There are currently almost 8 billion people on Earth. If every single person planted a tree each year for the next 20 years, that would mean roughly 160 billion new trees.
Could massive tree planting actually slow climate change?
Trees and carbon
Carbon dioxide is the main gas that causes global warming. Through photosynthesis, trees and other plants transform carbon dioxide from the atmosphere into carbohydrates, which they use to make stems, leaves and roots.
The amount of carbon a tree can store varies a great deal. It depends on the tree species, where it is growing and how old it is.
Let’s say the average tree takes up 50 pounds of carbon dioxide a year. If a person planted a tree every year for 20 years – and each one survived, which is highly unlikely – those 20 trees would take up about 1,000 pounds, or half a ton, of carbon dioxide per year.
The average person in the United States produces a whopping 15.5 tons of carbon dioxide a year compared with 1.9 tons for an average person in India. This means that if each person in the U.S. planted one tree per year it would offset only about 3% of the carbon dioxide they produce each year, after all 20 trees had matured. But, it would offset 26% for somebody in India.
Planting trees is certainly part of the solution to climate change, but there are more important ones.
Protecting existing forests makes sense. Not only do they absorb carbon dioxide in the trees and the soil, but they provide habitat for animals. Trees can provide firewood and fruit for people. In cities, they can offer shade and recreational spaces.
But trees should not be planted where they didn’t grow before, such as in native grasslands or savannas. These ecosystems provide important habitat for their own animals and plants – and already store carbon if they are left undisturbed.
It is important that everybody – businesses, politicians, governments, adults and even kids – do what they can to reduce fossil fuel emissions. I know it can seem pretty overwhelming to think about what you as one person can do to help the planet. Fortunately, there are many options.
Volunteer with a local conservation organization, where you can help protect and restore local habitats. Discuss with your family new lifestyle choices, like biking, walking or taking public transit rather than driving.
Two Girl Scouts take a stand against deforestation.
And don’t be afraid to lead an effort to protect trees, locally or globally. Two 11-year-old Girl Scouts, concerned about the destruction of rainforests for palm oil plantations, led an effort to eliminate palm oil in Girl Scout cookies.
Sometimes change is slow, but together people can make it happen.
Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.
And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.
Gregory Moore, The University of MelbourneThe housing market in most parts of Australia is notoriously competitive. You might be surprised to learn we humans are not the only ones facing such difficulties.
With spring rapidly approaching, and perhaps a little earlier due to climate change, many birds are currently on the hunt for the best nesting sites.
This can be hard enough for birds that construct nests from leaves and twigs in the canopies of shrubs and trees, but imagine how hard it must be for species that nest in tree hollows.
They are looking for hollows of just the right size, in just the right place. Competition for these prime locations is cut-throat.
Sulphur-crested cockatoos battling for spots
Sulphur-crested cockatoos, Cacatua galerita, are relatively large birds, so naturally the hollows they nest in need to be quite large.
It can take 150 years or more before the hollows in the eucalypts that many native parrot species nest in are large enough to accommodate nesting sulphur-crested cockatoos. Such old trees are becoming rarer as old trees on farms die and old trees in cities are cleared for urban growth.
In late winter, early spring you quite often find sulphur crested-cockatoos squabbling among themselves over hollows in trees.
It can take 150 years or more before the hollows in the eucalypts that many native parrot species nest in are large enough to accommodate nesting sulphur-crested cockatoos. Shutterstock
These squabbles can be very loud and raucous. They can last from a few minutes to over an hour, if the site is good one. Once a pair of birds takes possession and begins nesting, they defend their spot and things tend to quieten down.
The stakes are high, because sulphur-crested cockatoos cannot breed if they don’t have a nesting hollow.
In parts of southeastern Australia, rainbow lorikeets, Trichoglossus moluccanus (and/or Trichoglossus haematodus), have expanded their range over the past couple of decades. It is not uncommon to see sulphur-crested cockatoos in dispute with them over a hollow.
The din can be deafening and if you watch you will see both comedy and drama unfold. The sulphur-crested cockatoos usually win and drive the lorikeets away, but all is not lost for the lorikeets.
Sometimes the hollows prove unsuitable — usually if they are too small for the cockatoos — and a few days later the lorikeets have taken up residence. Larger hollows are rarer and so more highly prized.
It is not uncommon to see sulphur-crested cockatoos in dispute with rainbow lorikeets over a hollow. Shutterstock
How hollows form
Many hollows begin at the stubs of branches that have been shed either as part of the tree’s growth cycle or after storm damage. The wood at the centre of the branch often lacks protective defences and so begins to decay while the healthy tree continues to grow over and around the hollow.
