I was talking to some friends and one of them claims that it is more efficient to combat global warming by cutting the trees and replanting them because a growing forest consumes more CO2. (Firstly, a newly planted forest will require human effort planting the trees equipment that uses petrol and makes more CO2, also cutting equipment and such, but let's ignore that.)

I was looking for some data online that would show that a growing forest of the same kind of trees (let's say pines) will consume more CO2 than a fully grown one, but couldn't get any properly scientific data, just some anecdotal stories from biased parties.

Which consumes more carbon dioxide: a fully grown forest or a newly planted one that is growing (ignoring the planting CO2 costs)?

  • I am sending this notice to anyone who's expressed interest in this question, either by posting an answer or, by commenting. the bounty expires in approx. 23 hours. – Duckisaduckisaduck Dec 2 at 21:40
  • @Erik I am sending this notice to anyone who's expressed interest in this question, either by posting an answer or, by commenting. the bounty expires in approx. 23 hours. – Duckisaduckisaduck Dec 2 at 21:40
  • A recent article in Scientific American discusses the potential of forests (new and old) to fight climate change: The Best Technology for Fighting Climate Change Isn't a Technology – LShaver Dec 5 at 20:55
up vote 6 down vote accepted
+50

A forest of old trees sequesters more carbon per year than a forest with the same quantity of young trees.

When I first saw this question I thought I knew the answer -- trees grow faster when they're young, therefore they sequester carbon at a higher rate. When I went looking for the data to back this up, I found that this is still a somewhat controversial topic. However, based on some recent and comprehensive research, it seems that older trees actually sequester more carbon than young trees, up until their death.

In a 2014 study titled "Rate of tree carbon accumulation increases continuously with tree size" (pdf here), a group of 38 researchers from public and private institutions around the world

conducted a global analysis in which [they] directly estimated mass growth rates from repeated measurements of 673,046 trees belonging to 403 tropical, subtropical and temperate tree species, spanning every forested continent.

They found that

In absolute terms, trees 100 cm in trunk diameter typically add from 10 kg to 200 kg of aboveground dry mass each year (depending on species), averaging 103 kg per year. This is nearly three times the rate for trees of the same species at 50 cm in diameter, and is the mass equivalent to adding an entirely new tree of 10–20 cm in diameter to the forest each year.

Essentially, even though old trees are adding mass at a slower rate, kg per kg, than young trees, they are adding more net mass per year because they are already so much larger.

These lines from the abstract sum up the findings nicely:

Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree.

What about "mature" trees?

In our experience, species we're familiar with such as dogs, cats, and humans reach a maximum size when they've still got well over half of their lives left (barring dietary issues). Thus we might think the same is true of trees -- after a certain number of years, they "mature," and then no longer increase in mass -- meaning they stop sequestering CO2.

But in primary school we all learned that trees add a new ring each year -- this is how we can tell how old they are. So even when a tree stops getting taller, it keeps adding rings, meaning it's increasing mass throughout the trunk and branches.

In a 2008 study, "Old-growth forests as global carbon sinks" (pdf here), the authors look at the basis for the idea that forests "mature" and become carbon neutral (emphasis added):

The commonly accepted and long-standing view that old-growth forests are carbon neutral [...] was originally based on ten years’ worth of data from a single site. It is supported by the observed decline of stand-level NPP [net primary productivity] with age in plantations, but is not apparent in some ecoregions. Yet a decline in NPP is commonly assumed in ecosystem models. Moreover, it has led to the view that old-growth forests are redundant in the global carbon cycle.

The authors then go on to explain the findings of their study, which included analysis of data from over 500 sites in boreal and temperate forests:

If, however, the hypothesis of carbon neutrality were correct, the expected probabilities of observing a sink or source would be equal and around one-half, the average sink strength for a random ensemble of forests 200 years old and above would be zero, and the mean CO2 release from heterotrophic respiration would equal the mean CO2 sequestration through NPP [...]. However, we observe this ratio to be well below one on average and not to increase with age. Hence, all three quantitative tests fail to support the hypothesis of carbon neutrality. The currently available data consistently indicate that carbon accumulation continues in forests that are centuries old.

They go on to hypothesize a mechanism supporting their observations:

If old-growth forests reach high above-ground biomass and lose individuals owing to competition or small-scale disturbances, there is generally new recruitment or an abundant second canopy layer waiting in the shade of the upper canopy to take over and maintain productivity.

(The "if" here is important because the research excludes the impact of large-scale disturbances such as fire, insect outbreak, avalanches, etc, as these are not a function of the forest itself.)

Although tree mortality is a relatively rapid event (instantaneous to several years long), decomposition of tree stems can take decades. Therefore, the CO2 release from the decomposition of dead wood adds to the atmospheric carbon pool over decades, whereas natural regeneration or in-growth occurs on a much shorter timescale. Thus, old-growth forest stands with tree losses do not necessarily become carbon sources, as has been observed in even-aged plantations (that is, where trees are all of the same age).

While this study did not look at tropical forests, other studies provide smaller-scale evidence that the same analysis does hold in the tropics.

Young forests may actually be carbon sources

In the same 2008 study, the authors present some evidence that young forests may be carbon sources:

In fact, young forests rather than old-growth forests are very often conspicuous sources of CO2 because the creation of new forests (whether naturally or by humans) frequently follows disturbance to soil and the previous vegetation, resulting in a decomposition rate of coarse woody debris, litter and soil organic matter (measured as heterotrophic respiration) that exceeds the NPP of the regrowth.

