How the size and shape of dried leaves can turn small flames into colossal bushfires

Jamie Burton, University of Melbourne; Alexander Filkov, University of Melbourne, and Jane Cawson, University of Melbourne

The 2020-21 fire season is well underway, and we’ve watched in horror as places like K’gari (Fraser Island) burn uncontrollably, threatening people and their homes and devastating the environment.

To lessen the impact of fires, we need to know when they are likely to burn and how intensely. Central to this is the flammability of litter beds — the layer of dead leaves, needles, twigs and bark on the forest floor.

Every large fire begins as a small fire, igniting and initially spreading through the litter bed, but what makes some litter beds more flammable than others?

Aerated litter beds fuel bigger fires

Over the past few years, fire scientists across the world have been busy tackling this burning question. In tropical forests in the Amazon, oak forests in North America and eucalypt woodlands in Australia, they have been collecting leaf litter beds and burning them in the laboratory to understand why litter beds from some plant species burn differently to others.

Each of these studies focused on leaf litter beds made up of a single species, and each identified a range of drivers of flammability. These drivers relate to both the characteristics of the individual litter particle (leaf, needle or branch) and the litter bed itself.

Our new research sought to consolidate these studies to find the common drivers of flammability between different single-species litter beds from different parts of the world.

From our meta-analysis, we found “litter packing” and “litter bulk density” were key factors in litter bed flammability.

Litter packing is a measure of how many gaps are between the dried leaves, needles and branches, and is important for determining how much air is available for burning. Likewise, litter bulk density is a measure of how much litter there is, and is important for determining how quickly and how long litter burns.

Oak tree litter bed — The litter bed from oak trees. The curly leaves create air gaps throughout the litter bed, which lead to bigger fires.
Jamie Burton, Author provided

We found loosely packed litter beds spread fire faster, burned for shorter periods of time and were more consumed by the flames. Importantly, we found this was universal across different types of litter beds.

We also identified the characteristics of leaves, needles and branches that cause variations in litter packing and litter bulk density.

For example, if the litter particles are “curly” and have a high surface area to volume ratio, then they’ll form litter beds with low packing ratios which burn faster and have higher consumption. Examples include leaves from some oak (Quercus) species.

At the opposite end, small and less curly leaves form densely packed litter beds which are less aerated. Examples include coast tea tree (Leptospermum laevigatum) and conifers with small needles such as Larix and Picea. This results in slower moving fires, which do not consume all the litter.

For eucalypt litter beds, things are a little more complicated. Some species have thick and flat leaves which pack densely, so fire spreads more slowly and less litter is consumed. Other species, such as the southern blue gum (Eucalyptus globulus), have larger leaves which tend to pack less densely, so fires burn more quickly with taller flames.

Eucalyptus litter bed — The litter bed of eucalyptus trees.
Jamie Burton, Author provided

How can this information help us manage fires?

Of course, under extreme fire weather conditions, any litter bed will burn. However, at the beginning of a fire or under mild conditions, differences in litter characteristics may strongly influence how that fire spreads. Research on this can be useful for many aspects of fire management and planning.

For example, if we know which plants produce less flammable litter, we can select them for planting around houses, landscaping in fire-prone areas and also use them as green firebreaks to reduce the risk to people and homes. If a fire was to start, it may spread less quickly and be less intense, making it easier to contain and put out.

_Allocasuarina_ needle litter — *Allocasuarina* species with long thin needles tend to pack loosely, leading to faster flame spread and shorter burning times.
Jamie Burton, Author provided

But also it may not be that straightforward. When deciding which species to plant, the flammability of living plants needs to be considered, as well. Some plants that have less flammable litter may actually be highly flammable as a living plant. For example, although coast tea tree may form densely packed litter beds, the high oil content in the leaves makes it highly flammable as a living plant.

Our findings could also be used for predicting fire behaviour. For example, our results could be integrated into fire behaviour models, such as the Forest Flammability Model, which uses information on the composition and structure of the plant community to predict fire behaviour.

Next steps

Our study provides information on what leaf and litter characteristics affect flammability in litter beds composed of a single species. But in many forests, litter beds are made up of a variety of plant species, and more research is needed to understand what happens to litter packing and flammability in these multi-species litter beds.

