The lack of monsoon rains last year likely caused bark beetles and other diseases to kill trees across more than a half million acres of the Southwest, according to a U.S. Forest Service report.
The report was sourced from a combination of radar flyovers and ground surveying across 23.2 million acres of the Southwest region as a forest health status check for land managers. The survey reported a majority of the tree deaths from bark beetles were ponderosa pine and pinyon trees.
Of the areas that saw tree mortality, the Four Corners region of the Navajo Nation and Hopi Reservation saw the most widespread mortality in 2019. The Navajo Nation alone accounted for 64% of the acres where ponderosa pine tree death occurred and 69% of the acres where pinyon tree death occurred last year.
Each acre listed in the report experienced varying degrees of tree mortality, said Daniel Ryerson, a federal forest health specialist. Some acres experienced severe tree mortality while others could be seen more sparsely across the landscape, he said.
“Just because we see a hundred acres with tree mortality doesn’t mean all the trees are dying,” Ryerson said. “The four corners had varying levels of mortality, a lot of sparse reports over long parts of the landscape.”
In total, the Southwest experienced 663,500 acres of tree death as a result of bark beetles accelerated by drought in 2019, where 555,710 acres were exclusively ponderosa pine and pinyon trees.
Coconino National Forest reported 15,120 acres of ponderosa pine mortality, whereas Apache-Sitgreaves National Forest experienced the most ponderosa pine mortality throughout National Forest land in the Southwest, totaling 34,280 acres.
Authorities said the numbers show ponderosa pine trees withstood the dearth of monsoon rains, especially when compared to the weather in 2018.
In the 2018 report, 394,500 acres reported tree mortality, where just under 250,000 acres of tree death belonged to ponderosa pine in Arizona. Authorities reported 166,890 acres of ponderosa pine tree death in the state in 2019.
The 2018 report was the largest reported mortality of ponderosa pine since a drought in the early 2000s resulted in 1.7 million acres of total tree mortality in a single year, according to Joel McMillin, who led the forest health assessment in Arizona.
“That drought led to a large outbreak of bark beetles in ponderosa pine and pinyon pine and some other forest types as well. Since that time, there has not been as much mortality, we’re returning to low levels of background mortality up until 2017 and 2018,” McMillin said.
Arizona has several bark beetle species that are native to the area that decades ago caused little harm to the region. Due to increasing temperatures and more sporadic precipitation caused by climate change, these beetles have become notorious for their attacks on overpopulated and water-deficient tree stands — especially in Colorado, Washington and British Columbia.
Rich Hofstetter, an NAU professor who studies forest insects, said bark beetles are understood to be a natural part of an ecosystem where they attack dead or dying trees to provide nutrients for the rest of the forest beneath the trees. The majority of bark beetle species are geared toward a single type of tree, while some will attack multiple varieties.
Bark beetles have a short life cycle, and will last generally for three to five months depending on the season.
Many bark beetles either die or are eaten before they can make it to a host tree, Hofstetter explained. The current state of the majority of Southwestern forests has trees overpopulating the landscape, leaving less water to spread among the stands. Densely populated tree stands help beetles find host trees more easily, and less water provides trees less resources to resist beetles.
“One reason why we say we’re opening up a canopy by reducing the number of trees is the beetles have to search longer and harder to find an appropriate type of tree,” Hofstetter said.
In his mind, the Forest Service's monitoring is helpful to determine where bark beetle outbreaks are occurring across the millions of acres of the Southwest. In his mind, given the current state of our climate, he expects we will be seeing more outbreaks in the coming years.
“With climate change our forests are already getting stressed. It’s logical bark beetles are going to move in and potentially do well,” Hofstetter said. “When it’s going to happen, we don’t know, but likely it will.”