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.
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.




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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.

Tiny treetop flowers foster incredible beetle biodiversity



Hundreds of beetle species seem to be specialists that feed only from small white flowers on trees.
Susan Kirmse, CC BY-ND

Caroline S. Chaboo, University of Nebraska-Lincoln

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.

A tall metal structure emerging from the forest canopy in Venezuela.
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.

An image of a iridescent green-blue beetle.
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?

[You’re smart and curious about the world. So are The Conversation’s authors and editors. You can get our highlights each weekend.]

What’s next

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.The Conversation

Caroline S. Chaboo, Adjunct Professor in Insect Systematics, University of Nebraska-Lincoln

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

Every year in Australia, nature grows 8 new trees for you — but that alone won’t fix climate change



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Albert Van Dijk, Australian National University; Cris Brack, Australian National University, and Pablo Rozas Larraondo, Australian National University

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.




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Photos from the field: capturing the grandeur and heartbreak of Tasmania’s giant trees


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.

Typically, to be called a forest in Australia, a canopy of trees over two meters tall needs to shade 20% of the ground. If only 10-20% of the ground is shaded, we call it a woodland instead.




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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.

By international standards our emissions are massive, equivalent to the carbon stored in 24 standard trees per person per year. Even so, those eight new trees do us a big favour.

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.




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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.




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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.




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The Conversation


Albert Van Dijk, Professor, Water and Landscape Dynamics, Fenner School of Environment & Society, Australian National University; Cris Brack, Associate Professor, Fenner School of Environment and Society, Australian National University, and Pablo Rozas Larraondo, Research fellow, Australian National University

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

Photos from the field: capturing the grandeur and heartbreak of Tasmania’s giant trees



Steve Pearce/The Tree Projects, Author provided

Jennifer Sanger, University of Tasmania

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.




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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.




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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.

Chopped wood in a logging coupe.
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.




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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.




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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.

A portrait of an entire tree captured. Its canopy breaches the clouds.

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.

A woman appears tiny standing against an enormous felled tree.

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.

Nine people sit across the trunk of an enormous tree.
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.

A man wraps a measuring tape around a huge tree trunk, covered in moss.

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.

The tops of the giant tree canopies are higher than the clouds.

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.The Conversation

Jennifer Sanger, Research Associate, University of Tasmania

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

Gabon’s large trees store huge amounts of carbon. What must be done to protect them



Ivanov Gleb/Shutterstock

John Poulsen, Duke University

Large trees are the living, breathing giants that tower over tropical forests, providing habitat and food for countless animals, insects and other plants. Could these giants also be the key to slowing climate change?

The Earth’s climate is changing rapidly due to the buildup of greenhouse gases, like carbon dioxide, in the atmosphere as a result of human activities. Trees absorb carbon from the air and store it in their trunks, branches, and roots. In general, the larger the tree, the more carbon it stores.

Globally, tropical forests remove a staggering 15% of carbon dioxide emissions that humans produce. Africa’s tropical forests – the second largest block of rainforest in the world – have a large role to play in slowing climate change.

But large trees are in trouble everywhere. I carried out research to examine the distribution, drivers and threats to large trees in Gabon. Gabon has 87% forest cover and is the second most forested country in the world.

By carrying out this project, I was able to identify areas with a wealth of large trees (and therefore key carbon stores and sinks), what needed to be done to better protect them and eventually recommend those areas as a priority for conservation.

National inventory

In 2012, the government of Gabon began a national inventory of its forests to measure the amount of carbon stored in its trees – one of the first nationwide efforts in the tropics.

An inventory of this scale isn’t easy, especially in a heavily forested country. Technicians from Gabon’s National Parks Agency travelled to every corner of the country, sometimes hiking more than two days crossing swamps and traversing rivers, to measure the diameter and height of trees in plots a bit larger in size than a soccer field.

Using Gabon’s new inventory of 104 plots, we calculated the amount of carbon in 67,466 trees, representing at least 578 different species. We did this by applying equations to the tree measurements.