Besides different species, litter beds also contain different components such as twigs and bark. For example, in a mature wet eucalypt forest, bark and twigs can make up to 44% of the litter bed.

And for some eucalypt species, we already know bark burns differently to leaves. For example, the flaky bark of the Sydney red gum (Angophora costata) tends to take longer to ignite, but burns for a longer time compared to its leaves.

With fires becoming more frequent and fire seasons becoming longer, research into litter bed flammability has never been more needed.

Jamie Burton, PhD Candidate, University of Melbourne; Alexander Filkov, Senior research fellow, University of Melbourne, and Jane Cawson, Research Fellow in Bushfire Behaviour and Management, University of Melbourne

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

Why climate change will dull autumn leaf displays

File 20190402 177178 1o0ksl1.jpg?ixlib=rb 1.1 — Autumnal displays may be dimmed in the future.
Shutterstock

Matthew Brookhouse, Australian National University

Every autumn we are treated to one of nature’s finest seasonal annual transitions: leaf colour change and fall.

Most of the autumn leaf-shedding trees in Australia are not native, and some are declared weeds. Nevertheless, Australia has a spectacular display of trees, from the buttery tresses of Ginkgo biloba to the translucent oaks, elms and maples.

Autumn colour changes are celebrated worldwide and, when the time is right, autumn leaves reconnect us to nature, driving “leaf-peeping” tourist economies worldwide.

However, recent temperature trends and extremes have changed the growing conditions experienced by trees and are placing autumn displays, such as Canberra’s, at risk.

Autumn leaf colour changes and fall are affected by summer temperatures.
Shutterstock

This year, Canberra, like the rest of Australia, endured its hottest summer on record. In NSW and the ACT, the mean temperature in January was 6°C warmer than the long-term average. So far, autumn is following suit.

These extremes can interrupt the ideal synchronisation of seasonal changes in temperature and day length, subduing leaf colours.

In addition, hotter summer temperatures scorch leaves and, when combined with this and the previous years’ low autumn rainfall, cause trees to shed leaves prematurely, dulling their autumn leaf displays.

The subtlety of change

We learnt in childhood autumn colour change follows the arrival of cooler temperatures. Later we learnt the specifics: seasonal changes in day length and temperature drive the depletion of green chlorophyll in leaves. Temperature can also affect the rate at which it fades.

In the absence of chlorophyll, yellows and oranges generated by antioxidants in the leaf (carotenoids) as well as red through to purples pigments (anthocyanins), synthesised from stored sugars, emerge. Temperature plays a role here too – intensifying colours as overnight temperatures fall.

We’ve also come to understand the role of a leaf’s environment. Anthocyanin production is affected by light intensity, which explains why sunny autumns produce such rich colours and why the canopies of our favourite trees blush red at their edges while glowing golden in their interior.

However, early signs show this year’s autumn tones will be muted. After the record-breaking heat of summer and prolonged heat of March, many trees are shrouded in scorched, faded canopies. The ground is littered with blackened leaves.

Of course, we’ve seen it before.

During the Millennium Drought, urban trees sporadically shed their leaves often without a hint of colour change. Fortunately, that was reversed at the drought’s end.

But we’re kidding ourselves if we believe this last summer was normal or recent temperature trends are just natural variability. If this is a sign of seasons future, we need to prepare to lose some of autumn’s beauty.

Lost synchronicity

Long-term and experimental data show that the sensitivity of autumn colour change to warmer temperatures varies widely between species. While large-scale meta-analyses point to a delay in the arrival of autumn colours of one day per degree of warming, individual genera may be far more sensitve. Colour change in Fagus is delayed by 6-8 days per degree.

Warming temperatures, then, mean the cohesive leaf-colour changes we’re accustomed to will break down at landscape scales.

In addition, as warm weather extends the growing season and deep-rooted trees deplete soil moisture reservoirs, individual trees are driven by stress rather than seasonal temperature change and cut their losses. They shed leaves at the peripheries of their canopies.

The remainder wait – bronzed by summer, but still mostly green – for the right environmental cue.

For years, careful species selection and selective breeding enhanced autumn colour displays. This rich tapestry is now unravelling as hotter summers, longer autumns and drought affect each species differently.

