Mick Hanley– Associate Professor (Reader) in Plant-Animal Interactions, University of Plymouth
In highlighting how Britain lost half its hedgerow network in only 75 years following the post-WWII move to modernise farming, a recent report from the Council for the Protection of Rural England points out how hedgerows can reduce climate warming by naturally helping to remove CO₂ from the atmosphere.
Simply allowing existing hedges to get bigger could double overall carbon sequestration capacity (the process of capturing and storing atmospheric CO₂). The build-up of “greenhouse” gasses such as CO₂ causes the atmosphere to warm, which in turn increases the likelihood and severity of extreme weather events, such as flooding, wildfires and hurricanes.
The CPRE argue that a 40% increase in the UK’s hedgerow cover could provide a net CO₂ sequestration potential of 18.5 million tonnes. That may be a fraction of current UK CO₂ emissions 354 million tonnes in 2019, but the potential contribution to the climate problem is significant.
What can hedges do for the climate?
As ecologists we have to worry about many things: invasive species, habitat loss, overexploitation and pollution. But climate change is the big one.
Plants naturally capture carbon during the process of photosynthesis, the process of producing food for the plant to survive. The ability to couple CO₂ with water and generate sugar using solar energy is the most remarkable and important event in evolutionary history and might just help us prevent climate catastrophe by naturally capturing this key greenhouse gas.
The most extreme (yet wholly credible) scientific predictions for how climate change will affect the planet are so harrowing that I don’t even tell my final year undergraduate students. What is clear is that if humanity does not take responsibility at the COP26 meeting in Glasgow (the 2021 United Nations climate change conference), it is probably too late to completely mitigate the consequences. But it appears that there are some actions that can help – and growing more hedgerows may be one of the easiest ones.
Hedgerows should be the farmer’s friend, but after Hitler’s U-boats threatened Britain’s food supplies during the second world war, post-war governments impressed the need for agriculture to modernise. In came a raft of chemical fertilisers, pesticides and larger machinery. Out went intimate fields and the web of hedges established around them hundreds of years earlier.
Farmers actually received grants to grub them out, many doing it simply because they were paid to, rather than any obvious benefit to farming efficiencies. It is only since they disappeared that it has become apparent that they actually contributed a great deal to agricultural production and the wider rural landscape.
I am lucky enough to live in a part of the UK that still has hedgerows. They are among the numerous features that delight the summer influx of migrants to south-west England. Many older visitors recall the rural landscape of their youth and the traditional hedges around fields. Thanks to hedges, late spring brings a kaleidoscope of red campion, bluebell and the white flowers of stitchwort to the roadsides of Devon and Cornwall. With the flowers comes a posse of insects competing for the nectar and pollen these flowers provide.
Aesthetics are, however, only part of their value. Bees and hoverflies that feed from, and make their homes in, Britain’s hedgerows provide a key pollination service to farmers. Other insects spill out from the hedgerow to prey on crop pests and so contribute another critical ecosystem service to society. Simply retaining a living biological structure around the edges of fields helps retain soil and filter out agrichemicals that might otherwise leach into the wider environment.
A relatively modest taxpayer commitment of £310 million over 28 years (less than £10 per working adult in the UK) could attract a near four-fold return on investment, with additional benefits for job creation, cleaner air and enhanced water use.
The CPRE also argues that urban planting might also have major benefits for mental health and well being with plenty of scope to increase upon the mere 38% of urban roads with any form of hedgerow at present. It is also likely that reinstatement of rural hedgerows will make the countryside more attractive for visitors, says the CPRE.
The UK government will need to offer targeted subsidies to farmers and other land managers to replant hedgerow trees and then manage them in appropriate but well understood ways to maximise carbon-capture and biodiversity potential. There was a system of well-established and broadly successful EU agri-environment schemes in place in Britain before Brexit to encourage hedgerow expansion and management.
The National Farmers Union is lobbying government to replace and improve these schemes, but even with legislation and appropriate government support, it takes time to reinstate biodiversity.
That said, I planted a native hedge instead of an expensive, wind-catching fence when I moved house ten years ago, and saw that it was possible to grow a two-metre tall, biodiverse hedgerow within a decade. Surely if I can do it in my back garden, farmers can do even better.
Joshua Larsen– Senior Lecturer in Water Science, University of Birmingham Annegret Larsen– Assistant Professor in Geography, Wageningen University and Research
The Eurasian beaver, once a common sight across Europe, had disappeared almost entirely by the end of the 16th century thanks to hunting and river modification for agriculture and engineering.
But beavers are making a comeback across the UK and several other countries. They have already been released into the wild in Scotland and within enclosed river sections in England. Now expanding the wild release of beavers across England is on the cards.
Ecosystem recovery, increased biodiversity, flood protection and improved water quality are some of the upsides of having beavers around. But reintroducing wild animals to the landscape is always going to involve trial and error, and it’s vital to understand the possible consequences – both good and bad.