The results indicated that the density of carbon stored in Gabon’s trees is among the highest in the world. On average, Gabon’s old growth forests harbour more carbon per area than old growth forests in Amazonia and Asia.

Most of this carbon is stored in the largest trees – those with diameters bigger than 70cm at 1.3 meters from the ground. Just the largest 5% of trees stored 50% of the forest carbon. In other words, 3,373 trees out of the 67,466 measured trees contained half of the carbon.

Drivers of forest carbon stocks

Next, we examined the drivers of carbon stocks. What determines whether an area of forest holds many large trees and lots of carbon? Do environmental conditions or human activities have the largest impact on forest carbon stocks?

Environmental factors – such as soil fertility and depth, temperature, precipitation, slope and elevation – often influence the amount of carbon in a forest. During photosynthesis, trees harness energy from the sun to convert water, carbon dioxide, and minerals into carbohydrates for growth. Therefore, forests with low levels of soil minerals or that receive little rainfall should store less carbon than areas with abundant minerals and water.

Human activities – like agriculture and logging – also influence carbon stocks. Cutting down trees for timber, to clear land for farming, or for construction reduces the amount of carbon stored in forests.

We examined the amount of carbon in each tree plot in relation to the environmental factors and human activities associated with the plot. Surprisingly, we found that human activities, not environmental factors, overwhelmingly affect carbon stocks.

The impact of human activities on forest carbon was largely unexpected because of Gabon’s high forest cover (the second highest of any country) and low population density (9 people per square kilometer), 87% of which is located in urban areas. If human impacts are this strong in Gabon, what must their effects be in other tropical nations?

Although we don’t know for sure, we believe past and present swidden (slash-and-burn) agriculture is the principle cause for low carbon stocks in some areas. Forests close to villages had lower levels of carbon, probably because forest clearing for farming converts old growth forest to secondary forest.

Interestingly, forests in logging concessions held similar amounts of carbon as old growth forests. It is too early to conclude that timber harvest doesn’t reduce carbon levels by cutting large trees, but this finding gives hope that logging concessions can be managed sustainably to conserve carbon stocks.

Importantly, forests in national parks stored roughly 25% more carbon than forests outside of parks. Thus, protecting mostly undisturbed forests can effectively conserve carbon and biodiversity.

Saving Gabon’s giants

The critical role of humans in diminishing carbon stocks is both a blessing and a curse. One one hand, the future of forests are in our hands, giving us the power to choose our fate. On the other hand, we cannot ignore the responsibility to act collectively to secure these resources while considering the interests of the countries that host them.

Gabon is taking laudable actions to conserve its forests, including a protected area network of 13 parks. In addition, Gabon is reforming its logging sector and developing a nationwide land use plan. These actions are a great start, yet continued action is necessary to curb the effects of swidden agriculture and ensure that growing industrial agriculture does not reverse Gabon’s achievements.

Intact forests can pay returns. Norway recently committed to paying Gabon $150 million for stewardship of its forests. Conservation of forests requires sacrifice by the Gabonese people. Yet, this payment demonstrates that Gabon’s large trees are a national asset that can contribute to its development as well as an international resource requiring collective action to conserve.The Conversation

John Poulsen, Associate Professor of Tropical Ecology, Duke University

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

Are young trees or old forests more important for slowing climate change?



Jeremy Kieran/Unsplash, CC BY-SA

Tom Pugh, University of Birmingham

Forests are thought to be crucial in the fight against climate change – and with good reason. We’ve known for a long time that the extra CO₂ humans are putting in the atmosphere makes trees grow faster, taking a large portion of that CO₂ back out of the atmosphere and storing it in wood and soils.

But a recent finding that the world’s forests are on average getting “shorter and younger” could imply that the opposite is happening. Adding further confusion, another study recently found that young forests take up more CO₂ globally than older forests, perhaps suggesting that new trees planted today could offset our carbon sins more effectively than ancient woodland.

How does a world in which forests are getting younger and shorter fit with one where they are also growing faster and taking up more CO₂? Are old or young forests more important for slowing climate change? We can answer these questions by thinking about the lifecycle of forest patches, the proportion of them of different ages and how they all respond to a changing environment.




