An unexpected consequence of climate change: heatwaves kill plant pests and save our favourite giant trees



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Gregory Moore, University of Melbourne

Australia is sweltering through another heatwave, and there will be more in the near future as climate change brings hotter, drier weather. In some parts of Australia, the number of days above 40℃ will double by 2090, and with it the tragedy of more heat-related deaths.

In the complex world of plant ecology, however, heatwaves aren’t always a bad thing. Rolling days of scorching temperatures can kill off plant pests, such as elm beetles and mistletoe, and even keep their numbers down for years.

This is what we saw after the 2009 heatwave that reached a record 46.4℃ in Melbourne and culminated in the catastrophic Black Saturday bushfires. Years later, the trees under threat from the pest species were thriving. Here are a few of our observations.

Saving red gums from mistletoe

In the days following Black Saturday, botanists, horticulturists and arborists noticed a curious heatwave side-effect: the foliage of native Australian mistletoes (Amyema miquelii and A. pendula species) growing on river red gums lost their green colour and turned grey.

The two species of mistletoe are important in the ecology of plant communities and to native bird and insect species. But infestation on older trees can lead to their deaths, particularly in drought years.

Australian mistletoe is not related to the northern hemisphere mistletoes of Christmas kissing fame. They are water and nutrient parasites on their host tree and can kill host tissues through excessive water loss.

A eucalyptus tree trunk covered in leaves on a dried brown grass
The native mistletoe, Amyema miquelii, strangles this eucalyptus coolabah in the Burke River floodplain.
John Robert McPherson/Wikimedia, CC BY-SA

Often mistletoes go largely unnoticed, only becoming obvious when they flower. This is because many have evolved foliage with a superficial resemblance to the host species, a phenomenon known as host mimicry or “crypsis”.

During the Black Saturday heatwave, many mistletoes growing on river red gums died. The gums not only survived, but when record rains came in 2010, they thrived. A decade on, the mistletoe numbers are gradually increasing, but they’re still not high enough to threaten the survival of older, significant red gums.




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We want both mistletoes and red gums to persist. But often the old red gums are last survivors of larger populations that have been cleared — a seed source for future regeneration.

Under-appreciated elms

In many parts of Australia, the exotic English and Dutch elms are important parts of the landscapes of cities and regional towns. Elms provide great shade, are resilient and often low-maintenance. They also provide important environmental services, such as nesting sites for native mammals and birds.

Indeed, as Dutch elm disease decimates elm populations across North America and Europe, Australia can claim to have many of the largest elms and the grandest elm avenues and boulevards in the world, which we often under-appreciate.

A street lined by tall elms
Australia is home to some of the most beautiful elm avenues in the world.
denisbin/Flickr, CC BY-ND

But sadly, over the past 30 years the grazing of the elm leaf beetle, Xanthogaleruca luteola, has threatened the grandeur of our elms. These beetles can strip leaves to mere skeletons, and while the damage doesn’t usually kill the tree, it can make them look unsightly.

On Black Saturday, tens of thousands of elm leaf beetles fell from trees after prolonged exposure to high temperature. So many died, they formed what looked like a shadow under the tree canopies. Beetle numbers remained low for at least five years after that.




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Control programs, which often involve spraying chemical pesticides, were not required in that five year period. This was good for the environment as the chemicals can affect non-target sites and species. And we calculated that this saved well over A$2 million for Melbourne alone, money that could be better spent on parks and gardens (and of course, the elms looked splendid!).

Our iconic Moreton Bay figs

Then there are our magnificent, iconic Moreton Bay figs (Ficus macrophylla). Their large, glossy leaves, huge trunks, veils of aerial roots and massive canopies spread for more than 40 metres, and make them an Australian favourite.

Moreton Bay figs are prone to insect infestations of the psyllid, Mycopsylla fici, which can seriously defoliate trees under certain conditions. The fallen leaves can also stick to the shoes of pedestrians, causing a slipping hazard.

In Melbourne, psyllid numbers that were high before Black Saturday fell to undetectable levels in the following month.

Once again, a heatwave and hot windy weather had done an unexpected service. The incidence of psyllids has remained low for a decade or more now and, as with elm leaf beetles, control measures proved unnecessary and money was saved.

An enromous Moreton Bay fig trunk in a park
Moreton Bay figs are prone to insect infestations.
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Winners and losers

Many urban trees are renowned for their resilience to stress, both natural and human-caused. Climate change is proving a significant stress to be overcome, but we’ve observed how the stress can affect pests and disease species more than their hosts.

This gives the species growing in very tough urban conditions, where they lack space and are often deprived of water and good soils, a slight advantage, which may be the difference between living and dying under climate change.




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Climate change is bringing far more losses than gains. But, occasionally, there will be wins, and those managing pests in our urban forests must take advantage when they present.

If insect pest numbers fall we can direct resources to establishing more trees and ensuring our trees are healthier. The best way to avoid pests and diseases attacking trees is by providing the best possible growing conditions. That way we avoid problems before they arise rather than treating symptoms.

So as you swelter during this heatwave, remember it may not be all bad news for our urban and natural environments. Sometimes, positive outcomes arise when and where we least expect them.




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The Conversation


Gregory Moore, Doctor of Botany, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Burnt ancient nutshells reveal the story of climate change at Kakadu — now drier than ever before



S. Anna Florin., Author provided

S. Anna Florin, University of Wollongong; Andrew Fairbairn, The University of Queensland; Chris Clarkson, The University of Queensland; James Shulmeister, University of Canterbury; Nicholas R Patton, University of Canterbury, and Patrick Roberts, Max Planck Institute for the Science of Human History

Archaeological research provides a long-term perspective on how humans survived various environmental conditions over tens of thousands of years.