Other hollows develop after damage to the trunk or on a large branch, following lightning damage or insect attack. Parrots will often peck at the hollow to expand it or stop it growing over completely. Just a bit of regular home maintenance.
Sulphur-crested cockatoos can often be seen pecking at the top of large branches on old trees, where the branch meets the trunk. They can do considerable damage. When this area begins to decay, it can provide an ideal hollow for future nesting.
Sadly, for the cockatoo, it may take another century or so and the tree might shed the limb in the interim. Cockatoos apparently play a long game and take a very long term perspective on future nesting sites.
Every effort must be made to ensure old, hollow-forming trees are preserved. Shutterstock
Which trees are best for hollows?
In watching the local battles for parrot nesting sites, some tree species are the scenes of many a conflict.
Sugar gums, Eucalyptus cladocalyx, were widely planted as wind breaks in southern Australia and they were often lopped to encourage a bushier habit that provided greater shade.
Poor pruning often leads to hollows and cavities, which are now proving ideal for nesting — but it also resulted in poor tree structure. Sugar gums are being removed and nesting sites lost in many country towns and peri-urban areas (usually the areas around the edges of suburbs with some remaining natural vegetation, or the areas around waterways).
Many species need hollows for nests. Shutterstock
Old river red gums, (Eucalyptus camaldulensis) growing along our creeks and rivers are also great nesting sites. They are so big they provide ideal sites for even the largest of birds.
These, too, are ageing and in many places are declining as riverine ecosystems suffer in general. Even the old elms, Ulmus, and London plane trees, Platanus x acerifolia — which were once lopped back to major branch stubs each year, leading hollows to develop — are disappearing as they age and old blocks are cleared for townhouses.
Cavities in trees are not that common. Large cavities are especially valuable assets. They are essential to maintaining biodiversity because it is not just birds, but mammals, reptiles, insects and arachnids that rely on them for nesting and refuge.
If you have a tree with a hollow, look after it. And while some trees with hollows might be hazardous, most are not. Every effort must be made to ensure old, hollow-forming trees are preserved. Just as importantly, we must allow hollow-forming trees to persist for long enough to from hollows.
We consider our homes to be our castles. Other species value their homes just as highly, so let’s make sure there are plenty of tree hollows in future.
Three fresh books (chosen from a wider field of titles on the topic) exemplify how trees can be written about as more than just background or an incidental part of a landscape, but as integral to meaning.
My Forests: Travel with Trees by Janine Burke, The Heartbeat of Trees by Peter Wohlleben, and Tree Story, a collection curated by Charlotte Day and Brian Martin — are mixed in style and content. But all make clear the close relationships between people and trees and the vital importance of those connections.
It is not surprising that at a time of significant climate change, where natural ecosystems around the world are being devastated and after 18 months of a global pandemic, books on trees are proving popular.
There is an air of desperation in these three titles. Things are changing fast, trees and forests grow slowly, we are wasting time.
Books about trees are published every year. Some are beautifully illustrated with photos or hand-drawn images of special trees in large coffee table formats. Some, like J. R. R. Tolkien’s Lord of the Rings, have trees and forests as characters. Tolkien told a fan that his magnificent Ents were “either souls sent to inhabit trees, or else were folk who slowly took the likeness of trees owing to their inborn love of trees”.
Tolkien’s writing, including a story collection called Tree and Leaf, reminds us of the differences between tree time and human time — we humans are hasty folk. This is something I dwell upon often.
The Magic Faraway Tree by Enid Blyton was one of the first books I can recall reading where a tree played a major role and it helped set me on a path of lifelong reading and interest in botany.
That childhood favourite connects to Richard Powers’ The Overstory, which draws together a disparate fictional band of tree protectors. After his book became a hit, Powers recommended 26 other titles for tree-loving readers.
This library of tree books has served a wide and varied readership well and sustained those of us who despair at the wholesale clearing of forests and trees in our cities and suburbs.
In most Australian cities we are losing trees and canopy cover at a rate of about 1-1.5% per year. I’m still saddened by the loss of a lemon scented gum (Corymbia citriodora) that grew at the city end of the Tullamarine Freeway in Melbourne. I miss its shade in summer but also the delicious scent that wafted through the car window at certain times of the year.
In October last year, protesters mourned a sacred 350-year-old Djab Wurrung Directions Tree, cut down along Victoria’s Western Highway.
