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Planting trees won’t solve the climate crisis – but if it’s done right, it can help.

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Forest ecosystems are the largest terrestrial carbon sink on the planet, every year soaking up a total of 2.6 billion metric tons of carbon dioxide – around a third of total global emissions. But while it seems intuitive to infer from this that planting more trees is, broadly speaking, a good thing, recent high profile calls for an acceleration of tree planting programs as a means of mitigating the climate crisis have ignited surprisingly fierce debate among environmentalists.

Certainly, there’s every reason to question the motivations behind calls for tree-planting when they come from people intransigently opposed to taking meaningful action to curb carbon emissions in the first place – as was the case with the Trump administration’s otherwise inexplicable enthusiasm for the World Economic Forum’s Trillion Trees initiative. But that doesn’t change the fact that planting trees, provided it’s done with proper forethought and careful, science-based planning, is one of the most effective strategies we have for climate change mitigation.

Exactly how much carbon can be removed from the atmosphere through tree planting is hard to quantify, apart from anything else because of the wide variation in estimates of how much land it would realistically be possible to cover. A 2015 paper in the journal Nature Climate Change suggested that if roughly 20% of the current global area of arable land were to be forested (which would not be without some trade-offs in respect of food production, among other things) we could be looking at sequestering around 1.1 billion additional metric tons of carbon per year1 – a projection endorsed in the European Academies’ Science Advisory Council 2018 report on the potential of negative-emissions technologies in meeting the Paris Climate Agreement targets. By other estimates, expanding current forest cover across the total global area capable of naturally supporting forests could eventually store an extra 205 billion metric tons of carbon – roughly a quarter of the total amount currently in our atmosphere.

America’s forests already suck up the equivalent of more than 14% of U.S. CO2 emissions every year, and recent research suggests the potential to increase carbon sequestration capacity by a further 20% just by fully stocking all of the nation’s productive forestland (which again would bring its own challenges and trade-offs). That alone, according to a paper published in the Proceedings of the National Academy of Sciences last year, would remove an additional 51 million metric tons of carbon from the atmosphere each year. And according to the same paper, this is more than feasible: the U.S. has the federal, state and private capacity to produce and plant more than 1 billion seedlings per year.

Much of the current skepticism around tree-planting stems in part from the experiences of countries that have rushed to position themselves as leaders in this domain in recent years. Often, “rushed” has been the operative word, and as a result, these initiatives have in some cases done more to highlight the potential pitfalls of large-scale tree planting than its benefits.

Indeed, get it wrong, and you can end up doing more harm than good. The kind of monoculture plantations that have formed the backbone of several countries’ reforestation efforts, for example, can decrease biodiversity and modify the native wildlife, potentially precipitating a chain of adverse ecosystem effects. Moreover, the thirsty, fast-growing, non-native species of trees these kinds of plantations typically contain consume massive volumes of water, potentially leading to irreparable changes to soils and serious problems for water availability, and indeed the entire water cycle, as has been documented in plantations in China. Converting native grasslands to forests likewise brings its own risks, not least for the many species that depend on these ecosystems’ unique conditions to survive.

But none of this points to an inherent flaw in the concept of tree-planting itself. What it shows is simply that when it comes to putting the policy framework in place to actually do it, it is crucial that that framework is set up in such a way as to support the right kind of initiatives and prevent the kinds of problems that have played out elsewhere in the world with poorly-thought-out tree-planting programs. This means not just ensuring that the policies put in place don’t actively cause environmental damage, but also that what we’re doing is actually going to achieve what it sets out to achieve.

We know, for example, that spatially concentrating current tree planting capacity to fully stock non-stocked timberland, rather than planting the same number of trees over larger areas, provides the greatest potential to increase carbon sequestration capacity. We also know that, in addition to all their other risks, monoculture plantations only ever hold a small fraction of the carbon that could be captured by giving space for natural forest regrowth, and that restored natural forests absorb more carbon over time than plantations while also supporting greater biodiversity.

Should these kinds of lessons be taken onboard at the policy level, tree planting can be an effective component of a broader suite of measures to tackle climate change. Not only that, but do it right, and it becomes a win-win for both climate and conservation efforts – and a cost-effective one at that. It’s no magic bullet, and it certainly shouldn’t be mistaken for giving polluters license to keep pumping carbon into the atmosphere. But that doesn’t mean it’s not part of the solution.

Photo credit: Nathan Anderson, via Unsplash

 

 


  1. The paper projects 1.1–3.3 GtC/year (320 million to 970 million hectares or ~20–60% of the current global area of arable land). Estimate given here based on the low end of that range.↩︎