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Carbon dioxide is a fertiliser for plants, so if its concentration increases in the atmosphere then plants will grow better. So what is the problem? – a question from Doug in Lower Hutt
Rising atmospheric carbon dioxide (CO₂) is warming our climate, but it also affects plants directly.
A tree planted in the 1850s will have seen its diet (in terms of atmospheric carbon dioxide) double from its early days to the middle of our century. More CO₂ generally leads to higher rates of photosynthesis and less water consumption in plants. So, at first sight, it seems that CO₂ can only be beneficial for our plants.
But things are a lot more complex than that. Higher levels of photosynthesis don’t necessarily lead to more biomass production, let alone to more carbon dioxide sequestration. At night, plants release CO₂ just like animals or humans, and if those respiration rates increase simultaneously, the turnover of carbon increases, but the carbon stock doesn’t. You can think of this like a bank account – if you earn more but also spend more, you’re not becoming any richer.
Even if plants grow more and faster, some studies show there is a risk for them to have shorter lifespans. This again can have negative effects on the carbon locked away in biomass and soils. In fact, fast-growing trees (e.g. plantation forests) store a lot less carbon per surface area than old, undisturbed forests that show very little growth. Another example shows that plants in the deep shade may profit from higher levels of CO₂, leading to more vigorous growth of vines, faster turnover, and, again, less carbon stored per surface area.
The effect of CO₂ on the amount of water plants use may be more important than the primary effect on photosynthesis. Plants tend to close their leaf pores slightly under elevated levels of CO₂, leading to water savings. In certain (dry) areas, this may indeed lead to more plant growth.
But again, things are much more complex and we don’t always see positive responses. Research we published in Nature Plants this year on grasslands around the globe showed that while dry sites can profit from more CO₂, there are complex interactions with rainfall. Depending on when the rain falls, some sites show zero or even negative effects in terms of biomass production.
Currently, a net amount of three gigatons of carbon are thought to be removed from the atmosphere by plants every year. This stands against over 11 gigatons of human-induced release of CO₂. It is also unclear what fraction of the three gigatons plants are taking up due to rising levels of CO₂.
A hapless animal will swim by, triggering the sensitive hairs at the front of the bladderwort’s bladder, which opens like a trap door. The rush of water into the trap carries the animal inside. The door slams shut and digestion starts.
This all happens faster than the eye can see – in less than a millisecond, more than 100 times faster than a Venus flytrap.
The best habitat in all the (wet)land
The bladderwort is just one example of Utricularia. Australia’s Top End contains some 36 species of Utricularia, making it a a global centre for the genus. And the species count is still going up as researchers make new discoveries.
In particular, bladderworts can be found around the Howard River, about 30km east of Darwin, part of a 264 square km area of significant conservation value.
The Howard River area supports the largest and most continuous stretch of seasonally-flooded sandy wetlands in the Northern Territory, with extensive shallow lagoons and swamps.
The layer of fine sand is between 1 and 10 metres thick. The sand overlays less permeable material such as rock and clay, so the sand becomes completely waterlogged in the wet season. It makes a perfect home for bladderworts.
This highly dynamic environment provides a miniature topography of rises and depressions measured in just centimetres. As well as the alternating monsoonal dry and wet seasons, the topography is overlain with seasonal changes in water levels.
The species of Utricularia have adapted to different windows of opportunity in these seasonal changes and partition themselves within the habitat, often based on water height.
Within the same small area, species come and go during the season based on their tolerance of these habitat variables. This can be frustrating for the collector and observer, as not all species are found at the one time.
All shapes and sizes
A unifying feature of the Utricularia genus is the suction trap – or “bladder”. But the bladderwort species come in many shapes and sizes.
Flowers, for instance, can vary in size. Some bladderworts have flowers with large nectar-filled spurs. These can grow up to 15 millimetres long and attract insects with a long proboscis (an elongated “snout”). Other bizarre flowers on different bladderwort species have long antennae-like extensions and appear to involve insect mimicry to attract pollinators.
Twenty-two per cent of the sand sheet landscape in this region has been cleared for sand mining, as it holds a huge source of easily accessible, fine, high-grade sand used in concrete for building.
But it’s not all doom and gloom. A project, “Secret World: Carnivorous plants of the Howard sand sheets”, brought artists and scientists out into the field in a workshop setting.
Scientists explained the significance of the environment, the flora and the threats facing the habitat.
And the artists squelched about the waterlogged habitat and got down and dirty into this Lilliputian world. They set about interpreting the plants and with a diversity of approaches matching the diversity of the bladderworts, they produced a stunning portfolio of artworks.
