Planting Trees is Not Enough

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Globally, forests absorb large amounts of carbon dioxide and turn it into wood (in living and dead trees), and soil organic matter. This carbon is safely sequestered away from the atmosphere, where it can’t trap heat and contribute to climate change. Almost a third of the CO2 emitted by humans has been absorbed by the world’s forests over the past few decades, roughly the same amount that was taken up by oceans – this is obviously a very big deal if we want to slow climate change. So how can we use this to our advantage to address climate change? One way is planting trees.

Planting trees can have a huge impact on atmospheric carbon with relatively low cost, at least in theory (researchers have suggested we could plant up to a trillion trees). But will it work the way it’s supposed to? Or is it a convenient carbon offset that actually enables ongoing emissions, by letting them be written off against tree planting? Unlike emissions reductions, tree planting has a delayed benefit that mostly manifests decades later. Young trees initially absorb carbon very slowly and only reach their full potential after a century or more. This is far from ideal since we need immediate carbon reductions to prevent a climate crisis, though tree planting will hopefully pay future dividends. There are also unknowns that need further study – in some cases the young trees may actually absorb more of the sun’s heat by reducing the reflectivity (or albedo) of the land; or volatile compounds released by trees may act as greenhouse gases.

Deforestation: the elephant in the room

I’m not saying we shouldn’t plant trees – we should – but we need to be far more focused on reducing deforestation. Globally we are still losing some 7 million hectares of forest every year – that’s mature forest that is actively sequestering carbon now, not at some theoretical time in the future. We can and should also focus on restoring forest that has been lost, but if we can’t reduce the rate of deforestation it is practically meaningless.

Amazon deforestation (NASA Earth Observatory)

The ongoing cutting and burning of Amazon rainforests is particularly frightening. The annual rate of deforestation in Brazil increased by almost a third in 2019, to nearly a million hectares, the largest loss in a decade. Some researchers believe that if deforestation continues for another 10 to 15 years, large parts of the amazon will reach a tipping point where they are incapable of returning to rainforest. The cycle of evapotranspiration from trees and frequent rains could be permanently disrupted, fires would become more common, and large areas would shift to grassland or savanna. Not only would this cause a torrent of extinctions, it would also release billions of tons of carbon dioxide into the atmosphere. We need more international cooperation, including meaningful incentives and penalties, to address deforestation.

Old-growth forests: the best carbon banks

Developed countries should not feel self-righteous. We may not be permanently clearing large swaths of forest, but logging of old-growth forests in North America is contributing significantly to climate change, even if the forest is allowed to grow back. This is because old-growth forests store vast amounts of carbon, much of which may be released back to the atmosphere when they are disturbed. Worse still, this isn’t properly factored into carbon accounting, so old-growth forests are logged without any consideration that large amounts of carbon will be released into the atmosphere, exacerbating climate change. It’s a significant accounting error, especially in a country like Canada where old-growth forests are still commonly logged.

Until the 2000’s most scientists thought that only young, rapidly-growing forests actively accumulate carbon – old-growth forests store a lot of carbon, sure, but it was believed they stop absorbing new CO2 from the atmosphere. This idea was based on theory and models originating in the 1960’s. However by the turn of the millennium data was showing that old forests continue to accumulate carbon for many centuries, well into the old-growth stage. A 2007 article in the Journal Nature reported that “the currently available data consistently indicate that carbon accumulation continues in forests that are centuries old. 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. .” (Luyssaert et al. 2008)

Old-growth forests are not only treasure troves of biodiversity, they store large amounts of carbon

The evidence has continued to accumulate since then (as has the carbon banked in the world’s old-growth forests) and the current scientific literature clearly shows that leaving old-growth forests unlogged is the best way to maximize carbon sequestration. In an analysis of carbon sequestration forestry in the boreal region, Pukkala (2018) concluded that low rates of cutting or no cutting were the optimal strategies to sequester carbon, and that it was not optimal to commence cutting in older forests, even after the carbon biomass stopped increasing.

Intuitively you may think that a young, vigorously growing forest would sop up more carbon than an old, stable forest. There are a few reasons this is not the case, though. For one thing the massive three-dimensional architecture of a forest has vastly more photosynthetic surface area than a bunch of saplings, no matter how densely they are bunched together. One researcher noted that “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.” (Stephenson et al. 2014)

Old-growth pine forest near Rushbrook Lake, Ontario

Forests that appear stable in composition are constantly changing, with trees growing, dying and being replaced. Dead trees take centuries to decompose and some of the carbon is stored much longer than that in the soil. Logging has a double impact: it slows carbon accumulation by resetting the forest to a very low level of productivity compared to a mature forest, and at the same time it accelerates decomposition of the large amounts of banked carbon, releasing a lot of CO2 into the atmosphere at a time when it is critical that we take action to reduce emissions.

The scientific consensus indicates that the best way to mitigate climate change is to leave old-growth forests undisturbed. This is, as Al Gore would say, an inconvenient truth for the Canadian forest industry. This doesn’t only impact British Columbia; other provinces have significant tracts of old-growth boreal forest, as well as forests further south, especially in Ontario and Quebec.

We need to stop cutting old-growth forests in Canada, and work to fully include it in carbon accounting. British Columbia should be getting carbon credits for preserving old-growth temperate rainforest, Ontario should get credit when it stops logging old-growth forest in Algonquin Park, and many provinces should preserve old growth in the boreal forest, etc. If we’re going to allow carbon offsets, the first priority should be preserving the massive carbon banks in existing forests, and particularly in old-growth forest. Planting trees is necessary, but it is definitely not enough – it’s not even the right starting point.

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