There has been a growing disconnect between people and trees and vegetated spaces, particularly for those living in cities. Many people have become so focused on urban survival they have become distanced from the essential and intimate dependence that human beings have on plant life.
Earth as we know it, and the lifeforms it sustains, depend upon and have been shaped by plants and their evolution. Human beings can only survive on our planet because of the ecosystems made possible by plants and trees. If these systems are put in jeopardy because people fail to appreciate the importance of plants, then entire ecosystems are put in peril with profound consequences for humankind.
Climate change is giving us a glimpse of how these important relationships are affected by bushfires, stronger winds from unusual directions and more frequent storms with heavy rainfall that can lead of the loss of grand old trees that have stood as silent sentinels for decades and centuries.
All plants in an ecosystem are important to its function, but the large size and long lives of trees explain why they are often focused upon as representatives of their communities. Their size makes them obvious and contributes to the ambience of any landscape, but can also inspire a sense of awe and in some urban-dwellers, fear.
Their long life spans provide a sense of certainly and continuity in uncertain times of rapid change — their presence can link several human generations, when other connections have been lost. They also provide a tangible prospect, if they are left alone or are properly managed, for links to future generations. All of this can be very reassuring for people who feel vulnerable and oppressed by rapid change.
All three of the new books selected tend to anthropomorphise trees and aspects of their biology, attributing to them distinctly human qualities. Sometimes they are described by a mood, such as an upbeat growth in spring or by a willingness to share resources with other species. While this may be annoying to some scientists, it allows many people to relate or even identify more closely with trees, especially when there is complex biology and ecology involved.
Peter Wohlleben’s bestselling 2016 book The Hidden life of Trees, took readers on a voyage of discovery with a blend of science, philosophy and spiritualism.
Like that first book, his latest — The Heartbeat of Trees — can be enthralling and annoying almost in equal measure. But the author clearly relates the importance of using our senses when we are in forests to explore the complexity of tree biology. By doing so not only will we achieve a better understanding of trees, but also of ourselves and the importance of trees and vegetated places for human development, our physical and mental health and the sustainability of our societies. It will surely resonate strongly with readers after the pandemic lockdowns of the past year, which saw people flocking to parks, gardens and forests.
A personal and professional travelogue woven together by trees is the framework of My Forests: Travel with Trees, by Janine Burke. As an art historian Burke weaves her own experiences with trees with those depicted in paintings, ancient mythology and historic and literary texts.
It is a set of idiosyncratic connections that may not resonate with all readers, but the strong cultural links between trees and ancient human history are undeniable. The reader can learn a great deal about people but relatively little about trees themselves — they remain illusory, almost furtive.
Tree Story, curated by Charlotte Day and Brian Martin catalogues a recent exhibition at Monash University Museum of Art. It is an eclectic mix of style, content, form and media. Some of the images and text do not do justice to the works, but the book does provide a permanent and curated record of what was offered.
The book makes it clear that people see and connect with trees in different, varied and curious ways. While the works may look at the past, there are clear implications, messages and lessons for the present and importantly for the future. Indigenous voices and perspectives speak loudly, longingly and desperately. The works plead that we cannot go on treating trees in this way: for our own health and sustainable futures we must recognise that ultimately all earthly life is essentially one.
Strengthening the bond
The three books, in their own and different ways, challenge how we think about and interact with trees. They broaden the relationship that exists between trees and people and encourage an active and positive interaction. There is a unifying theme that healthy relationships will benefit both people and trees.
Authors and artists recount their personal stories of trees benefiting their own physical and mental well-being. Research shows that trees along streets and roadways have a traffic calming effect that results in slower speeds and more courteous driver behaviour. In a huge study of women’s health in the United States it was shown that green spaces (parks, gardens and trees) significantly correlated with many aspects of improved health.
Plants and trees are not passive participants in ecosystems. They actively contribute to the complexity, resilience and survival of these systems and while the environment affects and changes them, they also modify the environment. Shade from trees cools the understorey and soils, making it possible for a more diverse range of species to thrive. Shade on creeks and rivers helps native fish survive and breed.
These books highlight the complexity of the relationships that many of us have with trees – relationships that can bring change to both us and the trees.
Wohlleben asks that we use all our senses when we interact with trees and forests. There is more going on than meets the eye. Burke reminds us that culture and tradition influence our perception of trees and forests. The works exhibited in Tree Story help us to explore these influences and their meaning.
We are far from knowing all there is to know about plants, trees, forests and ecosystems. The scientific approach is but one method of questing for truth. The open-minded approaches explored in these books could stimulate new discoveries.
