PAST: Warming, drought, and pine beetle infestations in Larimer County, Colorado

2/15/2019

Average daily minimum temperatures in Larimer County, CO. Source: NOAA Climate at a Glance.

Climate changes and the local environment:

Average annual temperatures in heavily-forested Larimer County have steadily increased since the 1970s. Since the rate at which water evaporates rises as temperatures warm, drought has grown more common in the county. Drought places pine trees in the county under stress, leaving them vulnerable to infestation by mountain pine beetles.

Cold winter temperatures can kill these beetles and their larvae, especially if temperatures fall quickly in autumn. Yet rising winter temperatures have permitted huge numbers of beetles to survive into each spring. Meanwhile, by suppressing forest fires, foresters in Larimer County have made forests unnaturally dense with mature trees. Since mountain pine beetles prefer big trees with thick bark, these older trees are especially vulnerable to infestation.

Map showing the amount of forests in Colorado that have been affected by diseases or insects from 1996-2015. The grey shading indicates forests that were affected from 1996-2014, and the blue color indicates forests that were newly affected in 2015. This map was produced from aerial data and provided by the United States Department of Agriculture. Source: Colorado State Forest Service.

An attack begins with a single female beetle. The beetle burrows into the bark of a pine tree and releases a pheromone signal. The signal summons more beetles, which gather and attack the tree. Once the tree has been compromised, meaning that the flow of nutrients through the trunk has been impeded, the beetles deposit their larvae and continue onward, attacking nearby mature trees. While trees have numerous natural defenses to these beetles, the stress imposed on them by warming temperatures compromises their ability to use those defenses. To make matters worse, the mountain pine beetle has evolved to be able to ingest toxic resin that acts as one of the tree’s natural defenses.

There is a cyclical character to mountain pine beetle infestations. As mature trees die and their population declines, so do numbers of mountain pine beetles. Yet rising temperatures and increasing drought have nevertheless expanded the beetle's range and overall population size. Since dead trees release carbon dioxide as they decay, mountain pine beetles are turning the forests of Larimer County and the western United States into a source of greenhouse gases that contribute to global warming. They are therefore worsening the climatic trends that have contributed to their booming numbers.

How we know:

This information is based on reliable, easy to use tools provided by the National Oceanic and Atmospheric Administration (NOAA) State Annual and Seasonal Time Series and the NOAA Climate at a Glance app. Both tools use data collected at weather stations and stored in NOAA's U.S. Climate Divisions database. This article also draws on data from a 2015 aerial survey administered by the Colorado State Forest Service.

Graph showing the acreage of Colorado forest that was infested by the Mountain Pine Beetle and the Spruce Beetle from 1996-2015. Source: Colorado State Forest Service.

Human consequences:

Colorado forests purify the air and water; generate reliable sources of wood products; and provide opportunities for lucrative outdoor recreation activities. Some 90% of Colorado's inhabitants engage in outdoor recreation per year, and everyone benefits from the crucial economic and environmental resources generated by the state's forests. As of 2017, the state has a $17.3 billion tourism industry that relies on the aesthetic beauty of forested landscapes to attract visitors. Colorado is therefore a state that depends on the health of its forests. Thus, a threat to Colorado’s environment is a threat to its social and economic wellbeing.

Perhaps the most profound consequence of the mountain pine beetle epidemic for the people of Larimer Country involves the relationship between warming, beetle infestations, and wildfires. Dry, dying trees are much more vulnerable to wildfires, and length of the annual wildfire season in Colorado has therefore increased by more than two months since 1970. In their immediate vicinity, wildfires threaten human property, livelihoods, and safety. Even at range, wildfire smoke can threaten human health, as inhalation can compromise respiratory function.

Wildfires and declining overall forest health also reduce the quality of available water in Larimer County, and across Colorado. Forests naturally filter precipitation, maintaining clear waterways, but wildfires threaten water quality by introducing toxins into streams and watersheds. Since decaying trees release nitrates, mountain pine beetle infestations also increase the concentration of dangerous nitrates in the water supply.

Larimer County officials are therefore carefully monitoring water quality in their county. Their efforts - and those of officials across Colorado - have broad significance. Nineteen states obtain their water supply from river systems originating in Colorado. Warming will impact not only the supply, but also the safety, of the water they acquire.