An education kit produced from the project also took the story into local schools.
The Northern Territory Environment Protection Authority assessed the issues and determined areas of the sand sheets that should be set aside for conservation purposes. The art and science collaboration certainly played a pivotal part in this positive conservation outcome.
Tree planting has been widely promoted as a solution to climate change, because plants absorb the climate-warming gases from Earth’s atmosphere as they grow. World leaders have already committed to restoring 350m hectares of forest by 2030 and a recent report suggested that reforesting a billion hectares of land could store a massive 205 gigatonnes of carbon – two thirds of all the carbon released into the atmosphere since the Industrial Revolution.
Many of those trees could be planted in tropical grassy biomes according to the report. These are the savannas and grasslands that cover large swathes of the globe and have a grassy ground layer and variable tree cover. Like forests, these ecosystems play a major role in the global carbon balance. Studies have estimated that grasslands store up to 30% of the world’s carbon that’s tied up in soil. Covering 20% of Earth’s land surface, they contain huge reserves of biodiversity, comparable in areas to tropical forest. These are the landscapes with lions, elephants and vast herds of wildebeest.
Savannas and grasslands are home to nearly one billion people, many of whom raise livestock and grow crops. Tropical grassy biomes were the cradle of humankind – where modern humans first evolved – and they are where important food crops such as millet and sorghum originated, which millions eat today. And, yet among the usual threats of climate change and wildlife habitat loss, these ecosystems face a new threat – tree planting.
It might sound like a good idea, but planting trees here would be damaging. Unlike forests, ecosystems in the tropics that are dominated by grass can be degraded not only by losing trees, but by gaining them too.
Increasing the tree cover in savanna and grassland can mean plant and animal species which prefer open, well-lit environments are pushed out. Studies from South Africa, Australia and Brazil indicate that unique biodiversity is lost as tree cover increases.
This is because adding trees can alter how these grassy ecosystems function. More trees means fires are less likely, but regular fire removes vegetation that shades ground layer plants. Not only do herbivores like zebra and antelope that feed on grass have less to eat, but more trees may also increase their risk of being eaten as predators have more cover.
More trees can also reduce the amount of water in streams and rivers. As a result of humans suppressing wildfires in the Brazilian savannas, tree cover increased and the amount of rain reaching the ground shrank. One study found that in grasslands, shrublands and cropland worldwide where forests were created, streams shrank by 52% and 13% of all streams dried up completely for at least a year.
Grassy ecosystems in the tropics provide surface water for people to drink and grazing land for their livestock, not to mention fuel, food, building materials and medicinal plants. Tree planting here could harm the livelihoods of millions.
Losing ancient grassy ecosystems to forests won’t necessarily be a net benefit to the climate either. Landscapes covered by forest tend to be darker in colour than savanna and grassland, which might mean they also absorb more heat. As drought and wildfires become more frequent, grasslands may be a more reliable carbon sink than forests.
How have we reached the point where the unique tropical savannas and grasslands of the world are viewed as suitable for wholesale “restoration” as forests?
At the root of the problem is that these grassy ecosystems are fundamentally misunderstood. The Food and Agricultural Organisation of the UN defines any area that’s half a hectare in size with more than 10% tree cover as forest. This assumes that landscapes like an African savanna are degraded because they have fewer trees and so need to be reforested. The grassy ground layer houses a unique range of species, but the assumption that forests are more important threatens grassy ecosystems across the tropics and beyond, including in Madagascar, India and Brazil.
“Forest” should be redefined to ensure savannas and grasslands are recognised as important systems in their own right, with their own irreplaceable benefits to people and other species. It’s essential people know what degradation looks like in open, sunlit ecosystems with fewer trees, so as to restore ecosystems that are actually degraded with more sensitivity.
Calls for global tree planting programmes to cool the climate need to think carefully about the real implications for all of Earth’s ecosystems. The right trees need to be planted in the right places. Otherwise, we risk a situation where we miss the savanna for the trees, and these ancient grassy ecosystems are lost forever.
Planting almost a billion hectares of trees worldwide is the “biggest and cheapest tool” for tackling climate change, according to a new study. The researchers claimed that reforestation could remove 205 gigatonnes of carbon from the atmosphere – equivalent to about 20 years’ worth of the world’s current emissions. This has criticised as an exaggeration. It could actually be dangerous.
While the paper itself included no costings, the researchers suggested a best-case estimate of just US$300 billion to plant trees on 0.9 billion hectares. That’s just 40 US cents per tonne of carbon dioxide (CO₂) removed. More detailed studies on the costs of carbon removal through reforestation put the figure closer to US$20-50 per tonne – and even this may be optimistic at such large scales.