The books remind us of the pace of change being wrought on trees and forests by climate change and that the stakes, if we don’t reverse this decline, are very high.
Scientists should never dismiss what they don’t understand. Neither should readers. As climates change, the presence of trees and green space will be recognised as a priority. Trees will be a part of our futures no matter where we live because we cannot have economically viable, environmentally sustainable or liveable places without them.
Books can remind us what we have, and what’s at stake. Shutterstock
Gregory Moore, The University of MelbourneThe savage storms that swept Victoria last week sent trees crashing down, destroying homes and blocking roads. Under climate change, stronger winds and extreme storms will be more frequent. This will cause more trees to fall and, sadly, people may die.
These incidents are sometimes described as an act of God or Mother Nature’s fury. Such descriptions obscure the role of good management in minimising the chance a tree will fall. The fact is, much can be done to prevent these events.
Trees must be better managed for several reasons. The first, of course, is to prevent damage to life and property. The second is to avoid unnecessary tree removals. Following storms, councils typically see a spike in requests for tree removals – sometimes for perfectly healthy trees.
A better understanding of the science behind falling trees – followed by informed action – will help keep us safe and ensure trees continue to provide their many benefits.
We must try to stop trees falling over to prevent damage to life and property. James Ross/AAP
Why trees fall over
First, it’s important to note that fallen trees are the exception at any time, including storms. Most trees won’t topple over or shed major limbs. I estimate fewer than three trees in 100,000 fall during a storm.
Often, fallen trees near homes, suburbs and towns were mistreated or poorly managed in preceding years. In the rare event a tree does fall over, it’s usually due to one or more of these factors:
1. Soggy soil
In strong winds, tree roots are more likely to break free from wet soil than drier soil. In arboriculture, such events are called windthrow.
A root system may become waterlogged when landscaping alters drainage around trees, or when house foundations disrupt underground water movement. This can be overcome by improving soil drainage with pipes or surface contouring that redirects water away from trees.
You can also encourage a tree’s root growth by mulching around the tree under the “dripline” – the outer edge of the canopy from which water drips to the ground. Applying a mixed-particle-size organic mulch to a depth of 75-100 millimetres will help keep the soil friable, aerated and moist. But bear in mind, mulch can be a fire risk in some conditions.
Root systems can also become waterlogged after heavy rain. So when both heavy rain and strong winds are predicted, be alert to the possibility of falling trees.
A combination of heavy rain and strong winds can cause trees to fall. Shutterstock
2. Direct root damage
Human-caused damage to root systems is a common cause of tree failure. Such damage can include roots being:
cut when utility services are installed
restricted by a new road, footpath or driveway
compacted over time, such as when they extend under driveways.
Trees can take a long time to respond to disturbances. When a tree falls in a storm, it may be the result of damage inflicted 10-15 years ago.
This elm, growing very close to a footpath, fell in Melbourne during a 2005 storm. Author provided
3. Wind direction
Trees anchor themselves against prevailing winds by growing roots in a particular pattern. Most of the supporting root structure of large trees grows on the windward side of the trunk.
If winds come from an uncommon direction, and with a greater-than-usual speed, trees may be vulnerable to falling. Even if the winds come from the usual direction, if the roots on the windward side are damaged, the tree may topple over.
The risk of this happening is likely to worsen under climate change, when winds are more likely to come from new directions.
4. Dead limbs
Dead or dying tree limbs with little foliage are most at risk of falling during storms. The risk can be reduced by removing dead wood in the canopy.
Trees can also fall during strong winds when they have so-called “co-dominant” stems. These V-shaped stems are about the same diameter and emerge from the same place on the trunk.
If you think you might have such trees on your property, it’s well worth having them inspected. Arborists are trained to recognise these trees and assess their danger.
Storms can trigger falling trees which block roads. Shutterstock
Trees are worth the trouble
Even with the best tree management regime, there is no guarantee every tree will stay upright during a storm. Even a healthy, well managed tree can fall over in extremely high winds.
While falling trees are rare, there are steps we can take to minimise the damage they cause. For example, in densely populated areas, we should consider moving power and communications infrastructure underground.
By now, you may be thinking large trees are just too unsafe to grow in urban areas, and should be removed. But we need trees to help us cope with storms and other extreme weather.
Removing all trees around a building can cause wind speeds to double, which puts roofs, buildings and lives at greater risk. Removing trees from steep slopes can cause the land to become unstable and more prone to landslides. And of course, trees keep us cooler during summer heatwaves.