  • I think you've conflated "large" with "mature". – Tim Nov 27 at 7:00
  • Young trees also are supposed to use more O² than they produce. Can't find the source right now. – Erik Nov 27 at 14:15
  • @Tim I included some additional research discussing the idea of trees "maturing." – LShaver Nov 28 at 15:43
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    @LShaver Look, if forests were carbon-negative then in the 360 million years that they have existed they would have depleted the planet's atmosphere of carbon and humans wouldn't have evolved. That didn't happen, so they can't be carbon-negative. Since trees themselves are primarily made of carbon, being carbon-positive would have made forests self-terminating. That didn't happen either. The only rational explanation for forests and humans still/currently existing is that forests are — over the long term — carbon-neutral. That's how living systems work. All of them. – Tim Nov 29 at 19:37
  • @Tim forests are in competition with other ecosystems such as grasslands and deserts. The research also excludes large-scale events such as strong storms, fires, landslides, fungal/insect attacks, etc. – LShaver Nov 30 at 12:46

As LShaver writes, larger trees sequester more carbon than smaller trees, but only if they are still growing. In a "fully-grown" forest (as per the title) that process has ended — decay and regrowth are in equilibrium. Fully-grown forests are carbon-neutral. All of them.

The Amazon rainforest — all 5,500,000km² of it — is carbon neutral. If you plant a single apple tree in your back yard, you are sequestering more carbon each year (on a net basis) than the entire Amazon rainforest.

Thus any newly-planted forest will extract more CO₂ from the atmosphere than a "fully-grown" forest. Provided that by "fully-grown" you actually mean fully grown (the age of the forest is ≥ the lifespan of the species).

Carbon sequestration rates start off low when the trees are small, increase each year as the trees grow larger, and then reduce as they start dying off due to old age. If the lifespan of a particular tree species is, say, 50 years, then the rate of carbon sequestration for a newly-planted forest will typically peak in its 30s or early 40s before dropping to zero from age 50 onwards.

Tree planting — as a carbon sequestration strategy — thus depends almost entirely on the growth profile of the species. If you want to remove large amounts of CO₂ quickly, plant fast-growing species. If you want to remove the largest possible amount for the longest possible time, plant long-lived species that are capable of growing to great heights. Regardless of which approach you take, once the forest has matured it ceases to act as a carbon sink.

As far as carbon is concerned, a forest acts like a big sponge — hard to wet at first, then it gets really 'thirsty', but once it's saturated any additional water just leaks back out.

Once a tree dies (or is cut) then the decay process begins, and the carbon starts making its way back into the atmosphere/environment. Cutting/harvesting a forest is like squeezing a sponge. Burn it and the carbon is released quickly. Turn it into building materials for houses that last 40 years, and you delay its return to circulation by 40 years. Convert it to biochar and you can keep about half of it out of the atmosphere for centuries, if not millennia.

Cutting down forests is never good for short-term atmospheric carbon levels. Whether it is good for medium- and long-term atmospheric carbon levels depends on what you do with the timber (and roots), and whether the new species that is going in can ultimately sequester more kg/m² of carbon than the species it is replacing. Cutting and replanting with the same (or similar) species makes no sense whatsoever from a carbon-sequestration point of view.

tl;dr:

  • Clearing existing forests == carbon-positive
  • Replacing forests == carbon-neutral and pointless if a similar species is used
  • Planting new forests == carbon-negative
  • Fast-growing species == more short-term impact
  • Long-lived and tall species == more long-term impact
  • Immature forests == carbon-negative
  • Mature forests == carbon-neutral
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    There's no point confusing the issue by trying to factor in human activity (e.g. conversion of forest land to agriculture), because every single forest is different. For every example of a forest that's being cleared it is easy to find a forest that's not, or is being protected. The question doesn't ask about a specific forest at a specific point in history — it is asking about forests in general. It's best to understand forests in general and then, if you really, really want to try and account for specific human activity, do it separately. – Tim Nov 27 at 22:57
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    It seems your interpretation of a "fully grown" forest is one where the trees are past their maximum lifespan, meaning they are dead and decomposing. I think there's a bit more wiggle room between a tree being "fully grown" and dying. Trees are either growing or dead, so in the strictest sense, yes, a tree is only "fully grown" once it dies. But I think that definition might be a bit harsh here - it's not really fair to compare the carbon sequestering abilities of a sapling to a piece of dead wood. – Nuclear Wang Nov 28 at 17:22
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    @Nuclear Wang. That's not my interpretation at all. I didn't say that the age of all the trees is ≥ the lifespan of the species, I said the age of the forest is ≥ the lifespan of the species. A forest isn't mature until it has supported at least a generation of trees. Some may (with good reason) argue it is even longer, but it certainly isn't any less. Heavily diminishing returns on carbon sequestration are already evident by the time the forest's age reaches the lifespan of the species. – Tim Nov 29 at 16:25
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    Forests dedicated to sequestration should never be cut. Thus natural (new) growth will constantly occur. Any sustainable forest ends up being a well-balanced mix of ages. All the trees aren't one age. That would be called a plantation. We're not talking about plantations — we're talking about forests. – Tim Nov 29 at 16:34
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    @Tim Good point, sounds like the germ of another potential question regarding efficient management of plantations for carbon fixing. – Duckisaduckisaduck Dec 1 at 17:58

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