Global Warming and Pine Beetle Damage
On an environmental science field trip to the Cibola National Forest east of Albuquerque, the students noticed a large number of dead pine trees. The picture shows one that had been cut down, with the wood looking as if it had been shot full of holes with buckshot. The park ranger explained that the trees had been killed by the pine bark beetles which, because of climate change, have been extending their range to higher elevations in the park. The tree’s natural defense against the beetles is a secretion of pitch which forms a gooey protective barrier against beetles. The pine beetles do the most damage to mature trees that grow slower and produce less pitch, particularly when stressed by warmer temperatures and drought.
The quarter inch long beetles bore into the bark on mature trees and and create galleries in which they mate and lay eggs. The grubs feed in the inner bark and cambium layer, enlarging their tunnels as they grow. The “S” shaped tunnels can eventually girdle and kill the tree. Mature larvae move to the outer bark and create a cell in which they pupate. When the new adults hatch, they chew through the bark, leaving small, clearly visible exit holes. Most of the pine beetle damage is to the outer layer so the trees can still be used for lumber if harvested soon after the tree dies. Wood from beetle-affected trees retains its commercial value for 8 to 12 years after death, but the value drops rapidly during the first several months, as rapidly escaping moisture causes the wood to crack. The beetles also often introduce a blue stain fungus that can be lethal to the tree. Blue staining of the wood from the fungus is considered a defect, leading to a lower market price.
Normally, mountain pine beetles spend a winter as larvae in trees before emerging as adults the following summer. In the last two decades at the Colorado University Mountain Research Station, mean annual temperatures were 2.7 degrees Fahrenheit warmer than they were in the previous two decades. They found “The number of spring days above freezing temperatures increased by 15.1 in the last two decades, and the number of days that were warm enough for the beetles to grow increased by 44 percent since 1970.” This allows the beetles to produce an extra generation during most summers, leading to a rapid increase in the beetle population. The current bark beetle epidemic is the largest in history.
The bark beetles are a factor in wildfires as the large number of dead trees provide fuel for the fires. The needles stay on the trees for 2 or 3 years, contributing to canopy fires. After the needles fall, the bare trees contribute less to fires, but then they allow more sunlight through which melts the snow cover earlier and encourages the growth of underbrush, providing more fuel for wildfires. Compared with decades past, the traditional fire season now lasts two months longer in the West. The loss of trees also leads to earlier melting of snow which causes earlier runoff, contributes to heavier spring floods, and makes less water available later for streams and irrigation.
Because of global warming, the bark beetles have extended their range northward and to higher elevations, and their population is growing. Andrew Nikiforuk, a Canadian journalist, has chronicled the plague of bark beetles that in the last quarter-century has killed more than 30 billion pine and spruce trees from Alaska to New Mexico. Though there are other factors, such as how the forests are managed, climate change is by far the greatest factor. Trees stressed by heat and drought can’t mount a strong defense against the beetles, whose numbers grow exponentially when they produce more than one generation per season. In one area in Canada, the bark beetle hatch produced a cloud of the insects large enough to show up on radar.
And it is not just in the West. The southern pine beetle is now spreading through New Jersey’s famous Pinelands. According to the article, “The tiny insect, firmly entrenched, has already killed tens of thousands of acres of pines, and it is marching northward. Scientists say it is a striking example of the way seemingly small climatic changes are disturbing the balance of nature. They see these changes as a warning of the costly impact that is likely to come with continued high emissions of greenhouse gases.” Though New Jersey has warmed by about 2.3 degrees Fahrenheit over the past century, it is the winter low temperatures that are most important. Nights must get to about 8 degrees below zero to kill the beetles. The New Jersey climatologist’s office calculates that such bitter nights used to happen several times per decade in the state. The last night that cold in the Pinelands was in 1996, and the beetle outbreak was first noticed five years later.
Though global warming usually focuses on high temperatures, the graph shows that there is also an increase in the average nighttime temperatures. The coldest nighttime temperatures are the main determinant in the geographic range of bacteria, fungi, insects, and invasive species. Those who do not wish to address the problem of global warming often ignore those small things, one of which is the bark beetle. Besides the economic loss of billions of trees, there is an aesthetic loss in the beauty of the forests. The tree loss contributes to wildfires, problems with water management, and loss of food and habitat for many species that live in the pine forests. Those who claim it will cost too much to reduce our carbon emissions, do not consider the cost of not addressing the issue. The pine bark beetle is an example of but one of the many costly things that should be included in their balance sheets.
(c) 2013 J.C. Moore