In 2010, an estimated 675 million cubic metres of timber had been killed by mountain pine beetle in British Columbia. This mass destruction of forest becomes a feedback into the climate system because trees act as carbon sinks, storing carbon as they grow. The losses inflicted upon forests by the beetle have removed the future carbon sequestration potential of a multitude of trees.

Fortunately, after the dead trees' needles have dropped, sunlight can reach the forest floor and promote new growth, which stores carbon in the process. Unfortunately, needles often remain on the tree for a few years after death. Unless the dead trees are logged, it is difficult for new trees to grow and begin storing carbon anew.

Question: What are two ways that dead trees can contribute to atmospheric carbon dioxide concentrations?

Trees killed by mountain pine beetles begin to decompose, emitting the carbon that was stored inside of them during their lifespan. Dead trees and needles contain less moisture than their healthier, living counterparts. Combined with the recent drier climate in many beetle-infested forests, this is thought to increase the likelihood of wildfire. However, there is still some disagreement as to whether dead pine trees are actually better fuel sources than living pine trees. But, dead pine trees that still have their needles are believed to be good conductors for some types of fire. Although fires are a natural and often beneficial disturbance that regulates the health of an ecosystem, they also release the carbon that was stored within the forest.

Question: Briefly describe the mountain pine beetle feedback loop in your own words.

Congratulations on completing Lesson 7! Continue on to Lesson 8 to learn about the vital interactions between climate and the oceans.