Paradoxes and indirect effects

It seems logical warmer temperatures would mean shorter and less severe frost seasons. Paradoxically, observations suggest otherwise – the arrival of frost is unchanged or, worse, occurring earlier.

When not preceded by gradually cooling overnight temperatures, frosts can induce sudden, unceremonious leaf loss. If warm autumn temperatures fail to initiate colour change, autumn displays can be short-circuited entirely.

At the centre of many urban-tree plantings, our long association with elms faces a threat. Loved for the contrast their clear yellow seasonal display creates against pale autumn skies, elm canopies have been ravaged by leaf beetles this year. Stress has made trees susceptible to leaf-eating insects, and our current season delivered an expanse of stressed, and now skeletal, trees.

Autumn leaf displays drive tourism.
Norm Hanson/flickr, CC BY-NC-SA

Change everywhere?

This dulled image of autumn is far from universal. Climates differ between locations. So too will the climate changes we’ve engineered and their impact on autumn displays.

Increased concentration of anthocyanins associated with warmer summers has, for example, created spectacular leaf displays in Britain’s cooler climates.

Of course, we’ll continue to experience radiant autumn displays too.

In years of plentiful rain, our trees will retain their canopies and then, in the clear skies of autumn, dazzle us with seasonal celebrations. However, that too may be tempered by the increased risk of colour-sapping pathogens, such as poplar rust, favoured by warm, moist conditions. And there are also negative consequences for autumn colour associated with elevated carbon dioxide concentrations.

Of course, we need to keep it in perspective – the dulling of autumn’s luminescence is far from the worst climate change impacts. Nevetheless, in weakening our link with nature, the human psyche is suffering another self-inflicted cut as collective action on climate change stalls.

Matthew Brookhouse, Senior lecturer, Australian National University

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

Koalas sniff out juicy leaves and break down eucalypt toxins – it’s in their genome

File 20180702 116152 1ux1z28.jpg?ixlib=rb 1.1 — Koalas spend a large part of the day sleeping – while their digestive enzymes get to work.
emmanueleragne/flickr , CC BY

Jenny Graves, La Trobe University

News is out today that the entire genome of the koala has been sequenced. This means we now have a complete read-out of the genes and other DNA sequences of this iconic marsupial mammal.

Knowing the full set of koala genes deepens our knowledge of koalas (and other Australian mammals) in many ways. Now we can understand how koalas manage to survive on such a toxic diet of gum leaves. Now we can follow the fortunes of historic koala populations and make good decisions about how to keep remaining koala populations healthy. Now we have a new point of comparison that we can use to understand how the mammal genome evolved.

This is important for science – but also economically. Koalas are incredibly well loved, with their baby-faces, shiny noses and big fluffy ears. Millions of visitors line up each year to spot them snoozing in gum trees – indeed, they are worth A$3.2 billion in tourist dollars.

Koalas are listed as a vulnerable species in some parts of Australia, affected by habitat destruction, disease and other stresses.

We now have a high quality read-out of the koala genome. Video thanks to the Australian Academy of Science.

The koala genome

Koala DNA was sequenced with new “long-read” technology that delivers a complete and well-assembled genome. As far as quality of the read-out goes, it’s as good as the human genome, with continuous sequences now known over huge (almost chromosome-scale) spans. New technology enabled us to achieve this at a tiny fraction of the $2.7 billion it cost to sequence the first human.

The obtained koala genome sequence is much better quality than that for other sequenced marsupials – opossum, tammar wallaby and Tasmanian devil – and will really help us to assemble and compare genomes from all marsupials.

The koala has a genome a bit bigger than that of humans, with 3.5 billion DNA base-pairs. This amounts to about a metre of DNA, which is divided and packaged into eight large bits that we recognise as chromosomes.

An animal has a set of chromosomes from mother and a set from father, so koalas have 16 large chromosomes in each cell. This is similar to other marsupials; as a group they seem to have a low chromosome number and a very stable genome arrangement. In placental mammals the number and arrangement of chromosomes is much more varied: for example, humans have 46, and rhinos 82 chromosomes.

