Helping Your Forest Through Dry Times

NNRG | Posted on July 2, 2019 |

There are many things you can do to help your forest respond to dry times. The combination of warmer than normal temperatures and less precipitation contribute to low soil moisture—which is a major cause of water stress in trees and forests. Being aware of the signs of drought stress, managing tree density, and managing and planting appropriate species for site conditions can go a long way in improving a forest’s resilience to climate change.

It’s also important to maintain a diversity of tree species and age classes, retain large woody debris, and encourage a rich understory of native shrubs and flowers to contribute to soil moisture retention and overall forest ecosystem health. Below are some tips for helping trees and the entire forest adapt to drier, warmer times.

Recognizing drought stress in trees

Broadleaf trees will display obvious signs of water deficit 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 (though yellowing needles indicate drought stress, this does not necessarily indicate mortality)
- Bark beetle infestations, which prey on weakened trees
- Die-back or decline, or death of the crown of the tree

Understand the 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 among trees. For more information on transpiration, click here.
Diameter growth is 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 one that is simply dormant:

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)

What you can do for your forest during a drought

Watering: Watering trees, especially younger trees and saplings planted in the last 2 years, is an optional 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. Here are some considerations and observations from a project Amy Grotta of OSU Extension carried out last year on Douglas-fir seedlings.
Mulch: Leaves or chipped wood can be used as mulch and will help retain soil 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.
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.

What you can do for your forest in the future

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 sapling roots. The deeper and wider a tree’s root system, the better water uptake it will have. This increases its chance for survival through water-limited periods. If a seedling is planted in the fall, this gives the tree time to adjust from transplant shock and put energy into root growth. If a tree is planted in spring, growth efforts are directed to leaf area, and it is less-prepared for dry summer weather.
Consider your soil temperatures: Although ground temperatures can get too low for saplings to survive, temperatures in excess of 85 degrees F are too high for many species of trees such Douglas-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). In very dry times, it is probably best for seedlings if they have some cover provided by mature trees to ensure that the soil does not get too hot and/or 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.
Right tree, right site: 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. Planting drought-tolerant tree species—especially on fast-draining soils and south-facing aspects—is an important strategy. Talk to your nursery about the appropriate seed zone for your seedlings.
Thin highly stocked stands to reduce water stress: Denser forests are more prone to drought stress. As forest ecologist, Derek Churchill noted recently: Thinning stands reduces competition for water and nutrients, and thus the remaining trees have more resources to withstand drought, insects, and root rot. Thinning also benefits wildlife habitat by increasing non-tree plant diversity and abundance-

More resources on climate and drought adaptation and your forest

NW Climate Hub – https://www.climatehubs.oce.usda.gov/hubs/northwest
USDA portal for science-based Northwest-specific information and technologies to assist with climate-informed decision making.

Seedlot Selection Tool – https://seedlotselectiontool.org/sst/
Web-based mapping application that can be used to map current or future climates based on different climate change scenarios to inform the selection of seedlings for tree planting as part of reforestation and restoration efforts.

NW Climate Toolbox – https://climatetoolbox.org/
Suite of online tools to put a variety of climate data and information in reach of organizations and people for site specific locations – including: historical climate variability, temperature and precipitation normal, streamflow projections, climate projections, and data for Tribal Lands.

Adaptation Partners – http://adaptationpartners.org/
Researchers and resource managers provide scientific information on climate change effects and adaptation for specific regions in the Northwest and Intermountain West.

Climate Mapper – http://www.climatemapper.org/
The Nature’s Stage Climate Mapper allows users to explore the geoclimatic stability of HUC5 watersheds within the Pacific Northwest.

AdaptWest – https://adaptwest.databasin.org/
AdaptWest is a spatial database designed to help land management agencies and other organizations implement strategies that promote resilience, protect biodiversity, and conserve and enhance the adaptation potential of natural systems in the face of a changing climate.

Climate Impact Groups at UW – https://cig.uw.edu/
Providers of primary science and decision making tools.

Climate Impact Research Consortium at OSU – https://pnwcirc.org/
Primary science provider and resource tool developer.

Adaptation Workbook – https://adaptationworkbook.org/

Adaptation Clearinghouse – https://www.adaptationclearinghouse.org/