Temperatures in Washington and Oregon continue to be above average and much of the Pacific Northwest is experiencing severe drought. Go for a walk in the woods and chances are you’ll notice it’s a lot more dusty, the plants and trees are shedding leaves and dropping limbs; typically we’d expect dried leaves and dropped limbs after the first storms of autumn, but happening mid-summer these are are signs that the forest is trying to conserve water. Our forests are feeling the effects of these prolonged hot, dry days. At the landscape level there is increased risk of wildlife and at the individual tree drought stress decreases growth, increases susceptibility to pests and disease and contributes to mortality.
Woodland owners, especially those who planted young trees this spring, are looking at how to mitigate the threats that drought stress presents to their forests. Saplings, or trees between 1-3 years of age, are especially vulnerable, and they require special attention due to competition stress and the vulnerability that comes with not having a strongly established root system.
Fortunately, there are a few things you can do to help out your thirsty trees, including financially-frugal methods of watering and applying mulch. Below are ways to identify and mitigate drought stress in your forest.
According to Texas A&M Forest Service (source), landowners can identify drought stress using the following methods:
Identifying drought stress in trees
- Hardwood trees will display obvious signs of water defecit and drought stress. Keep in mind that drought stress from the previous year may have effects that continue on into the following years. During dry periods, you may see…
- wilting of leaves, which may become permanent and eventually kill the tree
- warped, scorched (brown), or curled leaves that fall excessively from the tree
- die-back or decline, or death of the crown of the tree.
- Conifers do not display wilting in the same way hardwoods do. Common symptoms for conifers include…
- Yellowing/red second-year needles that drop prematurely
- Note: although yellowing needles indicate drought stress, this does not indicate mortality.
- Bark beetle infestations, which prey on weakened trees and will ultimately cause death
- Die-back or decline, or death of the crown of the tree.
- Yellowing/red second-year needles that drop prematurely
Effects of drought stress on trees
- In normal conditions, plants release water through the stomata, small holes located on the leaf of the plant. This process, called transpiration, will occur during the day and eventually cease at night. During drought, however, the plant closes the stomata and ceases transpiration in an effort to retain the now-limited water in the soil. After prolonged periods of drought and no transpiration, photosynthesis eventually stops which causes energy loss and eventual death amongst trees. For more information on transpiration, click here.
- Diameter growth is powerfully affected by yearly water availability as well. The season’s tree rings will vary depending on water, and a drought can almost completely halt width-wise growth during the dry period and perhaps a few years beyond.
During prolonged drought, trees that experience die-back may either become dormant or die. It can be difficult to tell the difference visually, but two tests can be implemented to assist in differentiating a dead tree from a simply dormant tree:
- “Collect some small twigs about one-eighth inch in diameter and try to break the individual twigs. If the twigs snap easily and appear dry, the tree may be dead. If the twigs bend and don’t break with a snap, the tree may still be alive.
- Use your fingernail to scrape bark from a small twig or branch. If the tissue under the bark is green and moist, the tree may still be alive. If you are unsure of these results, you should wait until the drought ends, and the tree may sprout a new crop of leaves.” (Source)
Disease and drought
According to Dr. Barbara Fair of North Carolina State University, plants attempt to conserve energy in times of limited resources, which makes them vulnerable to infection. Surprisingly, however, the effects of the disease may not start until after the drought has ended. As the plant attempts to survive the dry conditions, its self-defense mechanisms and abilities to protect itself will diminish, and once rainfall returns, pathogens can then enter through the soil’s water system and attack the weakened plants. Common diseases in Oregon and Washington include…
- Canker fungi – affects many species
- Swiss Needle Cast – affects Douglas Fir
- Rhabdocline Needle Cast – affects Douglas Fir
During the drought
- Watering: Watering your plants, especially younger trees and saplings, is the best treatment for drought stress. Obviously, acres of forestland cannot be watered, but if you are especially concerned about a small grouping of trees or young saplings, watering may be a good option for you. Note: Trees do not grow well in constantly wet soil, so make sure to let the earth dry out in-between waterings.
- Mature trees: make sure to water the ground within the “drip line” of the tree, which is defined as the portion of ground that is beneath the edge of the tree canopy, where rainwater would fall naturally. Click here for more information regarding different watering methods and prescriptions for watering according to tree size.