In a paper published today in Nature Ecology and Evolution, we’ve tracked rainfall in northern Australia’s Kakadu region over the past 65,000 years. We wanted to know how major changes in rainfall may have affected the region’s Aboriginal communities through time.

Our findings suggest the Kakadu region wasn’t as prone to dry spells as surrounding areas — and it likely functioned as a place of refuge for early Australians as they struggled through harsh and arid conditions.

Learning lessons from leftovers

To generate a rainfall record spanning 65,000 years, we used ancient food waste left behind by the First Australians living at Madjedbebe, a rock shelter on Mirarr country in the Kakadu region.

This site boasts the earliest known evidence of humans living in Australia. It also boasts plenty of Pandanus spiralis, a native plant commonly called “pandanus” or “screw pine”.

This plant, known as “anyakngarra” to the Mirarr people — the Traditional Owners of Madjedbebe — is very important to them.

Anyakngarra fruit grows on a tree.
Anyakngarra (Pandanus spiralis) fruit. The tree is native to northern Australia and ubiquitous around the Top End’s waterholes and floodplains.
Author provided

Its leaves are used for weaving, its trunk to create dye, its fruit flesh is used in a drink and its nuts (the seed kernels within the fruit) are consumed as a rich source of fat and protein.

Anyakngarra’s nuts were also eaten by the First Australians 65,000 years ago. Discarded nut shells have been preserved as burned fragments, disposed of in fireplaces over time.

These small remnants have proven hugely useful for our research team, which includes archaeologists, environmental scientists and Traditional Owners.




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In a nutshell

By analysing the isotopic composition in ancient anyakngarra nutshells, we could track rainfall at Madjedbebe. Specifically, we detected how much water (and therefore rainfall) was available to anyakngarra plants in the past.

This analysis is possible due to photosynthesis – the process by which plants convert carbon dioxide in the air into sugars. Anyakngarra plants absorb two isotopes of carbon from the atmosphere: ¹²C and ¹³C. Isotopes are different types of atoms within a chemical element that have the same number of protons but a different number of neutrons. Chemically, the isotopes of carbon are the same, but each has a different atomic “weight”.




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When environmental conditions are favourable, an anyakngarra plant will preferentially absorb more ¹²C than ¹³C. But if a plant is stressed by its environment, such as when it’s waterlogged due to seasonal flooding, it begins to absorb more ¹³C.

The isotopic composition (the ratio of ¹²C to ¹³C) is recorded in the sugars used by the plant for new tissue growth, including for the seasonal growth of nuts.

A higher proportion of ¹³C in a nutshell indicates that the plant it came from was waterlogged during its growth season. From this, we can conclude it likely experienced higher levels of rainfall.

Anyakngarra fruit.
Pictured is the anyakngarra fruit, which has a fleshy section (now dried and fibrous) a hard nutshell and multiple white seeds (or nuts) inside.
Author provided

Like an oasis in a desert

Over the past 72,000 years, humans have lived through an ice age in which there were two particularly cold periods called “stadials”. During stadials, glaciers extended to cover parts of North America, northern Europe, northern Asia and Patagonia (in South America).

The height of the second stadial in this ice age was called the Last Glacial Maximum. While this occurred 22,000 to 18,000 years ago, intense cold and dry conditions in Australia started as early as 30,000 years ago.

During this time, water availability was the main challenge in arid northern Australia (rather than low temperatures). The country’s arid zone expanded dramatically and parts of central Australia may have been temporarily abandoned by Aboriginal people.

Yet the “palaeoclimatic” record we generated for Madjedbebe indicates that, although glacial stages did lead to less rainfall, the Kakadu region remained relatively well-watered during these periods.

Our records show that for as long as people have been around, rainfall at Madjedbebe is unlikely to have dropped below current levels. Thus, this area would have helped early Australians survive during long dry spells and may have also attracted others from surrounding areas.

A site at the Madjedbebe is rock shelter in the Northern Territory.
This is our research site, the Madjedbebe rock shelter in the Alligator Rivers region of the Northern Territory.
Dominic O’Brien/Gundjeihmi Aboriginal Corporation, Author provided

Changing with the seasons

Our findings are supported by other archaeological evidence from Madjedbebe. For instance, our research has revealed more stone tools were left at this site during the glacial periods. This implies more people gathered there at these times.

Also, because the Kakadu region was still drier during glacial periods as compared to inter-glacial periods, people had to travel further for food and other important resources.




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This is supported by evidence of an increased number of tools being brought to the site from further away. This points to increased mobility and new social arrangements being made as people adjusted to life in a harsher environment.

The challenge moving forward

Notably, over the past 65,000 years the driest time in the Kakadu region was not during the Last Glacial Maximum. It is today.

Rather than being the result of less rainfall occurring, this is likely due to higher evaporation caused by warmer inter-glacial temperatures. Aboriginal communities currently living in the Kakadu region are experiencing unprecedented aridity.

These difficult conditions are exacerbated by the threat of invasive plants and animals and disruption to cultural practices of landscape management, such as vegetation burning.

While the people of Kakadu have spent thousands of years adapting to environmental change, the scale and intensity of today’s anthropogenic impacts on regional climates and local landscapes poses an altogether different challenge.The Conversation

S. Anna Florin, Research fellow, University of Wollongong; Andrew Fairbairn, Professor of Archaeology, The University of Queensland; Chris Clarkson, Professor in Archaeology, The University of Queensland; James Shulmeister, Professor, School of Earth and Environmental Sciences, University of Canterbury; Nicholas R Patton, Ph.D. Candidate, University of Canterbury, and Patrick Roberts, Research Group Leader, Max Planck Institute for the Science of Human History

This article is republished from The Conversation under a Creative Commons license. Read the original article.