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Drought & Trees - Impact, Care, and Maintenance - By Toso Bozic

Image of Drought

Due to climate change, Western Canada and the USA are experiencing unprecedented levels of forest fires, with devastating effects on people, communities, the ecomony, and the environment. Record breaking temperatures and prolonged drought grealty impacts trees and forest communities. The direct impact of drought on trees is characterized by slowing or eliminating growth, serious health threats, and injury or death. Drought also impacts trees indirectly, by increasing their susceptibility to wildfire, insect pests, and disease. Severe droughts cause widespread tree mortality across the landscape (urban, acreage, farm, County, or Proveince-wide) with profound effects on the function of tree and forestry ecosystems, and the overall environment.

Alberta native plant communities (grass, shrubs, and trees) are well adapted for dry summer and fall weather, as well as for a period of prolonged drought, but the effect and impact of drought on trees is devastating and long-lasting.

How Drought Affects Trees

No differently than humans, trees need water to survive on hot, dry days. Many trees can survive for only short times under extreme heat and dry conditions. A trees first response to drought is to close its pores, called stomata. These pores are very important for photosynthesis, by controlling the amount of COtaken to produce sugar. Trees survive by moving water from their roots to their leaves, through small cylindrical vessels that are connected within the tree. Drought also disrupts water transport by reducing the amount of water available for the tree. Due to drought, the moisture in the air and soil decline and small air bubbles are formed in the vascular system, creating embolisms that block the water's flow.

Symptoms of Drought

There are many visible drought stress symptoms due to water deficiency. The effects are not always immediate and the full extent of the damage to trees can take one to three years to become apparent. In deciduous (hardwood) trees, some of the most common, recognizable drough symptoms are:

  • Scorching - the margins or edges of the leaves are browning
  • Wilting, curling, bending, rolling, and mottling of the leaves
  • Lighter green to yellow-green foliage
  • Leaves dropping or shedding, or early fall colour changes
  • Chlorosis
  • Smaller sized leaves, stunted shoots
  • Seed or cone production is increasing as the tree is under stress
  • Cracks on bark of young trees

In coniferous trees, drought symptoms are recognizable by shoots drooping, browning, second year needle yellowing, and an abundance of cone production the second year after a drought.

Picture 1: Leaf scorching (left and centre), and needle browning (right)

As drought intensifies and becomes prolonged, the effect on the whole tree is manifested in dieback of twigs, branches, and thinning of the crown. Leaves, twigs, and small branches in the topmost and large lateral branches are dying. In the interior of the tree, leaves are more concentrated around the trunk with many producing epicormic shoots. Roots are the 'engine' for trees and when drought conditions persists, the fine hair-like roots, whose primary function is to absorb water, begin to die back. Under prolonged droughts, even the larger, fibrous roots are lost.

Picture 2: Crown thinning (left), dieback (centre), and death (right)

What can be done to reduce the Impact of Drought?

To reduce the impact of drought, proper tree care includes:

  • Proper watering
  • Mulching - putting arborist wood chip mulch down to protect the roots from drying out
  • Do not prune or remove live branches
  • Do not fertilize trees
  • Control weeds to reduce competition from water
  • Do not disturb the soil using mechanical weed control, as you may damage roots and expose the soil to loose moisture
  • Pest management control, including spraying or insects, such as defoliators
  • Avoid any mechanical damage, such as cutting surface roots, damaging the root collar bark on the trunk
  • Consider planting a diverse range of trees and shrubs that are resistant to drought

Watering is crucial for tree survival during drought. There are several steps to waterng during a drought:

  1. Test your water for sodium before watering your trees. If it contains high levels of sodium, it will kill your trees fast, and not provide a chance for them to survive
  2. Check moisture levels in the soil by using a garden trowel or knife, to a depth of 4-6in. If you can easily push or insert a 6in screwdriver in the soil, there is enough water
  3. Amount of water - even today, science does not provide the exact amount of water needed for each tree, but there are some rules of thumb. During drought, trees grown in sites without lawn irrigation need approximately 10 gallons (38l) of water each week, per inch (2.5cm) of trunk diameter
  4. Timing - the optimal time to water trees is early in the morning. Try to avoid watering late at night due to the potential of developing fungus. Also, it is extremely important to water trees when the temperature is scorching during the day. If you trees are showing signs of water stress in the middle of the day by all means you should water them.
  5. Where to water - a very common mistake people practice is to water the water hose right next to the tree trunk. Trees should be watered at what an Arborist calls the 'drip line', an imaginary line extending from the outermost branch tips straight down to the ground
  6. Water delivery mechanism - drip irrigation is the best way to water trees, as you can control the amount of water delivered, as well as the speed of water droplets. If you don't have drip irrigation, use a hose, sprinklers, water gator bags, and buckets. It is extremely important to perform long and slow soaking at the outer edge of the drip line. Avoid any water run-off and water hitting the trunk.
  7. Frequency of watering - water trees once a week, with slow soaking. Avoid overwatering, especially if you have heavy clay in your soil.
  8. Do not forget to water trees in fall 

Mulching is a must, and provides a very important function during a drought, protecting the roots from extreme heat and keeping moisture around trees. Create a donut-shaped wood chip cover around your tree to keep water inside. Applying 4-6in (10-15cm) of arborist wood chip mulch will greatly reduce the loss of moisture in the soil. A layer of wood chip mulch will maintain constant soil temperatures and moisture.

Picture 3: Arborist wood chips is the best option long-term. It enhances water infiltration and retention, especially during drought, moderates temperature and reduces root drought stress, and provides nutrients.

Fertilization - Avoid adding any fertilizer during the drought, fertilize trees in the spring.

Weed Control - Remove any weed competition by either pulling or using herbicides.

Pruning - avoid cutting any live branches with leaves, remove dead branches next spring instead.

Pest Management - Use proper identification and pest management techniques to reduce stress to trees.

Mechanical Damages - Avoid any roots, root collar, or trunk damage by lawn mowers and weed whackers.

Tree Diversity - Plant tree species that are more resilient to drought.

Overall, drought conditions are very hard on trees and can kill them. Providing water, wood chip mulch, managing pest problems, and avoiding some common mistakes, will go a long way to helping trees survive, remain healthy, and avoid the long-term negative impact of drought stress. Many trees have survived extreme droughts in the past. With a little help, trees can survive and thrive.

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