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The forest carbon budget

Let’s start by imagining the world before humans began clearing forests and burning fossil fuels.

In this world, trees that begin growing on open patches of ground grow relatively rapidly for their first several decades. The less successful trees are crowded out and die, but there’s much more growth than death overall, so there is a net removal of CO₂ from the atmosphere, locked away in new wood.

As trees get large two things generally happen. One, they become more vulnerable to other causes of death, such as storms, drought or lightning. Two, they may start to run out of nutrients or get too tall to transport water efficiently. As a result, their net uptake of CO₂ slows down and can approach zero.

Eventually, our patch of trees is disturbed by some big event, like a landslide or fire, killing the trees and opening space for the whole process to start again. The carbon in the dead trees is gradually returned to the atmosphere as they decompose.

The vast majority of the carbon is held in the patches of big, old trees. But in this pre-industrial world, the ability of these patches to continue taking up more carbon is weak. Most of the ongoing uptake is concentrated in the younger patches and is balanced by CO₂ losses from disturbed patches. The forest is carbon neutral.

A misty forest scene.
New trees absorb lots of carbon, old trees store more overall and dead trees shed their carbon to the atmosphere.
Greg Rosenke/Unsplash, CC BY-SA

Now enter humans. The world today has a greater area of young patches of forest than we would naturally expect because historically, we have harvested forests for wood, or converted them to farmland, before allowing them to revert back to forest. Those clearances and harvests of old forests released a lot of CO₂, but when they are allowed to regrow, the resulting young and relatively short forest will continue to remove CO₂ from the atmosphere until it regains its neutral state. In effect, we forced the forest to lend some CO₂ to the atmosphere and the atmosphere will eventually repay that debt, but not a molecule more.

But adding extra CO₂ into the atmosphere, as humans have done so recklessly since the dawn of the industrial revolution, changes the total amount of capital in the system.

And the forest has been taking its share of that capital. We know from controlled experiments that higher atmospheric CO₂ levels enable trees to grow faster. The extent to which the full effect is realised in real forests varies. But computer models and observations agree that faster tree growth due to elevated CO₂ in the atmosphere is currently causing a large carbon uptake. So, more CO₂ in the atmosphere is causing both young and old patches of forest to take up CO₂, and this uptake is larger than that caused by previously felled forests regrowing.

The effect of climate change

But the implications of climate change are quite different. All else being equal, warming tends to increase the likelihood of death among trees, from drought, wildfire or insect outbreaks. This will lower the average age of trees as we move into the future. But, in this case, that younger age does not have a loan-like effect on CO₂. Those young patches of trees may take up CO₂ more strongly than the older patches they replace, but this is more than countered by the increased rate of death. The capacity of the forest to store carbon has been reduced. Rather than the forest loaning CO₂ to the atmosphere, it’s been forced to make a donation.

So increased tree growth from CO₂ and increased death from warming are in competition. In the tropics at least, increased growth is still outstripping increased mortality, meaning that these forests continue to take up huge amounts of carbon. But the gap is narrowing. If that uptake continues to slow, it would mean more of our CO₂ emissions stay in the atmosphere, accelerating climate change.

Overall, both young and old forests play important roles in slowing climate change. Both are taking up CO₂, primarily because there is more CO₂ about. Young forests take up a bit more, but this is largely an accident of history. The extra carbon uptake we get from having a relatively youthful forest will diminish as that forest ages. We can plant new forests to try to generate further uptake, but space is limited.

But it’s important to separate the question of uptake from that of storage. The world’s big, old forests store an enormous amount of carbon, keeping it out of the atmosphere, and will continue to do so, even if their net CO₂ uptake decreases. So long as they are not cut down or burned to ashes, that is.The Conversation

Tom Pugh, Reader in Biosphere-Atmosphere Exchange, University of Birmingham

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

Planting non-native trees accelerates the release of carbon back into the atmosphere



native forest.