The source of junk DNA

New findings from the koala genome help us to understand how mammal genomes evolved and how they work.

A lot (sometimes more than 50%) of animal genomes seem to be “junk DNA” – these are repeated sequences, many deriving from ancient viral infections. The koala, uniquely, seems to be in the middle of one such invasion. A DNA sequence derived from a retrovirus is present in different numbers and sites in different koala populations, testifying to its recent movement and amplification. This helps us learn how the genomes of humans and other mammals got so puffed up with junk DNA.

Like the human genome, the koala genome contains about 26,000 genes. These are stretches of DNA that code for or control proteins. Indeed, most koala genes are present in humans and other mammals – these are the same genes doing the same basic jobs in different animals.

So why is it important to sequence different species if their genomes are so similar? Well, it’s the special genes that have evolved to adapt the koala to its unique lifestyle that give us new and valuable information.

How to survive on gum leaves

How koalas exist on an exclusive low-calorie and toxin-laced diet of eucalyptus leaves has been somewhat of a science mystery.

The genome provides answers. The koala has multiplied a family of genes that code for enzymes (members of the cytochrome P450 family) that break down the toxins of gum leaves. Evolution of these additional gene copies has enabled the koala to outstrip its competition, even at the cost of sleeping most of the day.

The genome also gives us clues to the koala’s picky eating habits. The koala genome contains many additional copies of genes that enable them to taste and avoid bitter flavours and even to “smell” water and choose juicy leaves (they don’t drink water).

Koalas have 16 chromosomes per cell.
chrisfithall/flickr, CC BY

The genome also gives us new information about how koalas develop. Like other marsupials, they are born about the size of a pea, and complete most of their growth and differentiation in the pouch. Developing koalas are nurtured by milk with a complex composition that changes with the stage of development.

Saving an iconic Australian

Managing koala populations is very fraught, and there has long been a need for a holistic, scientifically-grounded approach to koala conservation.

Today’s koalas are the “last stand” of the marsupial family Phascolarctidae – and the koala genome contains new information about this evolutionary history. It also tells us that koala populations peaked about 100,000 years ago, then plunged to about 10% of their numbers 30-40,000 years ago, at the same time that the megafauna became extinct. This population was fairly stable until European settlement, when it plunged again to its present numbers (about 300,000).

Koalas once occupied a swathe of timbered habitat from Queensland to South Australia; now, only fragmented populations survive in the south. These are intensively managed, and small numbers of koalas are translocated to other sites, producing dangerously inbred populations. Bizarrely, one of the greatest problems is overbreeding in isolated populations – for example, on South Australia’s Kangaroo Island – which leads to animals eating themselves out of house and home.

The enemies of koalas in the north are habitat destruction and fragmentation by urbanisation and climate change.

The koala genome paper reports sequence comparisons of different populations and identifies barriers to gene flow. With the information from the koala genome, we can now monitor genetic diversity in the surviving populations, and maximise gene flow between connected populations.

Koalas are the sole surviving member of the *Phascolarctidae* marsupial family group.
Photo by Holger Link on Unsplash, CC BY

Maintaining genetic diversity is important because different animals can mount different responses to environmental threats and diseases such as chlamydia, a bacteria that affects koala reproduction and eye health.

The koala genome provides us with information about the immune genes of the koala, and the changes in activity of these genes in infected animals. This will help us understand the different responses of animals, vital for developing vaccines and treatments.

The koala genome also identifies powerful anti-bacterials in milk that protect the baby koala from disease – and may provide humans with the next generation of antibiotics.

So sequencing the koala genome is good for science and good for koalas, an iconic species at the top of the tree for conservation efforts.

Jenny Graves, Distinguished Professor of Genetics, La Trobe University

This article was originally published on The Conversation. Read the original article.

Curious Kids: Why are leaves green?

File 20171204 17064 t7ilgw.png?ixlib=rb 1.1 — The leaves of most plants are green because the leaves are full of green chemicals.
Marcella Cheng/The Conversation, CC BY-ND

Gregory Moore, University of Melbourne

This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!

Why are leaves green? – Indigo, age 6, Elwood.

The leaves of most plants are green, because the leaves are full of chemicals that are green.