Our research suggests that the promises implied in such studies could actually set back meaningful action on climate change. This is because of what we call “mitigation deterrence” – promises of cheap and easy CO₂ removal in future make it less likely that time and money will be invested in reducing emissions now.
Why would anyone expect governments or the finance sector to invest in renewable energy, or mass transit like high-speed rail, at costs of tens or hundreds of dollars a tonne if they – and shareholders and voters – are told that huge amounts of CO₂ can be absorbed from the atmosphere for a few dollars a tonne by planting trees?
Why should anyone expect energy companies and airlines to reduce their emissions if they anticipate being able to pay to plant trees to offset everything they emit, for the paltry price of less than 50 cents a tonne. If studies like this suggest removing carbon is cheap and easy, the price of emitting carbon for businesses – in emissions trading schemes – will remain very low, rather than rising to the levels needed to trigger more challenging, yet urgently needed, forms of emission reduction.
A false carbon economy
The promises of cheap and powerful tech fixes help to sideline thorny issues of politics, economics and culture. But when promises that look great in models and spreadsheets meet the real world, failure is often more likely. This has been seen before in the expectations around carbon capture and storage.
Despite promises of its future potential in the early 2000s, commercial development of the technology has scarcely progressed in the last decade. That’s despite many modelled pathways for limiting global warming still assuming – increasingly optimistically – that it will be deployed at a large scale in coming decades.
This model of tackling climate change goes hand in hand with another tool – pricing carbon emissions. This potentially allows companies to go on emitting by paying someone else to cut emissions or remove CO₂ elsewhere – an approach called climate offsetting. But offsetting makes exaggerated promises of carbon removal even more risky.
Tree planting financed through offset markets would guarantee the polluter could continue emitting carbon, but the market couldn’t guarantee removals to match those emissions. Trees might be planted and subsequently lost to wildfire or logging, or never planted at all.
Trusting in trees to remove carbon in future is particularly dangerous because trees are slow to grow and how much carbon they absorb is hard to measure. They’re also less likely to be able to do this as the climate warms. In many regions of the world but particularly in the tropics, growth rates are predicted to fall as the climate warms and devastating wildfires become more frequent.
Relying on trees to absorb CO₂ from the atmosphere in the future also appears misleadingly cheap because of the effects of economic discounting. Economists discount the current value of costs or benefits more deeply, the further in the future they occur. Models which determine the cheapest mix of policies available all use some form of discounting.
When researchers add carbon removal options like tree planting to these models, they tend to generate pathways for slowing temperature rise which reduce the role of short term action and replace it with imaginary removals late in the century.
This is because discounting over 30 to 60 years makes the removal options look incredibly cheap in today’s prices. Priming models to focus on minimising cost causes them to maximise the use of discounted future removals and reduce the use of more expensive near term emissions reduction.
I am not arguing against reforestation, nor for a purely technological response to climate change. Trees can help for many reasons – reducing flooding, shading and cooling communities, and providing habitat for biodiversity. Incentives for reforestation are important, and so are incentives for removing carbon. But we shouldn’t make trees or technology carry the whole burden of tackling climate change. That demands moving beyond technical questions, to deliver immediate political action to cut emissions, and to begin to transform economies and societies.
This article was amended on July 13 2019 to clarify the proposed costs of carbon removal by reforestation.
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On a warm evening in early 1802, Robert Brown sat aboard the HMS Investigator describing several plant specimens collected that day. Brown was the botanist on Captain Matthew Flinders’ expedition, and they had been anchored in King George Sound for nearly a month documenting the remarkable flora of the area.
He keenly awaited the return of their gardener, Peter Good, who had left earlier in search of a curious “pitcher plant” discovered the previous morning by botanical artist Ferdinand Bauer and landscape artist William Westall.
Unbeknownst to him, in minutes he would be gazing upon a uniquely wondrous plant: Cephalotus follicularis, the Albany pitcher plant.
Named after the southwestern Australian port city around which it occurs, the Albany pitcher plant stands out as an oddity even by the standards of carnivorous plants. The species is instantly recognisable, as it produces distinctive insect-trapping pitcher leaves that sit on the ground almost expectantly waiting for prey.
The toothed mouth and overarching lid of these pitchers look superficially similar to those of the tropical pitcher plants (Nepenthes) and North American pitcher plants (Sarracenia). However, these plants are not related; this similarity is a remarkable example of convergent evolution. The Albany pitcher plant is unique.