Lauren Waller and Warwick Allen, University of Canterbury

Large-scale reforestation projects such as New Zealand’s One Billion Trees programme are underway in many countries to help sequester carbon from the atmosphere.

But there is ongoing debate about whether to prioritise native or non-native plants to fight climate change. As our recent research shows, non-native plants often grow faster compared to native plants, but they also decompose faster and this helps to accelerate the release of 150% more carbon dioxide from the soil.

Our results highlight a challenging gap in our understanding of carbon cycling in newly planted or regenerating forests.

It is relatively easy to measure plant biomass (how quickly a plant grows) and to estimate how much carbon dioxide it has removed from the atmosphere. But measuring carbon release is more difficult because it involves complex interactions between the plant, plant-eating insects and soil microorganisms.

This lack of an integrated carbon cycling model that includes species interactions makes predictions for carbon budgeting exceedingly difficult.




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How non-native plants change the carbon cycle

There is uncertainty in our climate forecasting because we don’t fully understand how the factors that influence carbon cycling – the process in which carbon is both accumulated and lost by plants and soils – differ across ecosystems.

Carbon sequestration projects typically use fast-growing plant species that accumulate carbon in their tissues rapidly. Few projects focus on what goes on in the soil.

Non-native plants often accelerate carbon cycling. They usually have less dense tissues and can grow and incorporate carbon into their tissues faster than native plants. But they also decompose more readily, increasing carbon release back to the atmosphere.

Our research, recently published in the journal Science, shows that when non-native plants arrive in a new place, they establish new interactions with soil organisms. So far, research has mostly focused on how this resetting of interactions with soil microorganisms, herbivorous insects and other organisms helps exotic plants to invade a new place quickly, often overwhelming native species.

Invasive non-native plants have already become a major problem worldwide, and are changing the composition and function of entire ecosystems. But it is less clear how the interactions of invasive non-native plants with other organisms affect carbon cycling.




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Planting non-native trees releases more carbon

We established 160 experimental plant communities, with different combinations of native and non-native plants. We collected and reared herbivorous insects and created identical mixtures which we added to half of the plots.

We also cultured soil microorganisms to create two different soils that we split across the plant communities. One soil contained microorganisms familiar to the plants and another was unfamiliar.

Herbivorous insects and soil microorganisms feed on live and decaying plant tissue. Their ability to grow depends on the nutritional quality of that food. We found that non-native plants provided a better food source for herbivores compared with native plants – and that resulted in more plant-eating insects in communities dominated by non-native plants.

Similarly, exotic plants also raised the abundance of soil microorganisms involved in the rapid decomposition of plant material. This synergy of multiple organisms and interactions (fast-growing plants with less dense tissues, high herbivore abundance, and increased decomposition by soil microorganisms) means that more of the plant carbon is released back into the atmosphere.

In a practical sense, these soil treatments (soils with microorganisms familiar vs. unfamiliar to the plants) mimic the difference between reforestation (replanting an area) and afforestation (planting trees to create a new forest).

Reforested areas are typically replanted with native species that occurred there before, whereas afforested areas are planted with new species. Our results suggest planting non-native trees into soils with microorganisms they have never encountered (in other words, afforestation with non-native plants) may lead to more rapid release of carbon and undermine the effort to mitigate climate change.The Conversation

Lauren Waller, Postdoctoral Fellow and Warwick Allen, Postdoctoral fellow, University of Canterbury

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

Lemurs are the world’s most endangered mammals, but planting trees can help save them



Black-and-white ruffed lemurs are important indicators of rainforest health.
Franck Rabenahy, CC BY-ND

Andrea L. Baden, Hunter College

Madagascar, the world’s fourth-largest island, is a global biodiversity hotspot.
Andrea Baden

The island of Madagascar off the southeastern coast of Africa hosts at least 12,000 plant species and 700 vertebrate species, 80% to 90% of which are found nowhere else on Earth.

Isolated for the last 88 million years and covering an area approximately the size of the northeastern United States, Madagascar is one of the world’s hottest biodiversity hotspots. Its island-wide species diversity is striking, but its tropical forest biodiversity is truly exceptional.

