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.