How the climate is changing our trees

The climate is changing, and like us, plants are having to adapt to survive. Some are coping with the increased heat and dryness by evolving to meet these new conditions, others are migrating to more suitable climates, and some are simply unable to weather the weather.

So what does that mean for our backyards and gardens?
And what should we be looking for at the nursery and why?

The situation

According to Gregory Moore, doctor of botany at the University of Melbourne, all ecosystems, including our backyards and urban forests, will be affected by climate change.

“In Australia the rises in temperature are likely to be in the order of four degrees Celsius,” Moore says, “and the decreases in rainfall in places like Victoria will be in the vicinity of 10-15 per cent.

“Urban forests will experience increased temperatures, changed, often lower rainfall, greater storm intensities and droughts.”

40 per cent of trees lining the streets in parts of Sydney, 32 per cent in Melbourne and 85 per cent in Darwin will be vulnerable if current carbon emissions continue.

Moore highlights that while these changes will not be uniform across the country, it means that favoured environments of certain plants are becoming less hospitable.

One study from 2016 investigated the vulnerability of the City of Melbourne’s current tree stock. It found that 19 per cent of tree species planted were already vulnerable to climate change, and that as many as 35 per cent could become vulnerable by 2040 and 62 per cent by 2090.

Another 2017 study of 1.5 million trees in 29 council areas across Australia echoed this sentiment. It revealed that “nearly one in four trees in urban centres will be at high risk of dieback,” reported the Sydney Morning Herald. This includes wilting, browning of leaves and dead branches as a result of climate change.

Around 40 per cent of trees lining the streets in parts of Sydney, 32 per cent in Melbourne and 85 per cent in Darwin will be vulnerable if current carbon emissions continue.

Impact on Australia’s trees

“Generally trees from a wide geographic/climatic range will probably be fine,” says Moore, “as there are varieties/populations within these species that will cope with climate change.”

These include trees from genera such as:

Platanus (such as plane trees and sycamores),
Linden (such as lime trees and basswoods),
Pittosporum (such as lemonwood, mock orange and cheesewood),
Betula (such as birch),
Ulmus (such as elms)
and some coniferous genera.

Moore says these are well placed as urban trees because of their “environmental resilience and tolerance of a wide range of soil, rainfall and temperature conditions”.

“Most should cope with the changes in temperatures and rainfall that are projected for many cities,” Moore says.

Though it should be noted that this does not mean that all trees from these genera will be resilient to the impacts of climate change in all parts of Australia.

In fact, while some species of birch may continue to thrive in northern regions, they may be reaching the outer edges of their tolerance further south east.

Moore suggests that even if species’ ranges are limited, “there may be the option of selecting different but closely-related species”.

birch

Elm

This is the case with the eucalypt.

As a genera the eucalypt is well placed to withstand the effects of climate change because of the variety of species it contains – around 900 in total. But many of these species are facing extinction due to the increased temperatures and the adverse conditions of climate change.

a temperature increase of just 2 degrees would see 40 per cent of eucalypt species stranded in climate conditions they are not prepared for

Extreme heat in Tasmania saw thousands of eucalypts die out or start leaking sap from their bark in 2015, the ABC reported. A long and extremely hot spell of weather at the start of 2013 saw the sap begin “boiling out of the tree like a sore”.

A 2016 study estimated that a temperature increase of just 2 degrees would see 40 per cent of eucalypt species stranded in climate conditions they are not prepared for. These would have to move south or risk being wiped out.

Diversity is the key here, however, and as a study out of ANU has found, having over 700 species in its genera means greater genetic diversity and climate adaptation potential overall.

The stringybark is one example of eucalypt that will likely cope well with climate change, says Moore, given its “wide distribution and adaptations to fire and other stresses”.

Other adaptations that are likely to aid trees in the face of climate change include jacarandas saving water by dropping their leaves during dry times, and others releasing water through their leaves as an evaporative cooling system akin to sweating during periods of extreme heat.