The beaver is a gifted environmental engineer, able to create its own ecological niche – matching itself perfectly to its environment – by building dams. These dams are made from materials the beaver can carry or float – typically wood, stones and mud, but also fence posts, crops from nearby fields, satellite dishes and old kids’ toys.
The dam creates a peaceful, watery home for beaver families to sleep, eat and avoid predators. And the effects of dam building ripple outwards, with the potential to transform entire ecosystems.
Our review of beaver impacts considers evidence from across Europe and North America, where wild beaver populations have been expanding since around the 1950s.
There is clear evidence that beaver dams increase water storage in river landscapes through creating more ponds and wetlands, as well as raising groundwater levels. This could help rivers – and their inhabitants – handle ever more common weather extremes like floods and droughts.
If you observe beaver dams in the wild, water often comes very close to the top of their dams, suggesting they might not be much help in a flood. Nonetheless, some studies are finding that beaver dams can reduce flood peaks, likely because they divert water onto floodplains and slow downstream flow. However, we don’t know whether beaver dams reliably reduce floods of different sizes, and it would be unwise to assume they’re always capable of protecting downstream structures.
The good news is that it seems all the extra water dams store could help supplement rivers during dry periods and act as critical refuges for fish, amphibians, insects and birds during droughts.
Beaver dams increase the time it takes for things carried by rivers to move downstream. In some cases, this can help slow the spread of pollutants like nitrates and phosphates, commonly used in fertilisers, which can harm fish and damage water quality.
Beavers’ impact on phosphates is unclear, with just as many studies finding phosphorus concentrations increasing downstream of beaver dams as those finding a decrease or no change. But beavers seem especially skilled at removing nitrate: a welcome skill, since high concentrations of nitrates in drinking water could endanger infant health.
All that water storage means beavers create a wonderful mosaic of still-, slow- and fast-moving watery habitats. In particular, they increase the biodiversity of river valleys, for example helping macro-invertebrates like worms and snails – key to healthy food chains – to thrive.
But nuance is key here. Evidence of beaver dam impacts on fish populations and river valley vegetation, for example, is very mixed. Because they are such great agents of disturbance, beavers promote plants that germinate quickly, like woody shrubs and grasses.
While this can reduce forest cover and help some invasive plants, given time it can also help create valleys with a far richer mosaic of plant life. So although beaver presence is likely to bring benefits, more research is needed to get clearer on precisely how beavers change ecosystems.
Net zero carbon
Beavers are great at trapping carbon by storing organic matter like plant detritus in slow-flowing ponds. However, this also means beaver ponds can be sources of greenhouse gases, like CO₂ and methane, that contribute to the greenhouse effect. This led one author to wonder “whether the beaver is aware the greenhouse effect will reduce demand for fur coats”.
Can beavers still be helpful in achieving net zero carbon? The short-term answer is probably yes, since more carbon seems to be trapped than released by beaver activities.
However, long-term outcomes are less clear, since the amount of carbon that beavers keep in the ground depends on how willing they are to hang around in a river valley – and how willing we are to let them. A clearer understanding of where beavers fit within the carbon cycle of river systems is needed if we are to make best use of their carbon capture skills.
Beavers are reentering landscapes under human dominance, the same thing that originally drove them from vast swathes of European river systems.
In the UK, this means they’ll lack natural predators and may be in competition with cows and sheep for food: possibly resulting in unsteady wild population trajectories.
Although good data on long-term beaver activity is available from Sweden, Norway and Switzerland, our different climate and landscapes mean it’s hard to make a straightforward comparison.
Beavers’ use in rewilding can be incredibly cost-effective, as dam construction and the biodiversity benefits that flow from it is done largely for free. But we need to be tolerant of uncertainty in where and when they choose to do their work.
Working with wild animals – who probably don’t share our priorities – is always an unpredictable process. The expansion of beavers into the wild has a bright future so long as we can manage expectations of people who own and use beaver-inhabited land.
Emma White– Research Fellow in Environmental Psychology, University of Surrey
Sarah Golding– Research Fellow in Health Psychology, University of Surrey
COVID-19 has shown that pandemics can seriously affect people’s physical and mental health. Stress, anxiety and depression have increased around the world, with the greatest effects for those living under the strictest lockdowns. Many people’s physical activity levels also fell during lockdown. Gardens, though, can help us push back against these negative effects.
Before the pandemic, having a garden was associated with better health and wellbeing, and this pattern has continued during COVID-19. In our own research on garden use during the UK’s first lockdown – published as a working paper this summer – we found that more frequent garden visits were associated with better wellbeing. Other work has also found that gardens have helped reduce mental distress during the pandemic.
With that in mind, here are five ways of using your garden that research suggests can improve your mental health. If you have access to an outdoor space and have been finding things difficult, you could try these out to boost your mood.
