Heat stress

Heat stress is a concern for all outdoor workers. Dave Crispin, senior arborist at Treeswest Australia and Arboricultural Association of Western Australia (Arb West) committee member, highlights some of the effects and management of high temperatures.

It’s been so hot in WA even the ice-cream truck melted

Jokes aside, Western Australia is experiencing one its hottest Februarys on record.

I hate to use the word ‘unprecedented’ – it became part of our lexicon during Covid (and there’s another one – ‘Covid’) and like most people I’m over Covid and anything else that is associated with it – but unfortunately, it looks like it’s here to stay, just like our severe heatwave here in Western Australia

Temperatures in and around Perth in January and February were averaging between 40 ºC and 45 ºC, and it seems to be happening more frequently as our climate changes.

How do trees cope in the intense heat? Unlike us they cannot uproot and hide under the shade of one of their own.

Celtis Australis showing canopy decline due to heat stress. Image: Dave Crispin

Be careful when thinning

At a recent lecture the theme was, ‘Action – Reaction = Consequence’.

Over the preceding weeks I’ve seen this play out on several occasions, manifesting as canopy decline due to sun scorch.

Like most consultants, I’m asked to look at many trees, and one occasion recently I was asked to look at a stand of Celtis australis (European hackberry). Several of the trees had recently been crown thinned, (Action) at the client’s behest, resulting in minimal foliage in the upper crown, which ordinarily would protect it from the midday sun. The tree had tried to respond, (Reaction) but it was too little too late. Consequence – all the main extended lateral branches radiating out from the trunk, exposed to the intense sunlight, had suffered major sun damage. As a result, the upper side of the branches had delaminated, peeling back to reveal the xylem tissue. The periderm (outer bark) was peeling off in sheets.

We can cover up, trees cannot.

Celtis Australis delaminating due to sun scorch. Image: Dave Crispin

Retained heat

Consider this: when the ambient temperature on Perth streets reaches around 35 ºC, and the direct sunlight hits the bitumen road surface, it’s not uncommon for the road to heat up to 50 ºC and beyond. The impact this has on the root zone, especially the lower order roots, can be disastrous. Many shallow-rooted trees like the humble Corymbia calophylla (Marri) have been lost recently.

Various trees have different tolerance levels to heat. Some cope better than others.

When Sydney had its record-breaking heatwave in 2020, 12 days over 40ºC, some trees died, while others coped very well. Thinking that native trees would fare better than exotics was not the case. Both exotics and natives took a hit. Trees that had a low heat tolerance and died included the Acer rubrum (red maple), Liriodendron tulipifera (Tulip poplar), surprisingly, Banksia integrifolia (Coastal banksia), and Tristaniopsis laurina (Watergum). Others suffered severe heat stress, and eventually had to be removed.

Trees that were more resilient, and seemed to cope better, were the Ulmus parvifolia (Chinese elm), Fraxinus excelsior (European ash), Callistemon viminalis (Weeping bottlebrush), and the Brachychiton populeneus (Kurrajong).

The info came from Misha Ketchell’s 2023 We need urban trees more than ever, published on theconversation.com.

Heat stress on trees in the urban environment is more prevalent given the retained and reflected heat from buildings and infrastructure, more so than in a rural environment. Heat stress reduces the tree’s ability to photosynthesise and increases photooxidative stress (chloroplasts are damaged inhibiting photosynthesis). If prolonged, this can lead to premature leaf abscission, leaf chlorosis, and decreased growth.

Extended periods of intense heat like those we’ve been experiencing, coupled with drought-like conditions due to declining rainfall, are a major cause for concern, especially for our local amenity trees. Residential trees can be given supplemental water by the property owner. Street trees, not so much.

In future, local government will have to invest more time in planting heat- and drought-tolerant tree species, and allocate more funds towards tree watering to reduce heat stress and tree mortality during the hottest periods of the Western Australian summer.

Lateral branch delaminating due to sun scorch. Image: Dave Crispin

Looking after our own

Arboriculture, and horticulture, are outside professions, it is part of the allure when talking with young people ready to sign up for what I consider to be one of the most rewarding professions there is. And summer is typically a favourite time of the year for working outside, but it does come with its challenges. Heat exhaustion (when your body loses excessive amounts of water and salt), is a real threat, and one that must be taken seriously. Plenty of hydration and electrolyte replacement is as essential as sunscreen and long sleeves when working outside on high-temperature days.

Every surface exposed to the sun has the potential to give first- or seconddegree burns, so gloves are an essential part of PPE (Personal Protective Equipment).

First-degree burns affect the outer layer of skin – the epidermis. They are painful, are usually accompanied with swelling and redness.

Second-degree burns affect the outer and underlying layer of skin, epidermis and dermis. Second-degree burns cause pain, redness, swelling and blistering, and the person should seek medical advice.

Managing the situation

CPD Trees, run by Mick Byrne, operates in Perth Hills, an area of dense forest.

Mick is a long-standing Arb West member and has had to stand down his team on a number of occasions this year due to Harvest and Vehicle Movement Bans (HVMB). HVMB’s are the responsibility of the local government authority, and as a rule they’re only issued on the actual morning. I asked Mick how he managed to find work for the guys when there was a work ban in place.

“After ringing the clients and cancelling the work for the day, I have to do a quick ring around and try and find work where there isn’t a ban in place,” he explained. “If not, I must stand the guys down.”

He mentioned on high fire risk days when they can work, when the appropriate measures are put in place, electrical saws are used, regulation fire extinguishers are taken out, and the guys are rotated to take regular hydration breaks.

All that being said, you can’t beat working outside in this beautiful state, whatever the weather has to offer. Arriving back at the yard on 40 ºC day to be greeted with ice-cold icypoles, and iced-watermelon makes it all worth it.

Removal of a large Ficus with a baiting station in the foreground. Image: Dave Crispin

PSHB update

The Polyphagos Shot Hole Borer (PSHB) continues to cut a destructive path through Perth’s urban tree population. In a recent press release it was reported a number of Perth’s famous landmarks had reported infestations, and a number of culturally significant and historic trees would have to be removed – or at the least be heavily pruned.

Locations such as Hyde Park, established by European settlers in 1897 under the name ‘Third Swamp’, has a rich tree history, with records of Ficus and Platanus being planted in 1897, and Pinus sp. and Jacaranda planted in 1912 and 1921 respectively, 100-year-old trees – and some even older – have become infected.

Driving past Hyde Park recently I noticed one of our Arb West members, Morin & Sons, had two crews at the park removing a large Ficus macrophylla var. hillii. Such a magnificent tree meeting an untimely end! Speaking with Alex Morin, the business owner, he mentioned this was only one of many trees he had recently removed for the Department of Primary Industries and Regional Development.

Morin & Sons removing a large PSHB infested Ficus. Image: Dave Crispin

Iconic Kings Park

One of Perth’s most notable landmarks, Kings Park, so named in 1901 to mark the ascension to the throne of King Edward VII, and sitting on approximately 400 hectares, has a host of trees favoured by the borer. Unfortunately, there are now a number that have to be removed.

The thought of losing so many of our historically significant trees, part of our Western Australian heritage, is devastating.

Take note: as an association, it is incumbent on Arb West to disseminate current, factual, practical advice to our industry, members or not, to try and contain the spread of PSHB. I suggest other associations in other Australian states watch this space!

To learn more of Arb West and its membership, log on to arbwest.com.au.

Image: Arb West
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