The most important of these chemicals is called “chlorophyll” and it allows plants to make food so they can grow using water, air and light from the sun.

This way that a plant makes food for itself is called “photosynthesis” and it is one of the most important processes taking place on the whole planet.

One of the most important chemicals on Earth is called chlorophyll. It’s green and it allows plants to make food so they can grow.
Marcella Cheng/The Conversation, CC BY-ND

Without photosynthesis there would be no plants or people on Earth. Dinosaurs would not have been able to breathe and the air and oceans would be very different from those we have today. So the green chemical chlorophyll is really important.

All leaves contain chlorophyll, but sometimes not all of the leaf has chlorophyll in it. Some leaves have green and white or green and yellow stripes or spots. Only the green bits have chlorophyll and only those bits can make food by photosynthesis.

All leaves contain chlorophyll, but sometimes not all of the leaf has chlorophyll in it.
Marcella Cheng/The Conversation, CC BY-ND

If you’re really good at noticing things, you might have seen plants and trees with red or purple leaves – and the leaves are that colour all year round, not just in autumn.

These leaves are still full of our important green chemical, chlorophyll, just like any other ordinary green leaf. However, they also have lots of other chemicals that are red or purple – so much of them that they no longer look green. But deep down inside the leaves the chlorophyll is still there and it’s still green.

Even leaves that don’t look green have chlorophyll. However, they also have lots of other chemicals that are red or purple.
Marcella Cheng/The Conversation, CC BY-ND

Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to us. You can:

* Email your question to curiouskids@theconversation.edu.au

* Tell us on Twitter by tagging @ConversationEDU with the hashtag #curiouskids, or

* Tell us on Facebook

Please tell us your name, age and which city you live in. You can send an audio recording of your question too, if you want. Send as many questions as you like! We won’t be able to answer every question but we will do our best.

Gregory Moore, Doctor of Botany, University of Melbourne

This article was originally published on The Conversation. Read the original article.

New research unlocks the mystery of leaf size

File 20170831 9971 75oye6 — Leaf sizes vary according to a complex mix of temperature and water.
Peter Wilf/Supplied

Ian Wright, Macquarie University

Why is a banana leaf a million times bigger than a common heather leaf? Why are leaves generally much larger in tropical jungles than in temperate forests and deserts? The textbooks say it’s a balance between water availability and overheating.

But new research, published today in Science, has found it’s not that simple. Actually, in much of the world the key limiting factor for leaf size is night temperature and the risk of frost damage to leaves.

As a plant ecologist, I try to understand variation in plant traits (the physical, chemical and physiological properties of their tissues) and how this variation affects plant function in different ecosystems.

For this study I worked with 16 colleagues from Australia, the UK, Canada, Argentina, the US, Estonia, Spain and China to analyse leaves from more than 7,600 species. We then teamed the data with new theory to create a model that can predict the maximum viable leaf size anywhere in the world, based on the dual risks of daytime overheating and night-time freezing.

These findings will be used to improve global vegetation models, which are used to predict how vegetation will change under climate change, and also to better understand past climates from leaf fossils.

Conifers, which grow in very cold climates, grow thin needles less vulnerable to frost.
Peter Reich

From giants to dwarfs

The world’s plant species vary enormously in the typical size of their leaves; from 1 square millimetre in desert species such as common eutaxia (Eutaxia microphylla), or in common heather (Calluna vulgaris) in Europe, to as much as 1 square metre in tropical species like Musa textilis, the Filipino banana tree.

But what is the physiological or ecological significance of all this variation in leaf size? How does it affect the way that plants “do business”, using leaves as protein-rich factories that trade water (transpiration) for carbon (photosynthesis), powered by energy from the sun?

More than a century ago, early plant ecologists such as Eugenius Warming argued that it was the high rainfall in the tropics that allowed large-leaved species to flourish there.

In the 1960s and ‘70s physicists and physiologists tackled the problem, showing that in mid-summer large leaves are more prone to overheating, requiring higher rates of “transpirational cooling” (a process akin to sweating) to avoid damage. This explained why many desert species have small leaves, and why species growing in cool, shaded understoreys (below the tree canopy) can have large leaves.