And if you’re feeling good now, you could use this chance to get ahead. Just as world leaders are being urged to prepare for the next pandemic, you can prepare your garden and develop habits now to better support your wellbeing in the future should there be another lockdown.
You don’t have to be a gardener to get active in your garden (although we think you should give it a go). Gardens are great places to be creative and provide lots of opportunities to get moving. Play hide and seek, do yoga on the lawn, build a bug hotel for insects to live in – anything you like!
And remember, if there’s another lockdown, being physically active in your garden can make up for lost opportunities to be active in other parts of your life.
2. Do nothing
Gardens help restore the ability to concentrate on demanding tasks, providing the perfect space for a break when working from home in a pandemic. Natural things – such as trees, plants and water – are particularly easy on the eye and demand little mental effort to look at. Simply sitting in a garden is therefore relaxing and beneficial to mental wellbeing.
To get your garden ready for break time, create a space in which to unwind. Surround yourself with soothing things, such as flowers.
Garden seating also seems to be key. People in our research told us they enjoyed relaxing in hammocks, chairs and benches. So, make time to sit and watch the clouds, or relax with a book and a cup of tea. And don’t feel guilty about it – taking a break is important for avoiding psychological fatigue.
3. Be alone
Gardens are places to escape the anxieties and demands of the world around us. They are particularly restorative because they are places where we can get away from our day-to-day life. In our research, some people talked about needing space from other household members and got this by hiding in the garden shed. Others hid away in the bathroom or bedroom.
Should there be another lockdown, remember that the garden is a good space to get away from work and other people. Perhaps create a hidden nook in your garden that you can hide out in for a few minutes. You’ll most likely return to work and life feeling refreshed and more productive.
4. Be social
Research also highlights the value of spending time with others outdoors. There are lots of ways to use your garden for socialising and building relationships. You could play a game outside, have a barbeque, chat to a neighbour over the fence, or invite a friend to drink hot chocolate in the snow (the Norwegians can teach us much about enjoying the outdoors during winter).
Of course, not everyone has a garden. But even if you don’t have outdoor space of your own, you can still follow some of these tips. Indoor plants can be used to create a more “natural” environment and have been shown to improve mood.
If you want to spend more social time outdoors, you could help at an allotment or community garden, as these are often highly social pursuits that involve working together outside. And if you want to do nothing at all, you could find a small local park to just sit and relax in.
Good access to natural environments supports health and wellbeing and can reduce health inequalities – yet access to green space remains very unequal. With growing recognition of the role of green space as a social determinant of health, how should health professionals respond? This year’s online NHS Forest conference will explore and share green space solutions at both a strategic and practical level, including insights from keynote speakers Judy Ling Wong, Dr Becca Lovell and Suzanne Tarrant.
Connect with children and nature champions around the world in The Trailhead. This new online community is free and open to practitioners, educators, parents, researchers and anyone committed to connecting children, families and communities to the benefits of nature. The Trailhead is your place to: Connect with peers and grow your network Join a global community of changemakers working to increase equitable access to nature Learn, lead and increase your impact Find potential conference co-presenters in The Trailhead’s 2022 Inside-Out International Conference Group SIGN UP
Jaime Zaplatosch VP, Green Schoolyards for Healthy Communities Children & Nature Network
Depending on where you are in the world, the start of the 2021-22 school year and the spread of the COVID-19 Delta variant might feel like “déjà vu all over again.” But, in fact, we are in a much better place to support going back to school fully in person this year. Many partners over many years have developed resources for taking learning outdoors — an approach that can provide a safer return to school now and enhance children’s learning and well-being all of the time.
RICHARD LOUVLEARNING OUTDOORS: Keeping students and teachers safer, improving education, and bringing healing during the pandemic
Eighteen months ago, school districts were mostly at a loss when it came to dealing with a global pandemic. Today, educators and communities have everything they need — if not in money, then certainly in good examples — to turn their schools into safer environments for students and teachers, and at the same time, improve educational outcomes. READ MORE
Ian Mell– Reader in Environmental & Landscape Planning, University of Manchester
The original plans for the temporary Marble Arch Mound in London depicted an inviting green space with thick vegetation and mature trees. The reality is far from that.
The sad-looking geometric hill has been criticised for featuring little other greenery than than the squares of wilting grass hanging to it. The views of Hyde Park and Oxford Street also leave much to be desired with obscured sight lines to other London landmarks. It is unsurprising then that people have also baulked at the £4.50-£8 entry fee, which is now being dropped after many visitors were refunded.
The poor design and construction have led the Mound to be labelled London’s “worst attraction”. Built to tempt shoppers back to Oxford Street, the Mound has ended up costing Westminster council £6 million. This is double the forecasted cost and has led to the resignation of the council’s deputy leader.