C. follicularis is the only species in the genus Cephalotus, which is the only genus within the family Cephalotaceae. Its nearest living relatives are rainforest trees from tropical South America, from which it is separated by some 50 million years. Indeed, it is the only carnivorous plant among the 70,000 species, a quarter of all flowering plants, that make up one of the largest evolutionary plant groups, the rosid clade.
The Albany pitcher plant is more closely related to cabbages, roses and pumpkins than it is to other pitcher plants.
The Albany pitcher plant only grows in a very small area of Western Australia, and is thought to be an ancient Gondwanan relict from a period when this region was almost tropical. It grows in nutrient-poor soils of coastal swamps and lowlands, where it survives by luring insects into its traps to be digested in a pool of enzymes at the base of each pitcher. Each pitcher bears a lid to prevent rain from diluting the pool of enzymes, with translucent windows to disorient trapped prey and prevent escape.
Interestingly, one species of insect not only survives inside the fluid of the pitchers, but relies on it for survival. The wingless stilt fly Badisis ambulans lays its eggs in the pitchers, and the larvae develop in the pool of pitcher fluid, feeding on captured prey.
These stilt flies live only in the dense vegetation of the swamps inhabited by the Albany pitcher plant. They look more like an ant than a fly, which is probably a deliberate mimicry of the ant Iridomyrmex conifer, the primary prey of the pitcher plant. It is likely that these three species – plant, fly and ant – have co-evolved together over millions of years.
The Albany pitcher plant was probably widespread in the southwest corner of WA before European settlement, and almost 150 populations have been recorded throughout this region. However, the species has declined dramatically over the past century as extensive land has been cleared throughout the southwest for agriculture and urban development.
The Albany pitcher plant now occurs only as small, isolated populations in remnant habitat patches. It is thought that less than 3,000 hectares of habitat suitable for the species now remains in the greater Albany region. Recent survey efforts suggest that fewer than 20 populations of the Albany pitcher plant still exist, and fewer than 5,000 plants remain.
Despite the perilous state of the Albany pitcher plant, it still has no formal conservation status. Indeed, swamps containing the species have been bulldozed for housing development in the past 12 months. But habitat loss and changes to bushfire frequency and water flow are not the only threats to this amazing species. Current projections of a drying climate in the southwest of Western Australia may see the species pushed towards extinction in the coming decades.
Incredibly, the Albany pitcher plant is also at risk from poaching. The species is prized for its horticultural novelty, and unscrupulous individuals dig up plants from the wild either to grow or sell. At one accessible location where the species was known to grow in abundance, every single plant within reach has been removed. At other sites, entire populations have been dug up.
Without improved conservation measures, and tough penalties for removing this incredible species from its natural habitat, the Albany pitcher plant and its complex web of insect relationships face a potentially dire future.
We need trees in our lives. This past summer, Adelaide experienced the hottest temperature ever recorded in an Australian state capital, hitting 46.6 degrees on January 24. Trees beautify otherwise grey cities and cool our suburbs during heatwaves. But different species have different levels of tolerance of heat, lack of water and other threats posed by climate change.
In a newly published study, we investigated likely climate change impacts on 176 of the most common tree species planted across Australian cities. Our analysis showed more than 70% of these species will experience harsher climatic conditions across Australian cities by 2070. Some of the most commonly planted trees are unlikely to survive these conditions.
So which tree species are best suited to particular places? Which species are more likely to thrive, rather than just survive, under a changing climate? Which of our beloved tree species won’t make it?
Tree species growing in warmer cities are more likely to be affected than those in cooler cities. Some species, such as the golden wattle (Acacia longifolia) or the prickly paperbark (Melaleuca styphelioides), might not make it in northern cities, unless we invest precious resources – such as water – to maintain these civic assets. Other species, such as the native frangipani (Hymenosporum flavum) or the tuckeroo (Cupaniopsis anacardioides), will likely become more suitable for planting in southern cities.
Trees are wonderfully effective at improving the microclimate of our cities, which makes tree plantings an effective and efficient way to adapt to climate change. The leaves of trees absorb and dissipate much of the sun’s radiation.
Trees cool air and land by several degrees compared to areas of concrete and asphalt. Swipe the heat map below to see how effectively trees cool down our cities. (Red indicates hotter areas, blue cooler areas.)
Governments recognise the importance of trees and have developed vital initiatives, such as the national 20 Million Trees program and the 5 Million Trees program in New South Wales. These are important first steps to increase urban tree cover across Australia. But the question arises: are we planting the right tree species?
What does the science say?
Australian cities are blessed with a higher diversity of tree species compared to other cities globally. However, the 30 most commonly planted species make up more than half of Australia’s urban forests.
