Drought, or the more generic term water stress, has been a big issue for trees in our area, resulting in tree decline and mortality in forests, natural areas, and yards across the region. Starting in 2012, we have had a series of record-setting summers in terms of heat and days without rain. This is taking a cumulative toll on trees.
Water Transport in Trees
Trees have an amazing ability to transport water from their roots all the way up to their tops, which may be over 100 feet above the ground. The dominant theory of how this is done is called transpiration pull. A large portion of the stem of a tree is called sapwood, and this is a big plumbing system through which water (sap) flows upward. The sapwood comprises many tiny “pipeline” pathways for water. Water flows into the tree roots through osmosis, flows up the sapwood, and makes its way into the leaves. The leaves have tiny pores called stomata that open up for gas exchange to occur as part of photosynthesis. The pores open up to take in carbon dioxide, and in the process oxygen and a little bit of water vapor are released. This process is called transpiration.
Water transport in trees takes advantage of the fact that water molecules are polarized, which gives them a slight attraction to one another (cohesion) and other objects (adhesion). Surface tension, the “skin” on top of water, is an example of cohesion, and the way that water goes up slightly on the edges of a glass measuring cup (creating a U shape) is an example of adhesion. As a leaf pore opens up, the water molecule next to it evaporates out. As it does, its cohesive nature “pulls” the water molecule next to it forward, which pulls the water molecule next to it, and so forth. There is a continuous chain of water molecules, called the water column, which runs from the leaves down to the roots. They are stuck together by cohesion, and they cling to the sides of the narrow pathways in the tree by adhesion. So as a tree transpires, water molecules are pulled up the tree, hence the term transpiration pull.
The “pulling” by evaporation from the leaves creates a negative pressure that draws new water from the soil into the roots to replenish the bottom of the water column. As long as there is adequate water to be drawn in, all is well. In drought conditions, though, there may not be any soil water available. As water continues to evaporate from leaf pores, especially on hot, dry days, the tension on the water column gets tighter and tighter because no new slack is coming in through the roots. Under extreme water stress, the water column breaks at some point in the stem, which is called a cavitation. This can result in an air bubble in the pipe, which is called an embolism. The chain of water molecules is now hydraulically disconnected between the roots and the leaves. If the tree is unable to repair the disconnect, that water pathway no longer functions.
Direct and Indirect Impacts of Drought
Classic symptoms of water stress in trees are dead tops and dead individual branches. If there is a major failure in the collective water column, water can no longer flow beyond that point of failure. Everything above it dies, leaving the tree with a dead top (figure 1). It could be that the water pathway to a particular branch failed, in which case that branch dies. The entire tree may also die due to water stress.
Water stress not only kills trees directly as described above but also indirectly by causing them to be susceptible to other agents. When trees don’t have enough resources (e.g. water), they become stressed. The tree has to prioritize how to use its inadequate resources since it can no longer maintain all functions. One of the first two things a tree gives up is diameter growth. You can look at core samples from older trees and see spots where the diameter growth rings were particularly small, which may indicate that those were drought years. The other thing it gives up first is insect and disease resistance. This leaves the tree more vulnerable to things like root disease and bark beetle attacks.
In western Washington, bark beetles are not usually an issue with healthy, vigorous trees, as healthy trees can defend themselves against beetles. Rather, westside beetles tend to opportunistic and take advantage of trees that either just died or are severely weakened by some other factor. Thus, you may find bark beetles in drought-stressed or drought-killed trees. The beetles are not the underlying issue, though. Rather, the tree suffered from water stress which attracted opportunistic insects looking for an easy meal.
Healthy trees can even hold their own against root disease in some cases, compartmentalizing decay and keeping the disease at bay. If that tree becomes water stressed, though, it can no longer fight the disease and is overcome. We often see “pulses” of mortality in root disease areas during drought years.
Here is a real case study from Snohomish County. I visited a landowner who had a bunch of recently dead Douglas-fir trees. The first thing I noticed were white fungal conks all over the stems of the dead trees. Did this fungus kill the trees? Then I noticed pockets of insect frass in the bark crevices. I peeled back pieces of bark and found Douglas-fir beetles inside. Did the beetles kill the tree? Then I looked around the area and saw that it was in the middle of a laminated root rot pocket (Figure 2). What happened was this: the trees in the area were suffering from root disease but were holding their own to some degree. There were a couple of drought summers that tipped the balance in the disease’s favor, causing them to die from root disease. As they were dying (or right after), they were colonized by opportunistic bark beetles. The beetles bring in a fungus with them called pouch fungus, which causes white, pouch-like fungal conks to emerge from the beetle holes.
Water stress can set a whole chain of events into motion, and the stress can be cumulative over time. Some trees that endured the first few drought years finally succumbed in subsequent drought years, and we will see that continue. The result is a significant uptick in tree decline and mortality. Some people are convinced that there is some sort of disease or insect epidemic that is wiping out trees left and right. Insects and diseases may indeed be involved, but they are not new or unusual. Rather, they are simply taking advantage of trees that are beginning to succumb to water stress.
What is the solution? In some cases, there isn’t a good one. Some trees are going to continue to decline and die due to adverse summer weather, and there’s nothing we can do except let it play out. If these newly-formed snags do not pose a hazard, they will provide a huge benefit for wildlife. Forty percent of our wildlife species require dead wood (standing or down). If the dead tree is a hazard, removal does not have to be all-or-nothing. Leaving the bottom 10 to 15 feet as a short snag will still provide important wildlife benefits while minimizing potential hazards.
The number one defense against the direct and indirect impacts of water stress is to maintain tree vigor. There are three key things you can do to improve the vigor of your trees. The first is to make sure tree species are appropriately matched to their sites. Where we see the most mortality (direct and indirect) from water stress is on certain “droughty” soil types. These tend to be gravelly soils that are excessively well-drained such that they dry out quickly. Trees on these marginal soil types may do OK most of the time, but in drought conditions they quickly succumb. Planting species that are more drought tolerant may be needed for these soil types. Your local Extension or Conservation District office can help you select appropriate species if you have sites where tree survival is poor.
The second key thing is density management. When trees are too crowded, they compete for resources like water and become stressed. In drought conditions, this competition becomes acute. When there is a very limited amount of available water, the more trees there are, the less water each will get. The WSU Extension Forestry program has educational resources to help you assess whether or not your tree stand is too dense. The “Focus On” article in the March 2018 WSU North Puget Sound Extension Forestry E-Newsletter goes through some simple assessments you can do.
The third thing is to avoid grass near trees. Grass and trees are not a good mix. Grass robs trees of water, which may never penetrate the sod to reach the tree roots below. Grass also harbors voles, which are rodents that will chew, girdle, and kill young trees. Keep the area around trees grass-free beyond the dripline. In a landscaping setting, replacing the grass around the tree with a good mulch will help retain water and keep tree roots cool. Do not pile the mulch up against the tree, though. In a forest setting, the natural forest understory plants and duff layer are fine.
One other principle to keep in mind is diversity. By encouraging a diversity of tree species on your property, you end up with mixed levels of drought and other tolerances that allow you to hedge your bets against various conditions. You also hedge your bets against insect and disease issues that are host species specific.
By Kevin Zobrist, Associate Professor in Forestry with Washington State University Extension, based in Everett, Wash.