In its failure, the project undermines the Mayor of London’s strategic work to enhance the quantity and quality of green and open space across the metropolitan area. It is also comes at a time when green spaces and parks around the city (in Peckham, Bermondsey and Bromley) are under threat of being removed to make way for housing development.
Marble Arch Mound’s failure highlights a willingness by local councils to invest in green space if they promise to promote direct financial return while they decrease funds for existing parks, which are seen as economic burdens and low-money makers. And yet there are ways for cities to improve access to nature, play and community interaction with via investment in nature rather than focusing on tourist revenue.
Why this project in this location?
For those living in urban areas whose green spaces are under threat it might seem confusing why local councils would choose to fund tourist projects like the Mound when they are getting rid of permanent spaces that serve local communities in the long rather than short term.
Is it simply an economic choice, as it’s cheaper to sell assets and remove the costs of having to maintain them? Are local councils trying to raise long-term council and business tax revenue through increased property provision? Or do they view development as more worthy compared to the management of public parks?
Research in Liverpool has shown that local authorities are under significant pressures to meet budgetary demands meaning all these considerations are being made. In some cases, sales are deemed politically practical regardless of the problems associated with a lack of accessible parks and green space.
However, the pandemic has reinforced the need for local councils, and society more generally, to consider its health, wellbeing, and the promotion of social interaction provided by the natural environment – especially in areas of low income or diverse ethnicity. Over the last 20 months, enhancement and maintenance of local parks has been critical to public health. Evidence from the Office for National Stastistics, Natural England and Public Health England provides an argument for more and better provision of green space in support of this view.
Working with pop-up parks in the future
Pop-up parks are not a bad idea. The use of pop-up forests and forest bathing pods in Liverpool generated excitement when they were used in the summer of 2019. Additional examples from the US and Australia also highlight the ways in which pop-up parks can add a vibrancy to urban areas.
However, these examples were well thought through and didn’t cost £6 million. They were more discreet in their scale and had more interactive elements to their designs that drew people in. The Marble Arch Mound went big and bold and is now viewed as a folly, much like the Garden Bridge, for failing to meet expectations.
London, though, has numerous examples of innovative partnerships developed between businesses, local councils and the environment sector that have brought underused and undervalued spaces back into use. For example, the Wild West End partnership champions biodiversity and green space provision in the same area as the Marble Arch Mound, but has delivered projects that have been seen as far more successful.
The partnership’s small park in Baker Street, built around an area of decking with planters containing silver birch trees and perennial plants, was created to improve air quality and biodiversity. It also serves as a spot for people to sit and host events in a busy area of the city. Two years since it opened in 2019 the space has had tangible environmental benefits, including welcoming many pollinators, and is loved by local residents, those employed in the area and visitors.
Likewise, environmental charity Thames 21, has worked extensively with partners to improve London’s waterways and engage people with the environment. They, and other organisations including Groundwork, have created great green spaces via volunteering and corporate social responsibility programmes that focus on getting people to engage directly with nature.
Westminster council purports that the temporary structure is part of their bid for a “greener, smarter, future, together”. Programmes like Wild West End, Thames 21 and Groundwork are actually achieving this vision with their green spaces, which focus on engaging the local community and improving wellbeing while striving to provide better green urban spaces that aid the local environment.
The ways in which these organisations have worked over a prolonged period in dedicated locations show that the provision of additional amenities such as play areas, sports pitches, or space to socialise can thrive compared to gimmicks or projects that prioritise economic development over ecology and community. They also highlight that a range of green and blue spaces including parks, street trees, open grassed areas and canals can meet local and city-wide health and recreation needs.
Pop-up projects like the Marble Arch Mound can become places that people cherish and important green additions that work with urban environments. However, if green spaces, short and long term, are community driven and have accessible urban nature at their heart, rather than economic concerns like the Marble Arch Mound, local communities and local councils can benefit.
Kamran Mahroof– Assistant Professor, Supply Chain Analytics, University of Bradford
Sankar Sivarajah– Professor of Technology Management and Circular Economy, University of Bradford
The COVID pandemic has shown the fragility of our global food supply chains, with many supermarkets and restaurants in almost every country having experienced food shortages. Millions of people in the UK alone have experienced severe food insecurity during COVID-19, according to a recent report by the country’s Foods Standards Agency. But food shortages were prevalent long before the pandemic.
At the same time, one-third of all food produced each year is squandered or spoiled before it can be consumed. Research also suggests that high-income countries waste as much food as sub-Saharan Africa produces.
This food waste then ends up in landfills to rot – which releases greenhouse gases. And when this is combined with the amount of energy it takes to produce, manufacture, transport and store this food, it contributes a staggering 3 billion tonnes of carbon dioxide to our planet. To put that in context, if food waste was a country, it would be the third-highest emitter of greenhouse gases in the world, after the US and China.