Victoria’s spate of fallen trees is a concern, but removing them is not the answer. Instead, we must learn how to better manage and live with them.
Gregory Moore, The University of MelbourneNative deciduous trees are rare in Australia, which means many of the red, yellow and brown leaves we associate with autumn come from introduced species, such as maples, oaks and elms.
One native tree, however, stands out for its leaves with soft autumnal hues that drop in March and April: Australia’s red cedar. Don’t be fooled by its common name — red cedar is not a cedar at all, but naturally grows in rainforests throughout Southeast Asia and Australia.
You may be more familiar with its timber, which I’ve been acquainted with all of my life. My grandmothers had cedar chests of drawers they had inherited from their mothers or grandmothers, and I had assumed they were made from one of the Northern hemisphere cedar species. The wood still smelled of cedar after all this time in family homes – a scent I associate with grandparents and country homes.
By the time I was given one of these chests to restore, I knew much more about the tree and valued the chest of drawers all the more. So, with autumn putting a spotlight on Australian red cedars, let’s look at this species in more detail.
Majestic giants of the rainforest
I first encountered red cedar trees in the sub-tropical rainforests of Queensland and New South Wales in the 1980s. Then, its scientific name was called Cedrela toona and later Toona australis. Now, it’s recognised as Toona ciliata.
The various names reflect a taxonomic history in which the Australian species was once regarded as being separate from its Asian relatives, but all are now considered one.
Native red cedar trees can grow up to 60m tall. Shutterstock
The trees are awe-inspiring. Under the right conditions, it can grow to 60 metres tall (occasionally more) with a trunk diameter of up to 7m.
After losing its foliage in autumn, the new foliage in spring often has an attractive reddish tinge. In late spring it has small (5 milimetres) white or pale pink flowers, but they usually go unnoticed in the rainforest because of their height or the density of other tree canopies growing beneath.
Older red cedars have wonderful buttresses at the base of their trunk, a characteristic shared by many tall tropical trees. These buttresses have long been considered an advantage for species that can emerge above the canopy of a rainforest where winds are much stronger, with the buttresses and expanded root systems providing greater strength and resistance to the wind.
These buttresses also greatly increase the surface area of the base of the trees exposed to air, which facilitates the uptake of extra oxygen as the activity of micro-organisms in the soil can leave it oxygen-depleted.
Tiny white flowers are hard to see from the ground in a rainforest. Forest and Kim Starr/Wikimedia, CC BY-SA
Logged to near extinction
With a wide distribution throughout Asia and Australia, its uses in ancient times were many and varied. In traditional medicine, bark was used or digestive remedies as well as wound dressing and its resin was used for treating skin conditions.
Dyes, oils and tannins used for preparing leather could also be extracted by boiling various plant parts. Today the wood is used for culturing shiitake mushrooms, which are much in demand in restaurants.
But the recent history of red cedar is a typically sad colonial tale. The species belongs to the same family as mahogany (Meliaceae) and, not surprisingly, was exploited for its timber from the early days of colonisation.
You can find red cedar timber in many public buildings across Australia. denisbin/Flickr, CC BY-ND
The timber is durable, lightweight and suitable for naval use and so was very heavily logged, right along the east coast of Australia from the early 1800s until the early 20th century.
The rich deep red colour of its timber and the fact it was soft and easily worked meant it was used for furniture, ornate carvings in public buildings, town halls and parliaments, such as the State Library in Melbourne. It was also used for implements and handles, and for sailing and racing boats.
You’ve probably had a close encounter with the lovely red banisters on some of these old buildings that were made of red cedar, often darkened under the patina of so many hands.
The once common and widespread species was logged almost to extinction along the east coast by the mid-1900s, and to the point of practical commercial extinction with little timber available to industry by the 1960s.
So valued was the timber that in the late 1970s, a plan was hatched to remove red cedar from Queensland National Park rainforests using helicopters. Luckily, the idea did not fly and so some great trees persist. The species has a conservation status of concern, but is not considered to be endangered at present.
The leaves of red cedar begin to fall in late March. Peter Woodard/Wikimedia
A terrible pest
The fact they are deciduous makes them potentially very interesting and useful for horticultural use, but that potential remains largely unrealised. And given the value and quality of its timber, you may be wondering why it’s not being grown in plantations across the continent.
The reason is a native moth called the cedar tip moth (Hypsipyla robusta), which lays its eggs on the main growing shoot of the tree. When the eggs hatch the larvae bore down the shoot, which not only results in shoot dieback but also causes the trees to develop multiple stems and branches which reduce its timber value.
