The health of ponderosa pines and other trees is a growing concern across Sisters Country. photo by Sue Stafford
The health of ponderosa pines and other trees is a growing concern across Sisters Country. photo by Sue Stafford

Walking or driving around Sisters Country, one can't help but notice some pretty unhappy trees.

The most notable, of course, are the ponderosa pine trees along Highway 20 outside of town. Removal of the dead pines west of Sisters along the highway will start in October. That area was sprayed with an herbicide, which was later determined to be detrimental to the health of the ponderosas.

Many other ponderosas are experiencing abnormally large needle dieback on the inside of the limbs, according to Amy Jo Detweiler of the Oregon State University extension office. Every summer, some needle dieback is normal and doesn't indicate a problem with the health of the trees. There's just more than usual this year.

Several diseases affect the growth and survival of ponderosa pine in the Pacific Northwest and cause disturbances for the pines. These include root diseases, stem decays and diseases, foliage diseases, and dwarf mistletoes. These disturbances can result in dead trees, down wood, abnormal branches (witches' broomstick), dead branches, dead tops, and broken stems. Disease and drought also promote insect attack and increase risk of wildfire.

As Sisters grows and more housing and businesses are constructed, there can be adverse effects on the trees. Disturbance and compacting of the soil around the trees during construction, and installation of asphalt and concrete surfaces, can lead to eventual weakness in the trees, opening the pines up to insect infestations and disease, often resulting in dead trees which must be removed.

City Forester Dan Galecki said, "Construction and traffic can physically hurt trees. Folks run into them, scar them, and compact the soil."

Approximately 200 insect species can affect ponderosa pines from cone stage to maturity. Pinecone beetles cause tree death by transmitting blue stain fungus to the tree. Their larvae also consume the phloem, restricting the flow of nutrients to the top of the tree.

Western pine beetle is a common cause of death for older trees, drought-stressed trees, and even healthy, vigorous trees during epidemics. Pine beetles are naturally present in all stands and if there is a lot of slash lying around, the bark beetle population can explode and kill vigorous trees.

Dwarf mistletoe is the most widespread parasite that causes branch and stem deformity. It germinates on ponderosa branches and forces its roots into the phloem, creating stem cankers that leave the wood weak, making it susceptible to fungal infections and insect attacks.

Root diseases, rusts, trunk decays, and needle and twig blights also cause significant damage. With so many possible causes of unhealthy trees, it is often difficult to determine the main source of the problem.

OSU extension forester Nicole Stone said that density and climate issues are the two main factors affecting the ponderosas. She indicated that hotter, drier weather is definitely stressing them.

Likewise, as the stands of ponderosas age and the trees increase in size, without thinning, they end up too close together. Trees that used to be healthy when they were smaller in diameter become too crowded and don't get enough sunlight. Being crowded together also increases the chances of insects and diseases spreading from tree to tree.

Stone offered tips for keeping yard trees healthy. Start by selecting the appropriate tree for the location and growing conditions. Don't plant a tree too close to the house or other trees. Keep it watered, fed, pruned in the winter, and monitor for disease and insects. One caution from Stone: Don't fertilize ponderosas growing in the yard, as that can upset their natural balance.

Detweiler indicated that drought-stressed trees are visible all over the state due to lower precipitation and higher temperatures. The Douglas firs on the west side of the Cascades are suffering, too.

When any tree is drought-stressed, it is vulnerable to attack from insects and disease, which can result in stunted growth, discoloration and loss of bark and/or leaves, and potential death of the tree.

A number of the aspen trees around town are looking particularly unhealthy, with black leaves and branch dieback, oozing resin, and insect holes. Their golden leaves rustling in the winds of fall make them a popular landscape plant, but they need the right conditions. Be cautioned they can turn a yard into an entire grove as they send up shoots everywhere.

One recent ecological fact shared by Galecki is that trees are moving out of their respective regions and growing more in higher elevations. Colorado has an example of this where aspens are less likely to be found in historical locations mixed in with sagebrush. Rather, they are now established in lush higher elevations. He reports this is also happening in Oregon with more than just aspen.

There have been reports of alder trees along Whychus Creek suddenly dropping withered leaves unexpectedly. Detweiler said that could be attributed to lack of adequate moisture and/or attack by either the alder flea beetle or the alder borer. They haven't seen samples of these coming into the extension office, but that doesn't mean they aren't around. According to Detweiler, the alder borer is tougher to manage and sprays don't work to control them. Usually when a tree shuts down that quickly, it is a survival tactic.

Galecki said that mortality of trees is always present. One thing he is seeing is the mortality of recently planted small replacement trees around town. Good trees to plant instead of aspens, cottonwoods, and birches, which are all prone to several different fungi, are crabapples and Canadian chokecherries, both of which have beautiful spring blossoms and grow well in Sisters.

Diagnosing what is going on with the trees is difficult because there may be multiple factors at play. Everyone interviewed, however, indicated that drought conditions prevalent over the last several years are having a definite negative impact on the vegetation in Central Oregon.