But the good news is there are numerous techniques, technologies and policies that together could help reduce global food waste at every point in the process of producing and consuming it.
Why is food wasted?
According to the Food and Agriculture Organisation for the United Nations, lack of infrastructure, limited knowledge on storage and food handling, combined with unfavourable climatic conditions, can lead to a lot of food spoilage and waste in low-income countries.
On the other hand, in high-income countries, aesthetic preferences and arbitrary sell-by dates mean food easily becomes waste. Cosmetic blemishes, produce that is too ripe, too big, too little or even the wrong shape can lead to perfectly good fruits and vegetables going to waste.
As the global population continues to increase, it places real pressure on world food production. Indeed, the industry will need to grow by at least two-thirds by 2050 to ensure adequate nutrition for everyone in the world.
Yet, despite the dire need to become more resourceful, food waste and loss is at an all-time high. Making it clear that unless prompt action is taken, food shortages will soon become a long-term reality.
What can be done?
The key to tackling this issue is to have a resilient and resourceful “farm-to-fork” approach to help reduce food waste and to ensure the future of food security. Here are some things that can help combat food waste:
AI drones and precision farming
Collaboration with food producers and more investment in technological applications and overall infrastructure at the earlier stages of the food supply chain can drastically improve food waste and loss in low-income countries.
This is important because plant diseases and pests – along with poor harvesting techniques – can be a big factor in the high levels of food waste at this point in the food supply chain.
Our research also indicates that artificial intelligence (AI) powered drones can help farmers become more resourceful and reduce the overuse of pesticides in food production. This is important because pesticides can adversely affect the food ecosystem. They pollute water, deplete soil fertility and contaminate turf – all of which can result in food loss and waste. This approach also enhances crop yield and reduces operational costs as well as improves the health of livestock. So it’s also better for the environment.
Target shoppers’ wallets
A big part of the food waste problem is changing how we shop and view food and our mindset around what constitutes waste. But research shows the best way to tackle food waste among consumers is to highlight the potential money that can be saved as well as the “feel-good factor”, or moral value, of doing a good thing for the environment.
A recent study with households in London, UK and Ontario, Canada, found that a two-week money-based intervention – called “reduce food waste, save money” – helped participants to throw away 30% less food. Participants were given local information on food waste and costs, along with tips on how to improve food planning, efficiently purchase, store, and prepare food – and how to use leftovers to create new meals.
Similarly, new technology can help commercial kitchens reduce food waste by directly connecting behaviour changes to increased profits. For example, the Winnow software system calculates the costs of discarded food, correlating food waste to sales. This AI-powered system has allowed Ikea stores to reduce food waste by 50% in 2020, saving 1.2 million meals in the process.
While the problem of food waste can feel quite out of your hands as a consumer, there are things you can do to help.
Things like supporting businesses or restaurants that use waste foods in their products or meals. Planning your meals around sell-by dates. Not throwing out food if it’s a bit wilted or bruised and only buying what you need – especially on special occasions where food can often go uneaten and to waste.
You can also show supermarkets that “wonky” fruit and veggies are just as good as the “normal” shaped produce by buying these over the perfect looking pears or potatoes.
Ultimately, it’s not going to be any single thing that solves food waste, but a collective approach can enable us to make the changes that need to happen.
Bonnie Waring– Senior Lecturer, Grantham Institute – Climate Change and Environment, Imperial College London
One morning in 2009, I sat on a creaky bus winding its way up a mountainside in central Costa Rica, light-headed from diesel fumes as I clutched my many suitcases. They contained thousands of test tubes and sample vials, a toothbrush, a waterproof notebook and two changes of clothes.
I was on my way to La Selva Biological Station, where I was to spend several months studying the wet, lowland rainforest’s response to increasingly common droughts. On either side of the narrow highway, trees bled into the mist like watercolours into paper, giving the impression of an infinite primeval forest bathed in clouds.
You can listen to more articles from The Conversation, narrated by Noa, here.
As I gazed out of the window at the imposing scenery, I wondered how I could ever hope to understand a landscape so complex. I knew that thousands of researchers across the world were grappling with the same questions, trying to understand the fate of tropical forests in a rapidly changing world.
Our society asks so much of these fragile ecosystems, which control freshwater availability for millions of people and are home to two thirds of the planet’s terrestrial biodiversity. And increasingly, we have placed a new demand on these forests – to save us from human-caused climate change.
Plants absorb CO₂ from the atmosphere, transforming it into leaves, wood and roots. This everyday miracle has spurred hopes that plants – particularly fast growing tropical trees – can act as a natural brake on climate change, capturing much of the CO₂ emitted by fossil fuel burning. Across the world, governments, companies and conservation charities have pledged to conserve or plant massive numbers of trees.