Despite this, they are still planted as a quick-growing ornamental tree for their shade in other parts of the world, such Hawaii and Zimbabwe.
The moths are attracted to the scent of the tree, so they’re very difficult to control. The moth does not attack the tree in South America, for instance, because the moth has not established there, so there are large plantations of red cedar in Brazil.
It’s an interesting reminder: often it’s the little things in ecology that can affect success, or failure. When we humans meddle without knowledge, things don’t necessarily go to plan, usually to our cost.
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.
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.
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.
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.
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.
Moreton Bay figs are prone to insect infestations. Shutterstock
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.
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.
The Research Brief is a short take on interesting academic work.
The big idea
Biologists have long known that rainforest treetops support a huge number of beetle species, but why these canopies are so rich in beetle diversity has remained a mystery. New research by my colleague Susan Kirmse and me shows that flowering trees play a critical role in maintaining this diversity, and that beetles may be among the most diverse pollinators in the animal kingdom.
We carried out a one-year study in a remote part of the Amazon rainforest in Venezuela. We used a specially built crane to collect a total of 6,698 adult beetles representing 859 species. These were gathered from 45 individual trees of 23 different tree species.
We were surprised to discover that the majority of these beetles – 647, or 75.3% of species found – were living on flowering trees. In fact, 527 beetle species in 41 families were associated exclusively with flowers. Interestingly, the majority of these species – almost 60% – were exclusively found on trees that produce lots of small white flowers.
Overall, this discovery shows that flowering trees are likely among the most important drivers for maintaining the high diversity of beetles in rainforests. But this relationship goes both ways. Our study also suggests that beetles may be among the most underappreciated pollinators in tropical forests.
Using a specialized crane, the team was able to collect beetles from the very top of the forest canopy. Susan Kirmse, CC BY-ND
Why it matters
Tropical rainforests are the very heart of Earth’s biodiversity. They harbor about 65% to 75% of all terrestrial species, including the most tree species and the most insects.
After finding such a tight relationship between beetles and flowering trees, we wondered: How many beetle species could be involved in pollination in the Amazon? Our study found an average of 26.35 unique beetle species for every species of tree. With an estimated 16,000 Amazonian tree species, this suggests that there might be more species of flower-visiting beetles than any other insects on Earth, potentially surpassing by far the 20,000 species of bees and the 19,000 species of butterflies.
Our study shows that flowering tree species play an important role as diversity hotspots in tropical rainforest canopies. For policymakers and biologists hoping to preserve or restore rainforests, promoting the cultivation of trees and other plants – especially those with lots of small white flowers that beetles love – could help to maintain species-rich communities. Flowers are a very important resource, providing food and shelter for thousands of insects in addition to beetles. Thus, preserving plant diversity or selecting many different indigenous tree species for reforestation can enhance the diversity of insects.
Beetles like the Griburius auricapillus are just some of the hundreds of species that can be found in treetops. Susan Kirmse, CC BY-ND
What still isn’t known
Our research was the first to describe this tight relationship between beetles and rainforest trees, especially with trees that produce thousands of small, simple flowers. But how this association came to be is still unclear.
Many of the beetle species were found only on trees with this particular type of flower. The trees get an obvious benefit: pollination. But what specifically these trees offer to the beetles requires further study. The simpler flowers are easier for beetles to access, but is the appeal food, like petals, pollen or nectar? Or maybe a home to find mates or lay eggs for the young to grow?
To fight the worldwide rapid declines in insect diversity, researchers and conservationists must understand the ecological connections between insects and their food plants. Long-term studies, particularly in research plots like the one we used in Venezuela, allow researchers to collect layers of information that help unravel the complexity of diversity.
Yet such sites rely on political interest and stability. Political instability in Venezuela is preventing our fieldwork from continuing at the Venezuela plot.
While we can’t return to our study site in Venezuela, it is clear that researchers must work together to understand the mysteries of life on Earth. But biologists are racing the clock as large rainforests are destroyed forever.
From Tasmania’s majestic forest giants to the eucalypt on your nature strip, trees in Australia are many, varied and sometimes huge. But how many are there exactly? And how does their number change over time?
To answer such questions, we mapped changes in Australia’s tree cover in detail, using 30 years of satellite images. We published the results in a recent paper and made the data available for everyone in our new TreeChange web interactive.
Perhaps surprisingly, it turns out that since 1990 we’ve been gaining trees faster than we are losing them. On average, we’ve been gaining eight “standard trees” per year for every Australian.
