Can't see the Forest For the Tree Plantations

If you plant a tree in a forest and then cut it down, does it still make a carbon impact?

As I mentioned last time, we’re working on an analysis of forestation – planting trees – as a solution to climate change.

The topic turns out to be even more complex than I’d expected. The carbon impact of “one tree” is like the health impact of “one food”: you can’t say anything intelligent without specifying what kind, in what quantity, under what circumstances.

As a buildup to our review of the potential climate benefits of forestation, I’m going to spend today just walking through some of the things that influence a tree’s impact on the climate.

There’s More To a Tree Than What You Can See

I didn’t try to make sense of this, you don’t need to either. The point is simply that there’s a lot going on here! (Source)

The basic idea is, of course, very straightforward:

1. Trees are big.

2. Big things have a lot of stuff in them.

3. For living things, a lot of that stuff is carbon.

4. So, trees contain a lot of carbon.

5. They get that carbon from the air (splitting CO₂ via photosynthesis).

6. So, if you plant a tree, it will pull a lot of CO₂ from the air and store the carbon in its body.

However, it turns out that tree trunks and branches are not the only places where forests store carbon. There are at least five important categories:

• “Aboveground plant biomass” – the tree trunk, branches, and leaves.

• “Belowground plant biomass” – right, can’t forget the roots.

• Dead wood. This may eventually decay, but until then, there’s carbon in it.

• Litter. I haven’t found a straightforward definition, but I think this is basically fallen leaves, twigs, partially decomposed wood, and anything else with carbon in it that’s above ground and not part of the previous categories¹.

• Soil carbon: everything below ground, except for the living tree roots. This includes any sort of dead-and-buried material, which may come from trees, but also microbes and other organisms.

There are a couple of important implications. First, it’s really hard to measure the amount of carbon in a forest. Only one of the five categories shows up as a big obvious thing that you can hope to measure by just walking around and taking photos – let alone satellite photography. There are scientific models for analyzing and predicting the other four categories, but much work is still needed.

Second, a plot of land might already be storing a lot of carbon before you plant any trees on it. A forest contains a lot more obvious aboveground plant biomass than grassland, but by the time you’ve taken things like soil carbon into account, the results may surprise you. One source states, “North American grasslands can store as much carbon in soil as tropical forests store as biomass”. The soil carbon may have been accumulating for thousands of years!

The upshot is that planting a forest may not absorb as much carbon as you’d think. If the act of clearing, planting and managing the forest disturbs the existing soil carbon, it might not yield much benefit at all².

Diamonds Last Forever, Trees Maybe Not

The Earth’s carbon circulates in many different forms, from CO₂ in the air, to carbon-based chemicals in living organisms, to carbon-bearing minerals like limestone; not to mention carbon in soil or oceans. Some of these repositories hold onto their carbon for millions of years, others have short lives.

Diamonds are pure carbon, and very stable under normal conditions, which is why you’ve never heard of a diamond rotting. There’s a company that is literally pulling carbon out of the air and using it to make diamonds. Unfortunately, this is not in any way a solution to climate change; it’s far too expensive to be anything more than a gimmick.

People debate how long a carbon storage technique needs to last in order to be a constructive part of the climate solution. Typical suggestions range from 100 to 1000 years. Personally, I lean toward the idea that approaches which last less than 100 years can still be useful. In any case, some degree of durability is necessary.

Individual trees can live for over 1000 years; I’ve stood inside one. And forests can last for a very long time. But only under the right conditions. Fire, drought, disease, logging, and clearing all can release a forest’s carbon back into the air.

Unfortunately, human activity has been exacerbating every one of these threats to forest longevity. Climate change increases drought; hot, dry weather promotes fire; heat- and drought-stressed trees are more vulnerable to disease; international trade helps spread pests; population growth and increased demand for meat encourage logging and clearing; and so forth. Trees have never come with a guarantee of permanence, and statistics based on historical forests may not apply to newly planted forests in the decades to come.

Not All Trees Are Planted Equal

Tree-based climate solutions can be divided into four categories, some of which have more benefit than others:

• Proforestation, aka conservation: the preservation of existing forests, e.g. “Save the Rainforests” campaigns

• Reforestation: planting trees in an area which was historically forested, but lost its trees to logging, fire, etc.

• Afforestation: planting trees in an area which was not historically forested.

• Densification: “fill-in” planting in an area which already has some tree cover.

Each of these has different characteristics. Proforestation is generally “best”: undisturbed forests generally contain trees in a robust variety of ages, sizes, and species; substantial amounts of carbon stored in dead wood, litter, and soil carbon; and other helpful qualities. In other words, they store a lot of carbon, and they have the best chance of holding onto it.