Sadly, human activities are ravaging tropical forests worldwide. Habitat fragmentation, over-harvesting of wood and other forest products, over-hunting, invasive species, pollution and climate change are depleting many of these forests’ native species.

Among these threats, climate change receives special attention because of its global reach. But in my research, I have found that in Madagascar it is not the dominant reason for species decline, although of course it’s an important long-term factor.

As a primatologist and lemur specialist, I study how human pressures affect Madagascar’s highly diverse and endemic signature species. In two recent studies, colleagues and I have found that in particular, the ruffed lemur – an important seed disperser and indicator of rainforest health – is being disproportionately impacted by human activities. Importantly, habitat loss is driving ruffed lemurs’ distributions and genetic health. These findings will be key to helping save them.

Deforestation from slash-and-burn agriculture in the peripheral zones of Ranomafana National Park, Madagascar.
Nina Beeby/Ranomafana Ruffed Lemur Project, CC BY-ND

The forest is disappearing

Madagascar has lost nearly half (44%) of its forests within the last 60 years, largely due to slash-and-burn agriculture – known locally as “tavy” – and charcoal production. Habitat loss and fragmentation runs throughout Madagascar’s history, and the rates of change are staggering.

This destruction threatens Madagascar’s biodiversity and its human population. Nearly 50% of the country’s remaining forest is now located within 300 feet (100 meters) of an unforested area. Deforestation, illegal hunting and collection for the pet trade are pushing many species toward the brink of extinction.

In fact, the International Union for Conservation of Nature estimates that 95% of Madagascar’s lemurs are now threatened, making them the world’s most endangered mammals. Pressure on Madagascar’s biodiversity has significantly increased over the last decade.

A red ruffed lemur, one of two Varecia species endemic to Madagascar.
Varecia Garbutt, CC BY-ND

Deforestation threatens ruffed lemur survival

In a newly published study, climate scientist Toni Lyn Morelli, species distribution expert Adam Smith and I worked with 19 other researchers to study how deforestation and climate change will affect two critically endangered ruffed lemur species over the next century. Using combinations of different deforestation and climate change scenarios, we estimate that suitable rainforest habitat could be reduced by as much as 93%.

If left unchecked, deforestation alone could effectively eliminate ruffed lemurs’ entire eastern rainforest habitat and with it, the animals themselves. In sum, for these lemurs the effects of forest loss will outpace climate change.

But we also found that if current protected areas lose no more forest, climate change and deforestation outside of parks will reduce suitable habitat by only 62%. This means that maintaining and enhancing the integrity of protected areas will be essential for saving Madagascar’s rainforest habitats.

Warm colors indicate areas where lemurs can move about readily, which promotes genetic diversity; cool colors indicate areas where they are more constrained and less able to mate with members of other population groups.
Baden et al. (2019), Nature Scientific Reports, CC BY-ND

In a study published in November 2019, my colleagues and I showed that ruffed lemurs depend on habitat cover to survive. We investigated natural and human-caused impediments that prevent the lemurs from spreading across their range, and tracked the movement of their genes as they ranged between habitats and reproduced. This movement, known as gene flow, is important for maintaining genetic variability within populations, allowing lemurs to adapt to their ever-changing environments.

Based on this analysis, we parsed out which landscape variables – including rivers, elevation, roads, habitat quality and human population density – best explained gene flow in ruffed lemurs. We found that human activity was the best predictor of ruffed lemurs’ population structure and gene flow. Deforestation alongside human communities was the most significant barrier.

Taken together, these and other lines of evidence show that deforestation poses an imminent threat to conservation on Madagascar. Based on our projections, habitat loss is a more immediate threat to lemurs than climate change, at least in the immediate future.

In 1961 naturalist David Attenborough filmed ruffed lemurs for the BBC.

This matters not only for lemurs, but also for other plants and animals in the areas where lemurs are found. The same is true at the global level: More than one-third (about 36.5%) of Earth’s plant species are exceedingly rare and disproportionately affected by human use of land. Regions where the most rare species live are experiencing higher levels of human impact.