Rainforest plants under the tree canopy can grow huge, complex leaves.
Ian Wright

But still there were missing pieces to this puzzle. For example, the tropics are both wet and hot, and these theories predicted disadvantages for large-leafed species in hot regions. And, in any case, overheating must surely be unlikely for leaves in many cooler parts of the world.

Our research aimed to find these missing pieces. By collecting samples from all continents, climate zones and plant types, our team found simple “rules” that appear to apply to all of the world’s plant species – rules that were not apparent from previous, more limited analyses.

We found the key factors are day and night temperatures, rainfall and solar radiation (largely determined by distance from the Equator and the amount of cloud cover). The interaction of these factors means that in hot and sunny regions that are also very dry, most species have small leaves, but in hot or sunny regions that receive high rainfall, many species have large leaves. Finally, in very cold regions (e.g. at high elevation, or at high northern latitudes), most species have small leaves.

Understanding the mechanisms behind leaf size means leaf fossils – like these examples from the Eocene – can tell us more about climates in the past.
Peter Wilf/Supplied

But the most surprising results emerged from teaming the new theory for leaf size, leaf temperature and water use with the global data analyses, to investigate what sets the maximum size of leaves possible at any point on the globe.

This showed that over much of the world it is not summertime overheating that limits leaf sizes, but the risk of frost damage at night during cold months. To understand why, we needed to look at leaf boundary layers.

Every object has a boundary layer of still air (people included). This is why, when you’re cold, the hair on your arms sticks up: your body is trying to increase the insulating boundary of still air.

Larger leaves have thicker boundary layers, which means it is both harder for them to lose heat under hot conditions, and harder to absorb heat from their surroundings. This makes them vulnerable to cold nights, where heat is lost as long-wave radiation to the night-time sky.

So our research confirmed that in very hot and very dry regions the risk of daytime overheating seems to be the dominant control on leaf size. It demonstrated for the first time the broad importance of night-time chilling, a phenomenon previously thought important just in alpine regions.

Still, in the warm wet tropics, it seems there are no temperature-related limits to leaf size, provided enough water is available for transpirational cooling. In those cases other explanations need to be considered, such as the structural costs and benefits of displaying a given leaf area as a few large leaves versus many more, smaller leaves.

The view from a canopy crane at the Daintree in Queensland.
Peter Wilf

These findings have implications in several fields. Leaf temperature and water use play a key role in photosynthesis, the most fundamental plant physiological function. This knowledge has the potential to enrich “next-generation” vegetation models that are being used to predict regional-global shifts in plant nutrient, water and carbon use under climate change scenarios.

These models will aid the reconstruction of past climates from leaf macrofossils, and improve the ability of land managers and policymakers to predict the impact of a changing climate on the range limits to native plants, weeds and crops.

But our work is not done. Vegetation models still struggle to cope with and explain biodiversity. A key missing factor could be soil fertility, which varies both in space and time. Next, our team will work to incorporate interactions between soil properties and climate in their models.

Ian Wright, Associate Professor in the Department of Biological Sciences, Macquarie University

This article was originally published on The Conversation. Read the original article.

Autumn Leaf Colours

Leaves Changing Colour in Autumn

Kevin's Walk on the Wild Side

Environment, Wildlife and Wilderness – Articles and News

Tag Archives for leaves

How the size and shape of dried leaves can turn small flames into colossal bushfires

Aerated litter beds fuel bigger fires

How can this information help us manage fires?

Next steps

Why climate change will dull autumn leaf displays

The subtlety of change

Lost synchronicity

Paradoxes and indirect effects

Change everywhere?

Koalas sniff out juicy leaves and break down eucalypt toxins – it’s in their genome

The koala genome

The source of junk DNA

How to survive on gum leaves

Saving an iconic Australian

Curious Kids: Why are leaves green?

New research unlocks the mystery of leaf size

From giants to dwarfs

Autumn Leaf Colours

Leaves Changing Colour in Autumn

Aerated litter beds fuel bigger fires

How can this information help us manage fires?

Next steps

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The subtlety of change

Lost synchronicity

Paradoxes and indirect effects

Change everywhere?

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The koala genome

The source of junk DNA

How to survive on gum leaves

Saving an iconic Australian

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From giants to dwarfs

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