- Saplings: According to the Sacramento Tree Foundation, saplings should be watered to a prescription of 5-15 gallons a week per sapling. Saplings must receive enough water to their root ball, and the best way to accomplish this is to create a watering well around the tree of about 3 feet in diameter and 4-6 inches deep. More info can be found here.
- According to a study completed by Brooks et al., Douglas Fir and Ponderosa Pine use a process called hydraulic redistribution in order to store a reserve of water of 16-21 extra days after a 60-day drought. This means that these two species are able to keep better-hydrated than others and will use this store of water first before halting transpiration and experiencing other harmful effects of drought. Keep this in mind as you assess the condition of your forest and decide whether or not watering is necessary.
- Mulch: Leaves or chipped wood can be used as mulch and will help retain moisture. Mulch can also regulate soil temperature, which is essential for root-growth. Make sure to place the mulch 4-6″ deep and at least 4″ away from the base of the tree for best results. For detailed information on mulch types and benefits, visit the Green Seattle Partnership Field Guide (pg 14)
- Reduce competition: Removing weeds, invasive species, and other competing vegetation will increase the amount of nutrients available to trees, thereby increasing the likelihood of tree survival, especially for seedlings.
- Pruning, perhaps: If disease has stricken your trees already due to drought stress, prune limbs of trees where obvious cankers are present to prevent further spreading of the pathogen. Do not over-prune, however.
In the future…
1. Plant in the fall: In the Pacific Northwest, planting in the fall as opposed to the spring is particularly advantageous. The main reason for this is due to the extended growing period for saplings before the increasingly dry summers. The deeper and wider a tree’s root system is, the better water uptake it will have, which increases its chance for survival through water-limited periods. If a seedling is planted in fall, this gives the tree time to not only adjust to being in the ground (known as transplant shock), it also allows growth throughout the still-warm fall, temperate winter, and spring while nutrients are still widely available. If a tree is planted in spring, however, its growing period is drastically reduced before it has developed these vital root systems, and it is unprepared and vulnerable for the dry summers. To learn more about the benefits of planting in fall, visit the Green Seattle Partnership Field Guide (pg 16)
2. Consider your soil temperatures: Although ground temperatures can get too low for saplings to survive, temperatures in excess of 85 degrees fahrenheit are too high for many species of trees such as firs (i.e. Douglas Fir and Noble Fir), and this will inhibit growth. In general, most PNW species do best in temperatures of 68-70 degrees, although Ponderosa Pines may prefer the temperature slightly warmer (Source). The forest floor is insular and therefore resistant to temperature change, often making it an ideal place for sapling growth; therefore, in very dry times like these, it is probably best for seedlings if they have some cover from mature trees to ensure that the soil does not get too hot in addition to too dry. Different soil types also have different capacities to retain moisture and nutrients. For example, soil filled with clay has the best ability to hold water, but it is slow to drain and air cannot access the roots well, whereas sandy soil does not retain moisture well but has good aeration. In general, it is best to plant in a loam-type environment. Soil types can be identified using the Green Seattle Partnership Field Guide (pg 37) or you can obtain soil information from NRCS using the Web Soil Survey here.
3. Consider your species: Knowing the environments in which individual species grow best is vital to success of the tree in general. This guide provided by WSU Extension provides incredibly useful information on the environments in which individual native species thrive and the identification of these species. Also check out this comprehensive guide by WA DNR on PNW forest types, their distribution, environment, and more. Try to find out what conditions your sapling was grown in as well – knowing whether it was grown in low altitude or high altitude, the seed origin and the nursery location will help you determine whether or not the tree is a good fit for your property.
Brooks, R., Meinzer, F., Coulombe, R., & Gregg, J. (2002, October 1). Hydraulic redistribution of soil water during summer drought in two contrasting Pacific Northwest coniferous forests. Retrieved July 15, 2015.
Fair, Barbara (March 2013). Coping With Drought. Retrieved July 15, 2015.
Lopushinsky, W., & Max, A. (1990, May 10). Effect of soil temperature on root and shoot growth and on budburst timing in conifer seedling transplants. Retrieved July 16, 2015.
Texas A&M Forest Service (n.d.). Effects of Drought Stress on Trees and Landscape Plants. Retrieved July 15, 2015.
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