Crisis can drive conservation

Scientists have warned that the fate of Madagascar’s rich natural heritage hangs in the balance. Results from our work suggest that strengthening protected areas and reforestation efforts will help to mitigate this devastation while environmentalists work toward long-term solutions for curbing the runaway greenhouse gas emissions that drive climate change.

A young woman participates in reforestation efforts in Kianjavato, Madagascar.
Brittani Robertson/Madagascar Biodiversity Partnership, CC BY-ND

Already, nonprofits are working hard toward these goals. A partnership between Dr. Edward E. Louis Jr., founder of Madagascar Biodiversity Partnership and director of Conservation Genetics at Omaha’s Henry Doorly Zoo, and the Arbor Day Foundation’s Plant Madagascar project has replanted nearly 3 million trees throughout Kianjavato, one region identified by our study. Members of Centre ValBio’s reforestation team – a nonprofit based just outside of Ranomafana National Park that facilitates our ruffed lemur research – are following suit.

At an international conference in Nairobi earlier this year, Madagascar’s president, Andry Rajoelina, promised to reforest 40,000 hectares (99,000 acres) every year for the next five years – the equivalent of 75,000 football fields. This commitment, while encouraging, unfortunately lacks a coherent implementation plan.

Our projections highlight areas of habitat persistence, as well as areas where ruffed lemurs could experience near-complete habitat loss or genetic isolation in the not-so-distant future. Lemurs are an effective indicator of total non-primate community richness in Madagascar, which is another way of saying that protecting lemurs will protect biodiversity. Our results can help pinpoint where to start.

[ Like what you’ve read? Want more? Sign up for The Conversation’s daily newsletter. ]The Conversation

Andrea L. Baden, Assistant Professor of Anthropology, Hunter College

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

Entire hillsides of trees turned brown this summer. Is it the start of ecosystem collapse?



Rachael Nolan, CC BY-NC

Rachael Helene Nolan, Western Sydney University; Belinda Medlyn, Western Sydney University; Brendan Choat, Western Sydney University, and Rhiannon Smith, University of New England

The drought in eastern Australia was a significant driver of this season’s unprecedented bushfires. But it also caused another, less well known environmental calamity this summer: entire hillsides of trees turned from green to brown.

We’ve observed extensive canopy dieback from southeast Queensland down to Canberra. Reports of more dead and dying trees from other regions across Australia are flowing in through the citizen science project, the Dead Tree Detective.




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A few dead trees are not an unusual sight during a drought. But in some places, it is the first time in living memory so much canopy has died off.

Ecologists are now pondering the implications. There are warnings that some Australian tree species could disappear from large parts of their ranges as the climate changes. Could we be witnessing the start of ecosystem collapse?

Extensive canopy dieback in Kains Flat, NSW, January 2020.
Matt Herbert

Why are canopies dying now?

Much of eastern Australia has been in drought since the start of 2017. While this drought is not yet as long as the Millennium Drought, it appears to be more intense. Many areas have received the lowest rainfall on record, including long periods of time with no rainfall. This has been coupled with above-average temperatures and extreme heatwaves.

The higher the temperature, the greater the moisture loss from leaves. This is usually good for a tree because it cools the canopy. But if there is not enough water in the soil, the increased water loss can push trees over a threshold, causing extensive leaf “scorching”, or browning. The extensive canopy dieback we have observed this summer suggests that the soil had finally become too dry for many trees.

Widespread rainfall deciciencies and higher temperatures across many parts of Australia.
Bureau of Meteorology

Are the trees dead?

Brown or bare trees are not necessarily dead. Many eucalypts can lose all their leaves but resprout after rain.

Many parts of eastern Australia are now flushed with green after rain. In these areas, it will be important to assess the extent of tree recovery. If trees are not showing signs of recovery after significant rainfall, they’re unlikely to survive. In some cases carbohydrate reserves – which trees need to resprout new leaves – may be too depleted for trees to recover.

