The passage suggested three hypothesis that explain why yellow cedar have been steadily declining, but the professor in the lecture thought these are not adequate enough. 
Firstly, the professor disagree that insect parasites will cause the decline of yellow cedar. Actually, the yellow cedar can produce chemicals that's poison for cedar bark beetles. That's why most yellow cedar  will suffer the insect parasites only when it's already sick or dead. 
Secondly, according to the professor in the lecture, brown bears won't be a reason, too. The declining of yellow cedar happened both mainland of North America and also islands along the coast, however, there isn't any brown bear in these islands. 
Lastly, the last hypothesis, changes of climate, still won't be the reason. According to the professor, the climate indeed changed rapidly in nowadays, but if this really is the cause, trees that's in  higher elevation should die more than lower elevation, but it's opposite now.