We need to design our cities so we make good use of rainfall to water our trees.
COMMENTARY || Drought putting Edmonton's trees at risk unless watering improved
Forest ecologist offers tips for the city—and its citizens—to help take care of large urban trees.
By VICTOR LIEFFERS and TED HOGG
We should all be worried about Edmonton’s trees. A large urban tree is worth thousands of dollars, based upon the city’s valuation. Trees add enormously to the quality of life in our city. Over the last decades, there has been a gradual decline in the vigour and health of the average tree in Edmonton. Today most city trees have thin canopies and many have a large number of dead branches. Many trees are in a severely weakened state and are on the edge of death.
There are several species of insects damaging the leaves of our trees, but the main culprit for the decline in Edmonton’s trees is a long-term lack of water. Big trees are damaged by prolonged water stress. The cells of roots, stems and branches are damaged by periods of dry soil. In essence, air bubbles formed in the water conduction systems of the tree block these cells and prevent the recovery of the damaged tissues even after rainfall. Once weakened, it takes years for a tree to fully recover from a severe drought, even if well watered.
Edmonton has had a decline in precipitation over the last 20 years. Ground water that usually feeds the deep roots of big trees has also declined over this time. The long-term decline in soil moisture is most damaging to mature trees. Large trees still need plenty of water, but grow more slowly than young trees and therefore recover more slowly. The drought in the last half of the summer of 2017 and the continued drought in Edmonton in 2018 is the latest onslaught against our trees.
If Edmontonians wish to keep their trees healthy in the long term, we will have to change our attitude towards them. We can no longer take for granted the good health of our trees. Like more arid regions of the world, we need to design our cities so we make good use of rainfall to water our trees. We may also have to do supplementary watering to get trees through drought—a rain today is no reason to think trees will have sufficient water for the longer term.
In many circumstances people water lawns, but not trees in perimeter areas or boulevards. While grass can quickly recover following drought, trees have a much slower recovery and in a weakened state, they are more vulnerable to insects and diseases.
Here are some other issues to consider about our trees and what we might do to help them:
Many city trees are placed in areas with small volumes of soil, such as beside a compacted road or beside concrete or asphalt pads that drain away water. Such trees need careful water management.
In winter, bank salt-free snow beside trees so that the spring meltwater soaks the roots. Too often, we truck this clean snow away and the tree is too dry the following year.
Plan water management in your property to allow moderate rain storms to soak into the soil rather than be diverted to storm sewers. This will tend to raise the groundwater table and help the trees.
Minimize salt use on roads and sidewalks near trees. Salt ions bind to soil particles and harm plants for years.
Plant a variety of tree species and genetic types. Diversity allows more trees to survive drought, insects and diseases.
Water trees deeply if the grass turns brown, especially trees with limited soil volume. On deeply rooted trees such as elms, ash and oaks, watering can be heavily concentrated near the trunk.
City foresters could issue watering alerts to inform homeowners in regions of the city that miss most of the summer storms. The city should also water trees in parks.
City planners might encourage homeowners to water boulevard trees by reducing water rates during some summer months.
We cannot ignore our trees when creating water management systems or assume rainfall will be sufficient to keep them healthy during droughts. Get water to your trees.
Victor Lieffers is a forest ecologist in the Faculty of Agricultural, Life & Environmental Sciences.
Ted Hogg is a research scientist with the Canadian Forest Service.
This op-ed originally appeared July 17 in The Province.