Much of the Southwestern Region has periodically experienced severe drought conditions for the past decade, causing our forests to experience elevated water stress. As a result, several native bark beetles species attacked these drought-stressed trees, leading to elevated widespread tree mortality. When trees are healthy, and not stressed by drought or injured by fire, they are more resilient to bark beetle attacks. In addition, trees can be severely stressed in dense forest stands due to over competition for water and nutrients. Dense forest stands are also more conducive to bark beetle attacks, allowing pheromones to remain in the understory thus improving communication and providing a more suitable microclimate for beetle survival.
Several species of bark beetle, including western pine beetle, roundheaded pine beetle, and pine engravers, killed hundreds of thousands of ponderosa pine on the Lincoln National Forest, New Mexico during this drought period. Elevated tree mortality was recorded from 2011 to 2013. Tree mortality was mapped primarily on the west side of the Sacramento Ranger District and areas of the Capitan Mountains of the Smokey Bear Ranger District.
What happens once a tree dies?
Within 3 to 6 months after a tree dies, its needles will drop to the ground. The snag (a standing dead tree) will stand for a while, usually between 2 and 6 years before falling to the ground. How long a snag remains standing depends on a number of factors, but especially on the rate of decay of the tree's root system. Dead trees could pose a hazard if the path of their fall threatens structures or areas frequented by people.
What can an individual landowner do to protect trees on their property?
That depends on a handful of factors. For example, if a landowner can afford to, they could treat individual trees with insecticides to prevent future bark beetle attacks. Or, another strategy is to water trees to bolster their natural defenses. (Watering is no guarantee of protection, because a mass attack of bark beetles can overwhelm even a healthy, thriving tree and kill it.)
Another way to protect small groups of trees is to thin the smaller and less healthy trees in the group to give the remaining trees a better chance to regain enough vigor to fight off beetle attacks. Do not perform thinning unless slash can be removed, burned or chipped and dried (see following question). Thinning is the best long-term strategy for preventing bark beetle attacks and will also help to reduce fire danger on private property.
When the drought ends, will bark beetle populations return to normal levels?
Yes, but this may take some time, because trees depend on moisture to fight off beetle attacks. Because beetle populations are so high and geographically widespread, bark beetles have, in essence, a huge head start. In order for beetle populations to dwindle to normal levels across millions of acres, sufficient moisture over an extended period of time will be required.