But the fact is that there aren’t enough trees to offset society’s carbon emissions – and there never will be. I recently conducted a review of the available scientific literature to assess how much carbon forests could feasibly absorb. If we absolutely maximised the amount of vegetation all land on Earth could hold, we’d sequester enough carbon to offset about ten years of greenhouse gas emissions at current rates. After that, there could be no further increase in carbon capture.
Yet the fate of our species is inextricably linked to the survival of forests and the biodiversity they contain. By rushing to plant millions of trees for carbon capture, could we be inadvertently damaging the very forest properties that make them so vital to our wellbeing? To answer this question, we need to consider not only how plants absorb CO₂, but also how they provide the sturdy green foundations for ecosystems on land.
Plants convert CO₂ gas into simple sugars in a process known as photosynthesis. These sugars are then used to build the plants’ living bodies. If the captured carbon ends up in wood, it can be locked away from the atmosphere for many decades. As plants die, their tissues undergo decay and are incorporated into the soil.
While this process naturally releases CO₂ through the respiration (or breathing) of microbes that break down dead organisms, some fraction of plant carbon can remain underground for decades or even centuries. Together, land plants and soils hold about 2,500 gigatonnes of carbon – about three times more than is held in the atmosphere.
Because plants (especially trees) are such excellent natural storehouses for carbon, it makes sense that increasing the abundance of plants across the world could draw down atmospheric CO₂ concentrations.
Plants need four basic ingredients to grow: light, CO₂, water and nutrients (like nitrogen and phosphorus, the same elements present in plant fertiliser). Thousands of scientists across the world study how plant growth varies in relation to these four ingredients, in order to predict how vegetation will respond to climate change.
This is a surprisingly challenging task, given that humans are simultaneously modifying so many aspects of the natural environment by heating the globe, altering rainfall patterns, chopping large tracts of forest into tiny fragments and introducing alien species where they don’t belong. There are also over 350,000 species of flowering plants on land and each one responds to environmental challenges in unique ways.
Due to the complicated ways in which humans are altering the planet, there is a lot of scientific debate about the precise quantity of carbon that plants can absorb from the atmosphere. But researchers are in unanimous agreement that land ecosystems have a finite capacity to take up carbon.
If we ensure trees have enough water to drink, forests will grow tall and lush, creating shady canopies that starve smaller trees of light. If we increase the concentration of CO₂ in the air, plants will eagerly absorb it – until they can no longer extract enough fertiliser from the soil to meet their needs. Just like a baker making a cake, plants require CO₂, nitrogen and phosphorus in particular ratios, following a specific recipe for life.
In recognition of these fundamental constraints, scientists estimate that the earth’s land ecosystems can hold enough additional vegetation to absorb between 40 and 100 gigatonnes of carbon from the atmosphere. Once this additional growth is achieved (a process which will take a number of decades), there is no capacity for additional carbon storage on land.
But our society is currently pouring CO₂ into the atmosphere at a rate of ten gigatonnes of carbon a year. Natural processes will struggle to keep pace with the deluge of greenhouse gases generated by the global economy. For example, I calculated that a single passenger on a round trip flight from Melbourne to New York City will emit roughly twice as much carbon (1600 kg C) as is contained in an oak tree half a meter in diameter (750 kg C).
Peril and promise
Despite all these well recognised physical constraints on plant growth, there is a proliferating number of large scale efforts to increase vegetation cover to mitigate the climate emergency – a so called “nature-based” climate solution. The vastmajority of these efforts focus on protecting or expanding forests, as trees contain many times more biomass than shrubs or grasses and therefore represent greater carbon capture potential.
Yet fundamental misunderstandings about carbon capture by land ecosystems can have devastating consequences, resulting in losses of biodiversity and an increase in CO₂ concentrations. This seems like a paradox – how can planting trees negatively impact the environment?
The answer lies in the subtle complexities of carbon capture in natural ecosystems. To avoid environmental damage, we must refrain from establishing forests where they naturally don’t belong, avoid “perverse incentives” to cut down existing forest in order to plant new trees, and consider how seedlings planted today might fare over the next several decades.
Before undertaking any expansion of forest habitat, we must ensure that trees are planted in the right place because not all ecosystems on land can or should support trees. Planting trees in ecosystems that are normally dominated by other types of vegetation often fails to result in long term carbon sequestration.
One particularly illustrative example comes from Scottish peatlands – vast swathes of land where the low-lying vegetation (mostly mosses and grasses) grows in constantly soggy, moist ground. Because decomposition is very slow in the acidic and waterlogged soils, dead plants accumulate over very long periods of time, creating peat. It’s not just the vegetation that is preserved: peat bogs also mummify so-called “bog bodies” – the nearly intact remains of men and women who died millennia ago. In fact, UK peatlands contain 20 times more carbon than found in the nation’s forests.
