Fix the Planet newsletter: Can tree diversity help the climate?

4 months ago 98
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By Adam Vaughan

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Rainforest at dawn, Mgahinga Gorilla National Park, Uganda, Africa.

Dylan Garcia Photography / Alamy Stock Photo

This is Fix the Planet, the weekly climate change newsletter that reminds you there are reasons for hope in science and technology around the world. To receive it in your inbox, sign up here.

Hi, welcome back to Fix the Planet. A bit of joy this week, thanks to research showing there are far more tree species on Earth than we thought. The new estimate says there are more than 73,000 species globally, with about 9200 of those yet to be discovered by scientists.

As co-author Peter Reich at the University of Minnesota told me, on the one hand, this is a simple “celebration of life”. On the other, we can’t save what we don’t know about, he says.

But with tree planting and halting deforestation both seen as crucial tools for putting the brakes on global warming, this new finding means it’s also worth asking what greater tree diversity means for tackling climate change. Will preserving more species help us lock up more carbon?

How might a greater richness of species help store more carbon?

First, it’s worth remembering that if drawing down carbon is your primary concern, the main thing is having more trees and ensuring that they last. One high-profile analysis found there is space left on the planet to accommodate enough trees to lock up more than 200 billion tonnes of carbon dioxide, or about five years of humanity’s emissions (it’s worth noting that this research received a lot of criticism). But greater diversity could be part of the answer too, by making forests better at using resources and more resilient to threats.

Why are more diverse forests more effective?

Differences between tree species cause them to utilise available resources more effectively, says Martin Sullivan at Manchester Metropolitan University, UK. Varied canopy structures mean some intercept light better, while differences in root architecture influence how they draw resources from the soil. “This would mean that more [groups of trees] grow faster, and thus take in carbon more rapidly, because they can access more resources,” says Sullivan. Experimental studies have shown this mechanism in grasslands, and it could plausibly occur in forests too, he adds. Another factor is that tree species differ in their ability to absorb and store carbon, so by chance alone, a more diverse forest is more likely to have species that are very good at drawing down and sequestering carbon.

And why are they more resilient?

In addition to humans chopping them down and natural disasters damaging them, forests face threats from pests and pathogens, plus droughts and fires wrought by climate change itself. Research points to species-rich forests being less susceptible to bugs, disease and extreme climate events, says Florian Schnabel at the German Centre for Integrative Biodiversity Research (iDiv). “Diversity is an insurance,” he says.

“In a diverse forest, each species will have a slightly different niche it occupies, and there is a wide variety of survival strategies,” says Charlotte Wheeler at the Center for International Forestry Research. “If some species die due to a particular disease, then other species will be able to fill the gap they leave behind.” One example is ash trees being removed across the UK because of a fungal disease, with sycamores among the species proposed to replace them.

Tree diversity’s most important role in storing carbon could be the way it prevents forests flipping to other ecosystem types that hold less carbon, such as savannah, says Christian Wirth, also at iDiv. For example, different types of trees have different tolerances to drought, allowing for a turnover of tree species that keeps the forest intact.

As temperatures rise, from a global average of 1.1°C above pre-industrial levels now to at least 1.5°C in the next 20 years, that ability to survive and keep carbon locked up will be key. “Resilience is very important for forest and will be an important mechanism to allow forest to adapt to climate change, and inevitably there will be some winners and losers in the face of climate change,” says Wheeler.

How sure are we that diversity means more carbon absorbed and stored?

“There is still no definitive answer to say that more diverse forests will necessarily sequester carbon more quickly,” says Wheeler. Part of the problem is that it’s hard to conduct experiments to measure what impact diversity has, so most research to date has been observational. That makes attributing causality a challenge, says Sullivan.

Nonetheless, he says there is reasonable evidence that diverse temperate forests produce woody growth more rapidly than less diverse ones, drawing down and locking away more carbon. “But it is less clear whether these effects extend to diverse tropical forests,” he says.

What is clear is that newly planted forests with a mix of species usually sequester more carbon than monocultures, such as solely planting eucalyptus, as some tree-planting schemes have done. But again, whether that link holds true for natural forests is less understood, says Wheeler.

What difference will it make to climate efforts if the world has 9200 more tree species?

Not a lot in itself. Most of the predicted but undiscovered species are probably very rare. “What effect are these additional species likely to have on carbon sequestration?  Probably very little, as they’re mostly rare, so not making a major contribution,” says David Coomes at the University of Cambridge, a co-author of the paper.

Hang on, tackling climate change is far from the only thing forests offer us, right?

“Ecologists and land managers should not just have a singular focus on one ecosystem service but a balance of many: carbon, biodiversity, human well-being, hydrological regulation, economically useful plants,” says Justin Moat at Royal Botanic Gardens, Kew, in the UK.

“There are already compelling reasons to protect the biodiverse tropics: we know that species-rich forests are more resilient to pests, pathogens and a changing climate,” says Coomes. “The news that forests are even more diverse than previously supposed adds further weight to arguments that rainforests should be protected.”

More fixes

  • Does anyone really know what low-carbon heating means? A report out today from MPs in the UK says the government needs to undertake a public education campaign to explain why boilers and other heating systems in homes need to change. (See also the debate on whether one low-carbon option, heat pumps, needs a rebrand.)
  • What was the presentation that caused Boris Johnson to have an epiphany on climate change? The website Carbon Brief has got hold of the slides that convinced the UK prime minister to take the issue seriously.

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