In total, we found there is currently the equivalent of 1,000 standard trees for every Australian. But this doesn’t mean all our forests are doing well.
There are 24 billion standard trees in Australia
Counting trees is difficult, as there are always more small trees than big ones. So we defined a “standard”: imagine a gum tree with a trunk 30 centimetres in diameter, standing about 15 metres tall.
It’s the sort of good-sized tree you might find in your street or backyard — not huge, but not small either. It might have been planted 15 or 20 years ago. Cut it down and let it dry out, and it will weigh about half a ton.
To count the number of trees in Australia, we first estimated the total mass of trees by combining satellite and field measurements. Then we compared this result to the weight of a standard tree.
We found the total forest biomass across Australia holds the equivalent of about 24 billion standard trees.
If you want to know how forests and woodlands are faring in your state, council or on any property, you can use our TreeChange interactive.
What this means for forests and carbon emissions
If the total mass and number of trees has increased in Australia, does this mean the area of forests has expanded, too? To determine that, you need to decide how many trees make a forest.
By this definition, we gained a staggering 28 million hectares of forest over the last 30 years, plus another 24 million hectares of woodland.
So where did they come from, and why wasn’t it reported in the news? Probably because most of the trees were already there. They just grew larger and denser, and crossed the threshold of our definition of a forest, so were counted in.
Examples of standard trees, pictured outside my office.
And are eight new trees each year, per person, enough to soak up our greenhouse gas emissions? No.
And additional carbon is stored on the forest floor in, for example, logs and branches, as well as under the surface as organic matter. This is worth, perhaps, several more trees of carbon. But it is not clear how safe those carbon deposits are from fire and drought.
Still, if you wanted to set yourself a new year’s resolution, planting those additional 16 trees would be a great start.
Gains and losses
The increasing trend in forest extent has not been smooth — there have been big swings corresponding to wet and dry periods.
For example, the climate of northern Australia has become wetter over the last 30 years, which has helped tree growth. Changes in fire regime and the fertilising effect of our carbon dioxide emissions into the atmosphere may also have played a role.
And just like increased rainfall can help increase the area of forests, drought and bushfire can cause them to disappear.
Bushfires can thin vegetation so it falls short of the definition of a forest. Shutterstock
Bushfires may not remove or even kill most trees, but they can cause enough dieback, scorching or thinning for the vegetation to fall short of the definition of a forest or woodland.
Logging can also cause a patchwork of gains and losses when it goes through cycles of harvesting, regrowth and replanting. And land clearing of native forests still occurs in Australia, such as in the old growth forests of Tasmania, which are vital for native wildlife.
It’s not all good news
While we found the total area and biomass of forests and woodlands has been rising, quality can be more important than quantity when it comes to our ecosystems.
Many things are required to make up a high quality forest, such as a rich understory of perennial species, including grasses and shrubs, and even logs and branches on the ground. These features provide important habitats for many native animals.
Large old trees are also important. Some trees take hundreds of years to reach their greatest size, towering up to 100 meters tall.
These forest giants are an ecosystem in themselves, with birds and tree-dwelling mammals, such as sugargliders, relying on their nooks and crannies. Old growth forests also hold far more carbon than a new forest.
Many native birds rely on the nooks and crannies of old growth trees. Shutterstock
In some cases, a few remaining forests and woodlands are all that’s left of an endangered ecosystem, such as once-abundant box gum grassy woodlands.
Such old or rare forests are difficult or impossible to replace once lost. So creating new forests should never be seen as an alternative for protecting our existing ones.
Environmental scientists see flora, fauna and phenomena the rest of us rarely do. In this new series, we’ve invited them to share their unique photos from the field.
Tasmania’s native forests are home to some of the tallest, most beautiful trees in the world. They provide a habitat for many species, from black cockatoos and masked owls to the critically endangered swift parrot.
But these old, giant trees are being logged at alarming rates, despite their enormous ecological and heritage value (and untapped tourism potential). Many were also destroyed in Tasmania’s early 2019 fires.
Former Greens leader Bob Brown recently launched a legal challenge to Tasmania’s native forest logging. And this year, Forestry Watch, a small group of citizen scientists, found five giant trees measuring more than five metres in diameter inside logging coupes. “Coupes” are areas of forest chopped down in one logging operation.
These trees are too important to be destroyed in the name of the forestry industry. This is why my husband Steve Pearce and I climb, explore and photograph these trees: to raise awareness and foster appreciation for the forests and their magnificent giants.
Climbing trees is not just for the young, but for the young at heart. Kevin is in his early 70’s and helps us with measuring giant trees. Steve Pearce/The Tree Projects, Author provided
What makes these trees so special?