But in the late 20th century, some Scottish bogs were drained for tree planting. Drying the soils allowed tree seedlings to establish, but also caused the decay of the peat to speed up. Ecologist Nina Friggens and her colleagues at the University of Exeter estimated that the decomposition of drying peat released more carbon than the growing trees could absorb. Clearly, peatlands can best safeguard the climate when they are left to their own devices.
The same is true of grasslands and savannahs, where fires are a natural part of the landscape and often burn trees that are planted where they don’t belong. This principle also applies to Arctic tundras, where the native vegetation is covered by snow throughout the winter, reflecting light and heat back to space. Planting tall, dark-leaved trees in these areas can increase absorption of heat energy, and lead to local warming.
But even planting trees in forest habitats can lead to negative environmental outcomes. From the perspective of both carbon sequestration and biodiversity, all forests are not equal – naturally established forests contain more species of plants and animals than plantation forests. They often hold more carbon, too. But policies aimed at promoting tree planting can unintentionally incentivise deforestation of well established natural habitats.
A recent high-profile example concerns the Mexican government’s Sembrando Vida programme, which provides direct payments to landowners for planting trees. The problem? Many rural landowners cut down well established older forest to plant seedlings. This decision, while quite sensible from an economic point of view, has resulted in the loss of tens of thousands of hectares of mature forest.
This example demonstrates the risks of a narrow focus on trees as carbon absorption machines. Many well meaning organisations seek to plant the trees which grow the fastest, as this theoretically means a higher rate of CO₂ “drawdown” from the atmosphere.
Yet from a climate perspective, what matters is not how quickly a tree can grow, but how much carbon it contains at maturity, and how long that carbon resides in the ecosystem. As a forest ages, it reaches what ecologists call a “steady state” – this is when the amount of carbon absorbed by the trees each year is perfectly balanced by the CO₂ released through the breathing of the plants themselves and the trillions of decomposer microbes underground.
This phenomenon has led to an erroneous perception that old forests are not useful for climate mitigation because they are no longer growing rapidly and sequestering additional CO₂. The misguided “solution” to the issue is to prioritise tree planting ahead of the conservation of already established forests. This is analogous to draining a bathtub so that the tap can be turned on full blast: the flow of water from the tap is greater than it was before – but the total capacity of the bath hasn’t changed. Mature forests are like bathtubs full of carbon. They are making an important contribution to the large, but finite, quantity of carbon that can be locked away on land, and there is little to be gained by disturbing them.
What about situations where fast growing forests are cut down every few decades and replanted, with the extracted wood used for other climate-fighting purposes? While harvested wood can be a very good carbon store if it ends up in long lived products (like houses or other buildings), surprisingly little timber is used in this way.
Similarly, burning wood as a source of biofuel may have a positive climate impact if this reduces total consumption of fossil fuels. But forests managed as biofuel plantations provide little in the way of protection for biodiversity and some research questions the benefits of biofuels for the climate in the first place.
Fertilise a whole forest
Scientific estimates of carbon capture in land ecosystems depend on how those systems respond to the mounting challenges they will face in the coming decades. All forests on Earth – even the most pristine – are vulnerable to warming, changes in rainfall, increasingly severe wildfires and pollutants that drift through the Earth’s atmospheric currents.
Some of these pollutants, however, contain lots of nitrogen (plant fertiliser) which could potentially give the global forest a growth boost. By producing massive quantities of agricultural chemicals and burning fossil fuels, humans have massively increased the amount of “reactive” nitrogen available for plant use. Some of this nitrogen is dissolved in rainwater and reaches the forest floor, where it can stimulate tree growth in some areas.
As a young researcher fresh out of graduate school, I wondered whether a type of under-studied ecosystem, known as seasonally dry tropical forest, might be particularly responsive to this effect. There was only one way to find out: I would need to fertilise a whole forest.
Working with my postdoctoral adviser, the ecologist Jennifer Powers, and expert botanist Daniel Pérez Avilez, I outlined an area of the forest about as big as two football fields and divided it into 16 plots, which were randomly assigned to different fertiliser treatments. For the next three years (2015-2017) the plots became among the most intensively studied forest fragments on Earth. We measured the growth of each individual tree trunk with specialised, hand-built instruments called dendrometers.
We used baskets to catch the dead leaves that fell from the trees and installed mesh bags in the ground to track the growth of roots, which were painstakingly washed free of soil and weighed. The most challenging aspect of the experiment was the application of the fertilisers themselves, which took place three times a year. Wearing raincoats and goggles to protect our skin against the caustic chemicals, we hauled back-mounted sprayers into the dense forest, ensuring the chemicals were evenly applied to the forest floor while we sweated under our rubber coats.
Unfortunately, our gear didn’t provide any protection against angry wasps, whose nests were often concealed in overhanging branches. But, our efforts were worth it. After three years, we could calculate all the leaves, wood and roots produced in each plot and assess carbon captured over the study period. We found that most trees in the forest didn’t benefit from the fertilisers – instead, growth was strongly tied to the amount of rainfall in a given year.