Snowgums in the New England area resprouting in March 2020, following heavy rain. The trees lost most of their canopy during drought in 2019.
Trevor Stace, University of New England

The drought may also hinder post-fire recovery. Most eucalypt forests eventually recover from bushfires by resprouting new leaves. Some forests also recover when fire triggers seedlings to germinate.

But it’s likely that some forests now recovering from fire were already struggling with canopy dieback. So these two disturbances will test how resilient our forests are to back-to-back drought and bushfire.

Trees recovering from drought and/or fire may also enter the “dieback spiral”. The new flush of leaves following rain can make a particularly tasty meal for insects. Trees will then attempt to grow more foliage in response, but their ability to keep producing new leaves gradually declines as they deplete their carbohydrate reserves, and they can die.

Dieback spiral has led to extensive tree loss in the past, including in the New England area of NSW.

Should we be worried?

The capacity of eucalypts to resprout makes them naturally resilient to extended drought. There are some records of canopy dieback from severe droughts in the past, such as the Federation Drought. We assume (although we don’t know for sure) the forests recovered after these events. So they may bounce back after the current drought.

However, it’s hard not to be concerned. Climate change will bring increased drought, heatwaves and fires that could, over time, see extensive losses of trees across the landscape – as happened on the Monaro High Plain after the Millennium Drought.




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Australian research in 2016 warned that due to climate change, the habitat of 90% of eucalypt species could decline and 16 species were expected to lose their home environments within 60 years.

Such a change would have huge consequences for how ecosystems function – reducing the capacity for ecosystem services such as carbon storage, altering catchment water resources and reducing habitat for native animals.

Some trees resprouted new leaves after losing their canopy. But in some cases these leaves are now dying, like on these scribbly gums in the NSW Pilliga in August 2019.
Rachael Nolan

Where to from here?

Records of dead and dying trees on the Dead Tree Detective map.
Dead Tree Detective

Landholders can help bush on their property recover after drought, by protecting germinating seedlings from livestock and collecting local seed for later revegetation. Trees that appear dead should not be cut down as they may recover, and even if dead can provide valuable animal habitat.

Most importantly, however, we need to monitor trees carefully to see where they’ve died, and where they are recovering. A citizen science project, the Dead Tree Detective, is helping map the extent of tree die-off across Australia.

People send in photos of dead and dying trees – to date, over 267 records have been uploaded. These records can be used to target where to monitor forests during drought, including on-ground assessments of tree health and quantifying the physiological responses of trees to drought stress.

There is no ongoing forest health monitoring program in Australia, so this dataset is invaluable in helping us determine exactly how vulnerable Australia’s forests are to the double whammy of severe drought and bushfires.The Conversation

Rachael Helene Nolan, Postdoctoral research fellow, Western Sydney University; Belinda Medlyn, Professor, Western Sydney University; Brendan Choat, Associate Professor, Western Sydney University, and Rhiannon Smith, Research Fellow, University of New England

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

Thousands of city trees have been lost to development, when we need them more than ever


Thami Croeser, RMIT University; Camilo Ordóñez, University of Melbourne, and Rodney van der Ree, University of Melbourne

Climate change is on everyone’s lips this summer. We’ve had bushfires, smoke haze, heatwaves, flooding, mass protests and a National Climate Emergency Summit, all within a few months. The search is on for solutions. Trees often feature prominently when talking about solutions, but our research shows trees are being lost to big developments – about 2,000 within a decade in inner Melbourne.

Big development isn’t the only challenge for urban tree cover. During the period covered by our newly published study, the inner city lost a further 8,000 street trees to a variety of causes – vandals, establishment failures of young trees, drought, smaller developments and vehicle damage.

Still, thanks to a program that plants 3,000 trees a year, canopy growth has kept just ahead of losses in the City of Melbourne.




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Canopy cover is crucial for keeping urban areas liveable, shading our streets to help us cope with hot weather and to counter the powerful urban heat island effect. Trees can also be a flood-proofing tool.