Eualypytus regnans, known more commonly as Mountain Ash or Swamp Gum, can grow to 100 metres tall and live for more than 500 years. For a long time this species held the record as the tallest flowering tree. But last year, a 100.8 m tall Yellow Meranti (Shorea faguetiana) in Borneo, claimed the title — surpassing our tallest Eucalypt, named Centrioun, by a mere 30 centimetres.
Centrioun still holds the record as the tallest tree in the southern hemisphere. But five species of Eucalypt also grow above 85 m tall, with many ranking among some of the tallest trees in the world.
It’s not only their height that make these trees special, they’re also the most carbon dense forests in the world, with a single hectare storing more than 1,867 tonnes of carbon.
Our giant trees and old growth forests provide a myriad of ecological services such as water supply, climate abatement and habitat for threatened species. A 2017 study from the Central Highlands forests in Victoria has shown they’re worth A$310 million for water supply, A$260 million for tourism and A$49 million for carbon storage.
This significantly dwarfs the A$12 million comparison for native forest timber production in the region.
Chopping down old growth trees doesn’t make economic sense. Steve Pearce/The Tree Projects, Author provided
Tasmania’s Big Tree Register
Logging organisation Sustainable Timber Tasmania’s giant tree policy recognises the national and international significance of giant trees. To qualify for protection, trees must be at least 85 m tall or at least an estimated 280 cubic metres in stem volume.
While it’s a good place to start, this policy fails to consider the next generation of big, or truly exceptional trees that don’t quite reach these lofty heights.
That’s why we’ve created Tasmania’s Big Tree Register, an open-source public record of the location and measurements of more than 200 trees to help adventurers and tree-admirers locate and experience these giants for themselves. And, we hope, to protect them.
Last month, three giant trees measuring more than 5 m in diameter were added to the register. But these newly discovered trees are located in coupe TN034G, which is scheduled to be logged this year.
Logging is a very poor economic use for our forests. Native forest logging in Tasmania has struggled to make a profit due to declining demand for non-Forest Stewardship Council certified timber, which Sustainable Timber Tasmania recently failed. In fact, Sustainable Timber Tasmania sustained an eye watering cash loss of A$454 million over 20 years from 1997 to 2017.
The following photos can help show why these trees, as one of the great wonders of the world, should be embraced as an important part of our environmental heritage, not turned to wood chips.
Steve Pearce/The Tree Projects, Author provided
It’s not often you get to see the entirety of a tree in a single photo. This tree above is named Gandalf’s Staff and is a Eucalyptus regnans, measuring 84 m tall.
While Mountain Ash is the tallest species, others in Tasmania’s forests are also breathtakingly huge, such as the Tasmanian blue gum (Eucalyptus globulus) at 92 m, Manna gum (Eucalyptus viminalis) at 91 m, Alpine ash (Eucalyptus delegatensis) at 88 m and the Messmate Stringybark (Eucalyptus obliqua) at 86 m.
Steve Pearce/The Tree Projects, Author provided
This giant tree, pictured above, was a Messmate Stringybark that was felled in coupe, but was left behind for unknown reasons. Its diameter is 4.4 metres. Other giant trees like this were cut down in this coupe, many of which provided excellent nesting habitat for the critically endangered swift parrot.
The citizen science group Forestry Watch helps search for and measure giant trees in Tasmania. Steve Pearce/The Tree Projects, Author provided
Old-growth forests dominated by giant trees are excellent at storing large amounts of carbon. Large trees continue to grow over their lifetime and absorb more carbon than younger trees.
Steve Pearce/The Tree Projects, Author provided
The tree in the photo above is called Obolus, from Greek mythology, with a diameter of 5.1 m. Names are generally given to trees by the person who first records them, and usually reflect the characteristics of the tree or tie in with certain themes.
For example, several trees in a valley are all named after Lord of the Rings characters, such as Gandalf’s Staff (pictured above), Fangorn and Morannon.
Steve Pearce/The Tree Projects, Author provided
Giant trees are typically associated with Californian Redwoods or the Giant Sequoias in the US, where tall tree tourism is huge industry. The estimated revenue in 2012 from just four Coastal Redwood reserves is A$58 million dollars per year, providing more than 500 jobs to the local communities.
Few Australians are aware of our own impressive trees. We could easily boost tourism to regional communities in Tasmania if the money was invested into tall tree infrastructure.
Kevin's Walk on the Wild Side 
You must be logged in to post a comment.