Bark (or mountain pine) beetles have killed billions of trees across western North America. Since forests are large reservoirs of terrestrial carbon, it is important to measure how beetles impact carbon stocks. However, the severity of bark-beetle-caused tree damage often varies greatly within outbreak areas, so it can be difficult to quantify such a phenomenon. Because they are limited to quite small areas, conventional plot measurements can provide only limited information on how severe a beetle attack is. Remote-sensing techniques could come into their own here and are proving to be a promising alternative.

To better measure the effect of a bark-beetle outbreak on aboveground forest carbon stocks, we combined two remote-sensing methods and applied them in a beetle-infested forest in central Idaho. We used digital aerial photography to map tree mortality, and light detection and ranging (LiDAR) to measure aboveground tree-carbon stocks. By overlaying these maps, we were able to estimate the amount of carbon in trees killed by bark beetles.

As expected, we found a large variability in how severe tree damage was. Overall the tree mortality area was 39%, varying from no mortality in certain regions to 96% mortality in others.

For most locations, we discovered that at least 25% of aboveground tree carbon was in killed trees. Locations where more than 75% of aboveground tree carbon was in killed trees only made up a small part of the landscape. Despite this inconsistency, we found that beetles greatly impact aboveground carbon stocks across the entire landscape and that half of the total aboveground tree carbon was in trees killed by beetles.

Bark beetles affect many forests across western North America, yet the regional impact on the carbon cycle as well as other ecosystem processes is still unknown. We are continuing to develop methodologies for quantifying the effects of bark-beetle outbreaks on forested ecosystems in order to shed more light on this important phenomenon.

The present work is published in Environmental Research Letters (ERL) , as part of the ERL Focus on Extreme Events and the Carbon Cycle.