Tpo 23

The article proposes three explanations to illustrates some reasons for declining of yellow cedar trees in Northwestern of the North America. However, the lecturer challenges these theories and avers that none of those are adequate, also he mentions that it is better to confess that we don't know the real reason of this disappearnce of yellow cedars.
First, the professor states that although the insect parasites could damage the trees, the healthy yellow cedar are resistant to insects because of a poison in its leaves and barks. This hazardous chemical is in a high enough concentration which can kill the parasites. In addition, parasites can only harm the weak and sick trees. In account of this parasites could not be the root cause of this yellow cedar decline. This counter argument clearly rebuts the author first proposal.
Second, the lecturer points out even the bear has some negative effects on the yellow cedar tree, but it is not the main factore in this tree's decrease. The spokesman asserts that this decrease in number of trees has happened not only in the main land of Northwestern of the U.S. but also it has happened in the islands far from the shore, where bears do not leave. If the bear cause this problem for the tree how we can justify trees' decline in islands without bears. Thus, this theory about bears does not hold water.
Third, the lecturer explains that many more trees has died in the lower altitude, where the weather is warmer than the upper hight. He reopudiates the author's idea that climate change and seasonal and diel temperature fluctuation can be the reason for yellow cedar dieing out. He emphasize on this contrast that if the roots of yellow trees are sensitive to cold climate, the trees in the higher altitude should be damaged more than lower hight, but it has not been seen. Consequently, we cannot attribute this decline to the cold temperature, he says