Urban trees can get hot too

Urban trees give us welcome shade

With the Bureau of Meteorology declaring that the El Niño weather pattern is active over the Pacific Ocean and that a positive Indian Ocean Dipole has developed, the chances are we are in for a hot and dry summer. July this year was the Earth’s hottest month on record.

Some researchers are concerned that many species of native and exotic urban trees are not particularly heat tolerant.

Just like us when we sweat, trees lose water in order to remain cool. If a tree is unable to source enough water during hot weather, it becomes susceptible to dieback, insect infestation and pathogens.

During the 2019-2020 Sydney heatwave, researchers discovered that some urban tree species such as Red Maple, Tulip Poplar and Coast Banksia showed low tolerance of elevated temperatures and many actually died or became so debilitated they had to be removed.

Chinese Elm, Chinese Ash, Weeping Bottlebrush and Kurrajong on the other hand, all seemed able to avoid excessive dieback and death.

Trees control their water loss by opening and closing pores in their foliage. In dry times, they close their pores to conserve moisture but this means the leaves become susceptible to high temperatures. The leaves can become scorched and shriveled, placing the tree itself in a dire situation.

Eucalyptus stomata. Credit - Eye of Science photo library

Trees with thin leaves are generally more susceptible. Trees that lose foliage due to heat stress can take many years to fully recover.

Many of our native trees and shrubs are often more tolerant of hot, dry conditions. They have adapted to it over millennia. But they are not immune, and during hot weather most will appreciate a deep but less frequent watering.

Kurrajongs are ideal street trees that can tolerate drought

Mulching the root zones helps retain moisture. Regional and State authourities could install passive irrigation systems for street and parkland trees – runoff from buildings and roadsides is directed to the surface or sub-service - via a recharge pit – where soil moisture is retained at a depth where the tree roots can access it.

At a personal level, don’t forget to give that tree or shrub on the nature strip a good drink on a hot day.

References:

We Need Urban Trees More Than Ever - The Conversation

Summer of Severe Heat Predicted - ABC News