Why are there so many dead trees?

Dead trees, with Sunset Crater Volcano in background
Bettle-killed trees at Sunset Crater Volcano.

  NPS photo by Dallas Larsen

“Why are there so many dead trees?” has become a common question from park visitors.Entire hillsides of trees, green just weeks ago, are now rust-brown. Around Flagstaff, ponderosa pines are the most obviously affected. Elsewhere, at lower elevations, pinyon pines and junipers are also dying.Across the west, hundreds of thousands of forested acres have been transformed. The killer is the pine bark beetle - a tiny insect that is always here, but seldom noticed. Now, through a combination of extreme conditions, there are vast numbers of these insects, and they are successfully attacking vast numbers of trees.

Although the dead trees have suddenly become apparent, the problem and two underlying causes have been brewing for some time: there are too many trees, and they are too dry.

The ponderosa pine forest that blankets northern Arizona once burned regularly, with low intensity fires that kept it open and park-like. During the past century, when most fires were extinguished, the forest became crowded with an abnormal number of trees and underbrush. Until now, the major concern has been fire – and recent years have seen widespread, dramatic, destructive forest fires.

To compound the problem, Arizona is in the grip of the worst drought ever recorded here. For several years, summer rains and winter snows have been far below normal. By the summer of 2002, live standing trees contained less moisture than the cut lumber for sale at your local hardware store. Arizona forests are adapted for growth in dry conditions. But not this dry and not for this long.

Forest crowding and drought cause stressed and unhealthy trees. But these conditions are ideal for bark beetles, which enter through the bark to lay their eggs. A healthy tree fights back - its resin ejects the insects before harm is done. Because our dry, stressed trees are producing little or no sap or resin, beetles can enter freely.Their many eggs hatch into larvae, which feed on the inner bark, killing the tree. The beetles emerge as adults and fly off to infest additional trees.

Even if the drought were to end this year, signs of the bark beetle assault would continue. Many infested trees are not yet dead, but they will be; there is no known treatment. The impact of the beetles will be with us for centuries to come. The result may be the largest landscape-scale change most of us will ever see:the end of the vast pine forest. The aesthetic, ecological, social, and economic consequences are hard to imagine.

For more information:

University of Arizona Cooperative Extension

Interior West Bark Beetle Project

The Arboretum at Flagstaff

Last updated: February 24, 2015

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