Should we just plant trees everywhere to fix climate change?

In short, no. There are major negative consequences when planting programmes are poorly planned.

We need to stem deforestation and increase reforestation as a natural climate solution, but this needs to be done strategically and by listening to the science.

Climate change and biodiversity loss are two of the most pressing issues facing humanity. Left unchecked, they have the potential to threaten our very existence. Most of us, at least I assume it would be fair to say, realise they are interconnected: climate change exacerbates biodiversity loss (we lose species when conditions become unfavourable) and biodiversity loss exacerbates climate change (deforestation of major forests such as the Amazon releases large amounts of carbon to, and reduces sequestration from, the atmosphere). Yet, we tend to focus on them as separate issues a lot of the time. This is a major problem for reasons I describe below.

In an earlier post, I talked about the need for win-win biodiversity-climate solutions. The fact that the two are connected means this is possible, but it also means tackling each in isolation can lead to perverse outcomes. Focusing solely on climate mitigation can have direct negative consequences on biodiversity, ecosystems, and their contributions to society, like their ability to provide clean water for drinking and medicinal products.

So we need to tackle biodiversity loss and climate change together, searching for win-wins, or at least considering the potential for a narrow focus on either to be at best suboptimal, at worst detrimental.

Trees and forests as a natural climate solution

Trees and forests are a major tool in the climate mitigation space. In fact, they are one of the most effective tools. They capture carbon from the atmosphere and store it in their own biomass and soil. So it’s not surprising that large-scale tree planting is one of the first things we think about when we consider climate mitigation tools. And so it should be! This can be in the form of afforestation, where trees are planted in areas that were previously not forested, and reforestation of previously forested areas. The latter of which often involves replacing areas that were historically diverse native forests with monocultures of non-native species, such as pine forests in New Zealand. These two, afforestation and reforestation, should not be conflated because they are very different things.

The need for climate-resilient forests

High-profile efforts have considered how much of the Earth’s surface can support forests based on climatic suitability. So, we know where on Earth potentially suits forests, but this does not mean these areas should support forests. And, in fact, even where reforestation or afforestation is sensible, these forests themselves are coming under increasing risk from global change drivers like climate change and spread of pests and pathogens.

Many species that are used today in forestry will likely not be suitable in the coming century due to changing climatic conditions.

Any new planting programmes, whether reforestation or afforestation, need to account for the climatic suitability of species in their receiving habitat not only right now but for the next century. Doing so has shown that the available pool of species is reduced by up to 50% in Europe. So even the highly ecologically beneficial push to diversify production forests (as opposed to planting monocultures) is becoming harder because many species may no longer be climatically suitable in 50 years if trends continue. Regardless, it is important to design resilient ecosystems, given the increasing clarity of what climate change will look like.

Reforestation is good but needs careful planning

Even where reforestation is a good idea, we also have to accept that, for the purpose of carbon capture, there will be many cases where monocultures of productive forests will replace historically diverse native forests. This will lead to, and already has led to, major losses to ecosystems, including loss of biodiversity, decreased soil quality and increased erosion potential, and increases in vulnerability to pests and pathogens, fire risk, and water use. In certain cases, for the benefit of immediate carbon capture, this could be considered a justifiable trade-off. But, we should minimise such efforts by focusing instead on forests that work for carbon capture AND biodiversity.

Single species forests will also have a lower ability to adapt and respond to future threats. Think about an investor. Sensible investors invest in a diverse portfolio of stocks and shares to buffer themselves from unexpected shocks. Ecosystems are much the same — increased diversity of species or habitats increases their resilience to disturbances.

Worse still, monocultures of fast-growing species like pines tend to get cut down after a relatively short time period (or begin to die off) and so do not end up being long-term carbon sequestration solutions. Native forests, by contrast, often grow more slowly and can persist for centuries. Not to mention all of the social, ecological and recreational benefits we receive from healthy, functioning forest ecosystems!

So, in an ideal world, all reforestation projects would focus on restoring the original diverse native ecosystems or, secondarily, in cases where historically native species will struggle in the long-term, mixtures of climate-resilient species.

But should we plant everywhere that is potentially suitable?

OK, but what about afforestation you say? Well, this is a whole different story.

Before I go any further, let’s get one thing straight: Yes, we absolutely need to plant more trees, but no, these shouldn’t be everywhere that is potentially suitable.

Here is where we start to see the trade-off pendulum swing to the more problematic end of the scale. There are widespread consequences of planting trees (including natives) in treeless ecosystems. For instance, while there would obviously be increased above-ground carbon storage (treeless ecosystems don’t exactly have much above-ground biomass do they), this can lead to a reduction in below-ground carbon storage. In fact, it may even reduce existing carbon storage of the area.

Hold on, what are treeless ecosystems though? Treeless ecosystems are those that naturally lack trees, like tundra, alpine meadows, grasslands, deserts, shrublands and wetlands.

There are also a bunch of other consequences, including increased fire risk and intensity, increased albedo (trees absorb more solar energy and reflect less compared to bare ground or grasses), reductions in native biodiversity, and changes in the ability for the soil to hold nutrients and water. Indeed, planted areas tend to use more water than treeless equivalents.

What’s more, planting in treeless ecosystems often involves tolerant species that tend to be prone to spreading elsewhere. Yes, but aren’t we trying to mitigate climate change here? Who cares about invasive species? How about allowing or even promoting invasions into treeless ecosystems by non-native or invasive plant species for the purposes of carbon sequestration? Well, that’s very rarely a good idea for many of the same reasons as above. And the economic costs, not to mention the ecological costs, of invasive species are HUGE!

Martin Nuñez summed it up nicely in one tweet:

Climate Change is a huge problem

Invasive species are a huge problem

We should not fight Climate Change by promoting invasion!

In most places it is a BAD idea

Wrapping up

Don’t get me wrong we absolutely need to revegetate much of the planet. But we need to be careful how we do this and where we do this. And we should avoid doing so at the expense of naturally treeless ecosystems.

Win-wins are hard to find, but we’re better to search for them using the wealth of available science than to cause more damage with rash decision making.

Of course, we need to plug the leak first. While natural climate solutions like reforestation and, in some cases afforestation, continue, we must also stop deforesting much of the remainder of the planet’s forests. And the best way to fight climate change is to cut emissions first: we can’t plant our way out of the climate crisis.

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