Much of the Southwestern Region has periodically experienced severe drought conditions during portions of the past two decades, 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.
Central and Northern Arizona
Higher than normal levels of tree mortality have been reported in Arizona during the winter and spring of 2021. Most of the mortality appears to be related to drought stress and pine engraver attacks on ponderosa pines. In particular, increased pine mortality has been observed on the Prescott, Kaibab and Coconino National Forests, and along the Mogollon Rim on the Apache-Sitgreaves National Forests. Pine mortality has been concentrated in lower elevation sites, southern exposures and where trees are chronically stressed due to intertree competition and dwarf mistletoe infection. The photograph below shows some of the tree mortality.
Bark beetles are known to attack water stressed trees and tree mortality was anticipated given the low amount of precipitation received over the previous year. Attacked trees may have pitch tubes, however, during dry years only the boring dust or frass (circled in the image below) may be present.
Forest Health Protection will monitor the tree mortality with continued ground surveys and during our annual aerial surveys to determine the extent across the landscape. Results from these surveys will be available and reported in our annual conditions report.
Sacramento Mountains, Lincoln National Forest
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 from 2011 to 2013 resulting from a drought period in southern NM during this period. 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.
Above: Aerial image of ponderosa pine mortality along the western slope of the Sacramento Mountains of the Lincoln National Forest in 2013.
Sandia Mountains, Cibola National Forest
From 2012 to 2016, elevated white fir mortality caused by the fir engraver bark beetle has been widespread across the Sandia Ranger District of the Cibola National Forest, New Mexico. Although severe drought conditions have alleviated, tree mortality continues in the area. The white fir mortality has been widespread across much of the northern parts of the ranger district.
Above: Fir engraver bark beetle (Scolytus ventralis) (left) and aerial photograph from 2015 surveys of old and new white fir mortality on the north end of the Sandia RD (right).
Above: Map graphic of 2015 aerial survey results from the Sandia Mountains, showing areas with tree mortality (red shaded areas). Tree mortality has been mapped in this area from 2011 through 2016.
Effects from Recent Wildfires
Recent wildfire events, including the Wallow (2011), Los Conchas (2011), Thompson Ridge, and North Fire (2016) wildfires have also stressed trees and predisposed Douglas-fir to attack by Douglas-fir bark beetle on the Apache-Sitgreaves National Forests in Arizona as well as the Gila and Santa Fe National Forests in New Mexico. In Arizona, we have also documented increased mountain pine beetle attacks in the southwestern white pines after the Wallow Fire on the Apache-Sitgreaves National Forests, and in the Schultz Fire on the Coconino National Forest. In the Wallow Fire, Forest Health Protection has worked collaboratively with the Apache-Sitgreaves National Forests to limit Douglas-fir and southwestern white pine mortality by removing infested trees as well as hanging bark beetle pheromone interruptants in developed recreation sites and Mexican spotted owl protected activity centers.
Above: Aerial photograph of tree mortality in an area affected by the Wallow Fire.
For current information on bark beetle activity, see our most recent annual conditions report available on the Publications page.