This poses a great risk for our cities. If we were to lose one or two of these common species, the impact on our urban tree cover would be immense. Consequently, our best insurance is to increase the diversity of our trees.
Our quest to find climate-ready tree species is only just beginning. Supported by Hort Innovation Australia, the NSW Department of Planning, Industry and Environment, and the Commonwealth government, our team embarked on a project called Which Plant Where in conjunction with researchers at Western Sydney University. Our mission is to find the best plant species for urban landscapes that will be resilient to climate change.
We work with the nursery industry to provide evidence on species’ resilience to extreme heat and drought by testing plants to their limits in research glasshouses. Our work with plant growers and nurseries will inform them on how to adapt their business, by identifying the new challenges posed by climate change, as well as selecting highly diverse palettes of climate-ready species. We advise landscape architects, designers and urban planners about not only the best planting choices, but also how to increase the biodiversity of our cities.
We are committed to do more science in coming years, but you can start making a difference today. Australia’s National Tree Day will be celebrated again this year on Sunday, July 28. It’s a great opportunity to teach our families, communities and businesses about the importance of tree planting and environmental stewardship as key elements of adapting to climate change.
An old Chinese adage says:
The best time to plant a tree was 20 years ago. The second best time is now.
This weekend is your time. The game is simple – head to your closest plant nursery. Ask your local grower about which tree species are suitable for the local growing conditions and pick one you like. Then, plant a tree in your yard, or join one of the many planting events across Australia.
Teach your kids, family and friends about the difference they can start making today – for their future and our common good – one tree at a time.
Our results suggest the type of green space does matter. Adults with 30% or more of their neighbourhood covered in some form of tree canopy had 31% lower odds of developing psychological distress. The same amount of tree cover was linked to 33% lower odds of developing fair to poor general health.
We also found poorer mental and general health among adults in areas with higher percentages of bare grass nearby, but there’s likely more to that than meets the eye.
Our research involved tracking changes in health over an average of about six years, for around 46,000 adults aged 45 years or older, living in Sydney, Newcastle or Wollongong. We examined health in relation to different types of green space available within a 1.6 kilometre (1 mile) walk from home.
Our method helped to guard against competing explanations for our results, such as differences in income, education, relationship status, sex, and age. We also restricted the sample to adults who did not move home, because it is plausible that people who are already healthier (for instance because they are more physically active) move into areas with more green space.
So is the answer simply more trees and less grass? Not exactly. Let’s get into the weeds.
Imagine you’re walking down a typical street on a summer’s day in the middle of an Australian city. It’s full of right angles, grey or dark hard surfaces, glass structures, and innumerable advertisements competing for your attention. Then you turn a corner and your gaze is drawn upwards to a majestic tree canopy exploding with a vivid array of greens for as far as you can see.
Let’s get the obvious out of the way. Walking down this green street, you may instantly feel some relief from the summer heat.
But as the minutes of walking beneath this natural umbrella of lush foliage accumulate, other things are happening too. The vibrant colours, natural shapes and textures, fresh aromas, and rustling of leaves in the breeze all provide you with effortless distraction and relief from whatever it was you might have been thinking about, or even stressing over.
You walk past groups of people on the footpath taking time to catch up over coffee in the shade. Some research has found that tree cover, rather than green space more generally, is a predictor of social capital. Social capital, according to Robert Putnam, refers to the “social networks and the associated norms of reciprocity and trustworthiness” that may have important influences on our life chances and health.
You walk further and a chorus of birdsong soars through the neighbourhood noise. Trees provide shelter and food for a variety of animals. Research suggests tree canopy tends to be more biodiverse than low-lying vegetation.
Green spaces with tree canopy are settings where communities can come together to watch birds and other animals, which can also be catalysts for new conversations and developing feelings of community belonging in the neighbourhoods where we live … just ask dog owners.
Furthermore, large areas of bare grass in cities can make built environments more spread-out and less dense. Without tree canopy to shield from the midday sun, this may increase the likelihood of people using cars for short trips instead of walking through a park or along a footpath. The result is missed opportunities for physical activity, mental restoration, and impromptu chats with neighbours. Previous work in the United States suggests this might be why higher death rates were found in greener American cities.
Large open areas of grass can be awesome for physical activity and sport, but let’s make sure there is also plenty of tree canopy too, while also thinking about ways to get more people outdoors in green spaces. Here are some suggestions.
Making Australia greener and healthier
As the density of Australian cities continues to increase and more of us live in apartments and/or work in high-rise office blocks, it is great to see strategies to invest in tree cover and urban greening more generally across Australia. Cities with such plans include Sydney, Melbourne, Brisbane, Bendigo, Fremantle, and Wollongong.