This suggests that nitrogen pollution won’t boost tree growth in these forests as long as droughts continue to intensify. To make the same prediction for other forest types (wetter or drier, younger or older, warmer or cooler) such studies will need to be repeated, adding to the library of knowledge developed through similar experiments over the decades. Yet researchers are in a race against time. Experiments like this are slow, painstaking, sometimes backbreaking work and humans are changing the face of the planet faster than the scientific community can respond.
Humans need healthy forests
Supporting natural ecosystems is an important tool in the arsenal of strategies we will need to combat climate change. But land ecosystems will never be able to absorb the quantity of carbon released by fossil fuel burning. Rather than be lulled into false complacency by tree planting schemes, we need to cut off emissions at their source and search for additional strategies to remove the carbon that has already accumulated in the atmosphere.
Does this mean that current campaigns to protect and expand forest are a poor idea? Emphatically not. The protection and expansion of natural habitat, particularly forests, is absolutely vital to ensure the health of our planet. Forests in temperate and tropical zones contain eight out of every ten species on land, yet they are under increasing threat. Nearly half of our planet’s habitable land is devoted to agriculture, and forest clearing for cropland or pasture is continuing apace.
Meanwhile, the atmospheric mayhem caused by climate change is intensifying wildfires, worsening droughts and systematically heating the planet, posing an escalating threat to forests and the wildlife they support. What does that mean for our species? Again and again, researchers have demonstrated strong links between biodiversity and so-called “ecosystem services” – the multitude of benefits the natural world provides to humanity.
Carbon capture is just one ecosystem service in an incalculably long list. Biodiverse ecosystems provide a dizzying array of pharmaceutically active compounds that inspire the creation of new drugs. They provide food security in ways both direct (think of the millions of people whose main source of protein is wild fish) and indirect (for example, a large fraction of crops are pollinated by wild animals).
Natural ecosystems and the millions of species that inhabit them still inspire technological developments that revolutionise human society. For example, take the polymerase chain reaction (“PCR”) that allows crime labs to catch criminals and your local pharmacy to provide a COVID test. PCR is only possible because of a special protein synthesised by a humble bacteria that lives in hot springs.
As an ecologist, I worry that a simplistic perspective on the role of forests in climate mitigation will inadvertently lead to their decline. Many tree planting efforts focus on the number of saplings planted or their initial rate of growth – both of which are poor indicators of the forest’s ultimate carbon storage capacity and even poorer metric of biodiversity. More importantly, viewing natural ecosystems as “climate solutions” gives the misleading impression that forests can function like an infinitely absorbent mop to clean up the ever increasing flood of human caused CO₂ emissions.
Luckily, many big organisations dedicated to forest expansion are incorporating ecosystem health and biodiversity into their metrics of success. A little over a year ago, I visited an enormous reforestation experiment on the Yucatán Peninsula in Mexico, operated by Plant-for-the-Planet – one of the world’s largest tree planting organisations. After realising the challenges inherent in large scale ecosystem restoration, Plant-for-the-Planet has initiated a series of experiments to understand how different interventions early in a forest’s development might improve tree survival.
But that is not all. Led by Director of Science Leland Werden, researchers at the site will study how these same practices can jump-start the recovery of native biodiversity by providing the ideal environment for seeds to germinate and grow as the forest develops. These experiments will also help land managers decide when and where planting trees benefits the ecosystem and where forest regeneration can occur naturally.
Viewing forests as reservoirs for biodiversity, rather than simply storehouses of carbon, complicates decision making and may require shifts in policy. I am all too aware of these challenges. I have spent my entire adult life studying and thinking about the carbon cycle and I too sometimes can’t see the forest for the trees. One morning several years ago, I was sitting on the rainforest floor in Costa Rica measuring CO₂ emissions from the soil – a relatively time intensive and solitary process.
As I waited for the measurement to finish, I spotted a strawberry poison dart frog – a tiny, jewel-bright animal the size of my thumb – hopping up the trunk of a nearby tree. Intrigued, I watched her progress towards a small pool of water held in the leaves of a spiky plant, in which a few tadpoles idly swam. Once the frog reached this miniature aquarium, the tiny tadpoles (her children, as it turned out) vibrated excitedly, while their mother deposited unfertilised eggs for them to eat. As I later learned, frogs of this species (Oophaga pumilio) take very diligent care of their offspring and the mother’s long journey would be repeated every day until the tadpoles developed into frogs.
It occurred to me, as I packed up my equipment to return to the lab, that thousands of such small dramas were playing out around me in parallel. Forests are so much more than just carbon stores. They are the unknowably complex green webs that bind together the fates of millions of known species, with millions more still waiting to be discovered. To survive and thrive in a future of dramatic global change, we will have to respect that tangled web and our place in it.