Reforestation can hope to recreate this state, but it takes many decades (perhaps centuries) of careful management to get there. So preserving existing undisturbed forest is most important, but reforestation – with appropriate tree species – can also be very constructive.

Afforestation can be quite problematic: if an area wasn’t historically forested, there's probably a reason. The land may be vulnerable to fire, disease, predation from plant-eating insects or animals, etc. Densification also may push a forest unsustainably out of balance³. Some analyses focus on carbon capture potential per tree planted, ignoring the fact that the ecosystem may not be able to support as many trees as were planted⁴.

A related term, “improved forest management”, covers a range of practices such as restoration of degraded forests, “fuels treatment” to address over-managed forests now at risk of catastrophic burns, better scheduling of harvests in working forestlands, and others. This doesn’t fit neatly into the previous categories, but helps illustrate how many dimensions there are to forestry.

The upshot is that planting an acre of trees is helpful only to the extent that those trees will survive and thrive in the long run. And the historical state of the land provides a strong hint as to how that will turn out.

Does a Tree Count if You’re Just Going To Cut It Down?

It lacks a certain old-growth je ne sais quoi

Not all tree-planting projects are even attempting to create natural forests. A common alternative is the “tree plantation”. Per Wikipedia:

A tree plantation, forest plantation, plantation forest, timber plantation or tree farm is a forest planted for high volume production of wood, usually by planting one type of tree as a monoculture forest.

In other words, rather than attempting to reproduce the complex diversity of natural forests, a plantation treats trees as crops, where a monoculture of identical trees will be periodically harvested and replanted. According to a paper in Nature, of acres pledged for forestation under the Bonn Challenge pledges (a major international forestation initiative), 45% are for plantations. The paper goes on to say:

…plantations hold little more carbon, on average, than the land cleared to plant them. Clearance releases carbon, followed by rapid uptake by fast-growing trees such as Eucalyptus and Acacia (at up to 5 tonnes of carbon per hectare per year). But after such trees are harvested and the land is cleared for replanting — typically once a decade — the carbon is released again by the decomposition of plantation waste and products (mostly paper and woodchip boards).

And later:

…we find, on average, that natural forests are 6 times better than agroforestry and 40 times better than plantations at storing carbon.

On the other hand, from Project Drawdown:

Degraded lands present potential locations for tree plantations. Managed well, they can restore soil, sequester carbon, and produce wood resources in a more sustainable way.

Or Wikipedia:

Because tree farms are managed to enhance rapid growth, a tree farm tends to sequester carbon more quickly than an unmanaged forest, considering only the sequestration side of the equation and not the carbon release due to rot, fire, or harvest. The fact that managed woodlands tend to be younger and younger trees grow faster and die less contributes to this distinction.

The point being, there’s no obvious consensus regarding the carbon impact of tree plantations. I’m not sure why there are such a wide range of takes. It may simply be that the impact depends heavily on where the plantation is sited and how the wood products are used, there may be some bad analysis floating around (I certainly wouldn’t want to rely on Wikipedia for this sort of thing), or the science may still be unsettled. In any case, it’s clear that broad claims of the global potential for tree plantations need to be treated with caution.

(Barbara Haya notes that plantations can have indirect benefits, such as replacing the need to harvest old-growth forests, and even displacing carbon-intensive concrete and steel as building materials.)

Shocking Conclusion: Forests Are Complicated

The carbon impact from planting a tree can be positive, neutral, or even harmful. It depends enormously on the sort of environment it’s planted in, how the land will be managed, the previous condition of the land, and other factors.

(As I mentioned last time, “albedo effects” can further complicate the climate story: planting dark-leaved trees on light-colored land can absorb more heat, overwhelming any carbon benefit.)

As a result, we can’t think about climate impact in broad terms of “billions of trees planted”. The details are hugely important.

Next time, I’ll hopefully be ready to actually present what we’ve learned about the potential of forestation to help combat climate change. Now that we all understand some of the important factors that influence the climate impact of planting trees, we’ll be ready to understand how that impact can be estimated at a global level – and the limits of such estimates.

Thanks to Barbara Haya of the Berkeley Carbon Trading Project for feedback on a draft of this post. All errors are of course my own.

1. One paper defines litter as “the pool of organic C above the mineral soil (i.e., litter (Oi), fulvic (Oe), and humic layers (Oa)) including woody fragments with large-end diameters of up to 7.5 cm”.

2. In some cases, it seems that forestation can actually release more carbon than it absorbs. For instance, planting trees on a peat bog can go badly wrong.

3. In fact, wildfire suppression and other “modern” forest management practices have already pushed many existing forests into an unnaturally dense state, increasing the risk of massive fires. For instance, see https://www.usgs.gov/publications/land-management-explains-major-trends-forest-structure-and-composition-over-last.

4. Source: https://www.nature.com/articles/s41598-021-99395-6.