’t what’s killing them.