High on Flagstaff Mountain, a ponderosa pine had quietly succumbed during the summer of 2020, but its green needles lingered another year to blanch in the fall of 2021. The tree was now conspicuous, for its light tan needles contrasted with the deep greens of the remaining pines. Why did that one tree die?
To assign cause for a ponderosa pine death, a number of factors must be considered. Drought, mistletoe infestation, squirrel herbivory and bark beetle attack come to mind. Although a single one of these factors might kill a tree, the factors interact so that the impact of one factor predisposes the tree to other factors.
Drought has plagued western forests in recent years, increasing both numbers and magnitudes of forest fires. Drought compromises a pine’s production of monoterpenes and maintenance of resin pressure, the primary defenses against bark beetles.
Southwestern dwarf mistletoe, Arceuthobium vaginatum, is a parasitic plant that sinks its haustoria (analogous to roots) into tree branches to extract water and sugars. During dry conditions losses of water and sugars to a plant parasite make pines more susceptible to bark beetles and Abert’s squirrels, because both beetles and squirrels preferentially utilize trees with low resin pressure.
Abert’s squirrels, Sciurus aberti, eat the inner bark of branches as their primary source of nutrition during the winter. Their impact on most ponderosa pines is minor, but when they find a poorly defended, highly nutritious ponderosa pine they focus their feeding on that tree. Sustained bark consumption diminishes a tree’s photosynthetic tissues and its ability to produce both carbohydrates for metabolism and the resin and terpenes needed for its defense, making the tree more susceptible to bark beetle attack.
Something had removed bark from many of the smaller branches throughout the crown. Because I have watched Abert’s squirrels near this site on Flagstaff, I suspected that squirrels had focused their feeding on this tree. But squirrels usually feed on small twigs, not branches, and I did not see the tooth scrapings that squirrels would leave on naked wood as they removed bark. What else might have removed the bark?
Careful examination of the branches revealed the stubs of many mistletoes. Many branches had mistletoes, and the upper portions of the canopy had several distorted branches, an indication of a heavy mistletoe infection.
Mountain pine beetles, Dendroctounus ponderosae, attack only live trees, and they must kill the tree by draining resin pressure to zero to create the conditions that allow eggs to hatch and larvae to feed and grow. An attack begins when a female bores into a tree trunk to reach the inner bark, where she will excavate a gallery and lay eggs. She emits an aggregation pheromone that diffuses from the tree, recruiting other females to join in the effort to reduce resin pressure to zero. The bore hole cuts through resin canals, and resin floods the hole and dribbles out of the tree, drying when it reaches the air. These conspicuous pitch tubes mark a tree that has been attacked.
The trunk of the dead ponderosa has pitch tubes on its trunk and round exit holes dug when the next generation of beetles emerged.
Knowing that the tree had died after the mountain pine beetle attack, I thought more critically about the conspicuous herbivory. Inspection of branches with a hand lens revealed that insects had been feeding on the inner bark and excavating pupal chambers. I found tiny round holes where bark was intact — twig beetles were reproducing under the bark.
Twig beetles are tiny, 1.5 to 3.5 mm in length, and are able to become established in the phloem of pine, spruce and fir branches under drought and mistletoe stress. They can damage one or many branches, but rarely kill trees.
As I considered what species might remove bark to eat twig beetles, a downy woodpecker, Dryobates pubescens, tapped in the branches above me. It was working at the edges of raw wood, flaking off more bark. The naked patches had no evidence of scraping by squirrel teeth, but they had tiny punctures made by claws and larger punctures made by a pecking bill. Downy woodpeckers had been foraging for twig beetles beneath the bark.
In conclusion, twig beetles colonized a live tree already stressed by drought and mistletoes. Compromises by drought, mistletoes and twig beetles interacted and combined to make the tree highly susceptible to bark beetles. A bark beetle attack sealed the tree’s fate. After the last bark beetle infestation, which began in 2000 and dwindled in 2015, bark beetle populations diminished to the point that attacked trees were rare, but not absent. The death of this ponderosa reminds us that the beetles are still here, sustained by the occasional compromised tree that allows them to reproduce successfully.