Trees add beauty and character to our streets, and (so far) they’re not a political wedge issue in the ongoing culture war that is Australian climate policy. In short, they’re a very good idea, at just the right time.

Counting the trees lost to development

The thing is, this good idea happens in the midst of a construction boom. In Melbourne alone, this includes thousands of new dwellings and billions of dollars of new infrastructure. Many of the new buildings are very large – there’s a handy open database that shows these developments.

This map shows the scale of development under way in the inner city.
City of Melbourne

Next time you’re walking past a large construction site, look for empty tree pits – the square holes in footpaths where trees have been removed. Maybe you’ve already seen these and wondered what all the construction means for our trees. Well, now we know.

Our study puts a number on the impact of major development on city trees. In the City of Melbourne – that’s just the innermost suburbs and the CBD – major developments cost our streets about 2,000 trees from 2008-2017.

Using council databases and a mapping tool, we tracked removals of trees within ten metres of hundreds of major developments. We found much higher rates of tree removal around major development sites than in control sites that weren’t developed.

An example of our analysis, comparing tree losses around sites with major development (orange) to control sites (blue). Trees within 10m of major developments were much more likely to be removed.
https://doi.org/10.1016/j.scs.2020.102096

Even with the City of Melbourne’s robust tree-protection rules, trees can be removed or damaged due to site access needs, scaffolding, compacted soil, root conflicts with services access, and even the occasional poisoning.

The City of Melbourne invited artist Louise Lavarack to create a roadside memorial to a poisoned plane tree, which was then shrouded in gauze bandages.
Tony & Wayne/Flickr, CC BY-NC



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Tree protection limits losses

The silver lining in this story is that the city council’s tree-protection policy seems to be quite effective at saving our bigger trees. The vast majority of removals we saw were of trees with trunks less than 30cm thick. Only one in 20 of the trees lost was a large mature tree over 60cm thick.

This may partly reflect the fact that the council charges developers for not only tree replacement but also the dollar equivalent of lost amenity and ecological values. It gets very expensive to remove a large tree once you factor in all the valuable services it provides. When a tree is a metre thick, costs can exceed $100,000 – and that’s if there are no alternatives to removal.

The protection of bigger trees means Melbourne retained canopy fairly well, despite losing over 2,000 trees. Only 8% of city-wide canopy losses during our study period happened near major development sites. This modest loss is still serious, as removals are having more of an impact on future canopy growth than current cover.




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Lessons for our cities

While Melbourne-centric, there are lessons in this study for cities everywhere. Robust policies to protect and retain trees backed up by clear financial incentives are valuable, as even well-resourced councils with strong policy face an uphill battle when development gets intense.

Our findings highlight that retaining and establishing young trees is especially difficult. This is troubling given these are the trees that must deliver the canopy that will in future shelter the streets in which we live and work.




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Improved investments in how young trees are planted and how long we look after them can help. For example, in a promising local study, researchers showed that trees planted in a way that catches rainwater run-off from roads grow twice as fast, provided planting design avoids waterlogging.

Finally, in the context of rapid development, buildings themselves can play a positive role. Green roofs, green walls and rain gardens are just a few of the ways developments can help our cities deal with both heat and flooding.

There are plenty of precedents overseas. In Berlin, laws requiring building greening have resulted in 4 million square metres of green roof area – three times the area of Melbourne’s Hoddle Grid. In Singapore, developments must include vegetation with leaf area up to four times the development’s site area, using green roofs and walls. Tokyo has required green roofs on new buildings for nearly 20 years.

The solutions are out there, and urban greening is rising in profile. Recent commitments in Melbourne, Canberra and Adelaide are promising. Our study findings are a reminder that, even for the willing, we’ll have to take two steps forward, because there’s inevitably going to be one step back.




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The Conversation


Thami Croeser, Research Officer, Centre for Urban Research, RMIT University; Camilo Ordóñez, Research Fellow, School of Ecosystem and Forest Sciences, University of Melbourne, and Rodney van der Ree, Adjunct Associate Professor, School of BioSciences. National Technical Executive – Ecology, WSP Pty Ltd, University of Melbourne

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