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 province-wide) with profound effects on the function of tree/forestry ecosystems and the overall environment.
Alberta’s native plant communities (grass, shrubs, and trees) are well adapted for dry summer and fall conditions as well as for periods of prolonged drought, but still, the effect and impact of drought on trees are devastating and long-lasting. A drought may be short-lived (a few weeks or a month) or last for the full growing season or multi-year events, but its impact on a tree’s health can last much longer. Trees that were already stressed by some other issue, like harsh winter/winterkill, poor soils, salt, herbicides, and mechanical damage or insect infestation, are likely to decline even more following a drought.
No different than humans, trees need water to survive on hot, dry days. Many trees can survive for only a short time under extreme heat and dry conditions. The first response of a tree to drought is to close the pores (stomata). These pores are very important to the process of photosynthesis because they control the amount of CO2 the tree takes in 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 declines, and small air bubbles are formed in the vascular system creating embolisms that block the water’s flow.
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 the trees can take one to three years to become apparent.
In deciduous (hardwood) trees, some of the most common recognizable drought symptoms are:
- Scorching – browning of the margins/edges of leaves
- Wilting, curling, bending, rolling and mottling of leaves
- Lighter green to yellow-green foliage
- Leaves dropping/shedding or early autumn colour changes
- Smaller size leaves, stunted shoots
- Seed/cone production increases as the tree is under stress
- Cracks on the bark of young trees
In coniferous trees, drought symptoms are recognizable by drooping shoots, browning, second-year needle yellowing, and an abundance of cone production during the second year of a drought.
As drought intensifies and prolongs, the effect on the whole tree is manifested in diebacks of twigs, branches, and thinning of the crown. Leaves, twigs and small branches in the topmost portion, 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 persist, the fine hair-like roots whose primary function is to absorb moisture, begin to die back. Under prolonged drought, even the larger, fibrous roots are lost.
To reduce the impact of drought, proper tree care includes:
- Proper watering
- Mulching - placing arborist wood chip mulch to protect roots from drying out
- Do not prune or remove live branches
- Do not fertilize trees
- Control weeds to reduce competition for water
- Do not disturb soil through mechanical weed control as you may damage roots and expose soil causing moisture loss
- Pest management control including spraying of insects such as defoliators
- Avoid any mechanical damage such as cutting surface roots, damaging the root collar or bark on the trunk
- Consider diversity in the planting of trees and shrubs that are resistant to drought
Watering is crucial for tree survival during drought. There are several steps to consider regarding watering during the drought:
- Test your water for sodium before watering your trees. If it contains high levels of sodium, it will kill your trees faster.
- Check moisture in the soil by using a garden trowel/knife to a depth of 4-6 inches. If you can easily push/insert a 6-inch screwdriver into the soil, there is enough water.
- Amount of water - still today, science does not provide an exact amount of water for each tree but there is some rule of thumb. During drought, trees grown in sites without lawn irrigation need approximately 10 gallons (38 litres of water each week per inch (2.5 cm) of trunk diameter measured.
- 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 temperatures are scorching during the day. If your trees are showing signs of water stress in the middle of the day, you should water them.
- Where/area to water – a very common mistake people practice is to put the water hose right next to the trunk. Trees should be watered along what arborists call a “drip line” – an imaginary line extending from the outermost branch tips straight down to the ground.
- 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 and are using a hose, sprinklers, water gator bags, or buckets, it is extremely important to perform long and slow soaking at the outer edge of the drip line. Avoid any water run-offs and water hitting the trunk.
- Frequency of watering – water trees once a week with slow soaking water. Avoid overwatering if you have heavy clay in your soil.
- Do not forget to water trees in Fall
Mulching is a must and provides a very important function during drought – it protects the roots from extreme heat and keeps moisture around trees. Create a doughnut-shaped wood chip cover around your tree to keep water inside. Applying 4-6 inches (10 -15 cm) of arborist wood chip mulch will greatly reduce the loss of moisture in the soil. A layer of woodchip mulch will maintain more consistent soil temperatures and moisture.