The summer drought of 2015 was the most severe in Washington state in the last 16 years. According to the U.S. Drought Monitor, the entire state was under severe or extreme drought conditions from late July through early October. Yet many affected conifers did not show signs of stress until the following year.
Summer months are a critical time for a tree’s water needs. Water lost through needles on hot days is normally replaced by water from the soil. Lack of soil water can result in failures within the tree’s circulatory system. Sometimes, the whole tree dies. Sometimes, all the tissue beyond the point of failure dies, causing a dead top or scattered dead branches. Trees growing in rocky or well-drained soils may experience more drought stress. Young trees and trees with shallow, less-developed root systems are especially vulnerable to damage.
Fortunately, some conifer species are more drought tolerant than others. In general, pines are the most tolerant while Douglas-fir is somewhat tolerant and grand fir, western hemlock and western redcedar are the least tolerant. This tolerance can vary widely depending on the quality of the site.
Damage and mortality in Douglas-fir, western redcedar and pines was immediately noticeable during the drought in 2015. Symptoms included entirely red crowns, red tops and scattered red branches. But many affected conifers remained green for months as the weather cooled over winter. Then, with record-breaking heat in spring 2016, delayed symptoms became more noticeable and widespread. This was hard to notice in western hemlock, because many dying hemlocks dropped foliage without color change.
At the end of summer 2016, unusual levels of western redcedar mortality were reported. These trees likely had a delayed response to the previous year’s drought conditions. The 2016 annual insect and disease survey showed increases in ponderosa pine killed by western pine beetle, Dendroctonus brevicomis, and grand fir killed by fir engraver, Scolytus ventralis. Attacks by these bark beetle species often increase following drought events.
The Washington State Department of Natural Resources (DNR) Forest Health Program examined conifers with drought symptoms at sites throughout the state and many showed little indication of being killed by primary pathogens, insects or other animals. In many cases, there were not even signs of opportunistic wood infesting insects in the lower stem. When galleries were found in the main stem, they were most often wood borers, which only enter conifers that are dead or dying from other causes.
In Douglas-fir and ponderosa pine, secondary bark beetles were rarely seen in the lower stem of mature trees, only in small-diameter trees. In the rare cases where mature trees were felled, they were often found in small-diameter tops and branches. In pines, these were primarily Ips species pine engravers. Evidence of other minor opportunistic bark beetles was also found, such as the so-called “itty bitty pitys”: Pityogenes, Pityokteines or Pityophthorus species.
In northeast Washington and north Idaho, damaged Douglas-firs had high numbers of Scolytus monticolae, a secondary engraver beetle. In western Washington, secondary bark beetles found in Douglas-fir were either Douglas-fir engraver, Scolytus unispinosus, or Douglas-fir pole beetle, Pseudohylesinus nebulosus. Galleries and adults of secondary bark beetles were found in lower stems of western redcedar (cedar bark beetles, Phloeosinus species) and western hemlock (silver fir beetle, Pseudohylesinus sericeus).
Some drought-stressed conifers that survived with compromised defenses may have already been attacked by primary tree-killing bark beetle species in 2016. In normal weather conditions, trees killed by bark beetles often do not turn red until the following summer. So any further increases in bark beetle activity may not be evident until 2017. Trees with root disease already have compromised root systems and drought conditions can exacerbate this, further decreasing water uptake. Drought stress may also increase symptoms of otherwise minor foliar diseases.
Caring for Stressed Trees
If dead trees pose a falling hazard near people or property, they should be removed as soon as possible. Also consider the potential hazard from falling dead tops and large branches. But before removing damaged conifers (those with some green foliage) in areas where hazard is low, monitor for a normal flush of green buds in spring as a sign that the tree may survive. Dead tops and branches will eventually fall and top-killed trees can grow new leaders, but they may become deformed as a result.
Surviving trees should be monitored for evidence of bark beetle attack, such as pitch tubes, pitch streaming or red boring dust. To prevent a build-up of bark beetle populations, remove and destroy any large amounts of freshly killed breeding material or infested logs and slash. Irrigation and mulching around landscape trees during future droughts may reduce damage. Take care not to over-water and avoid fertilizing as this can increase the tree’s foliage growth and need for more water.