By Rachel Chan
Tree planting (ironically) is hot right now. In a bid to tackle climate change, countries and organisations worldwide have pledged to plant more trees. The UK government has pledged to plant a million trees between 2020 and 2024, and Ethiopia claimed to have planted 350 million trees in a day in 2019 (BBC News, 2019). Trees have a lot of potential for carbon sequestration and supporting biodiversity. Trees can be introduced to previously unforested areas, termed afforestation, or they can be replanted where they once grew, which is reforestation. This seems like an obvious course of action when deforestation is rampant and greenhouse gas emissions are increasing. However, many factors affect how effective afforestation is – it can do more harm than good in some cases.
The case for tree planting to mitigate climate change is seemingly clear: forest ecosystems are the largest terrestrial carbon sink on the planet (Domke et al., 2020). The management of these areas has been acknowledged as a cost-effective way to mitigate greenhouse gas emissions. Furthermore, deforestation accounts for 6-17% of carbon emissions by releasing stored carbon (van der Werf et al., 2009). It feels only natural to reverse this by doing the exact opposite.
To what extent tree planting could contribute to mitigation varies greatly. A bold estimate for the potential of reforestation and afforestation has recently been published, claiming that the earth could support an extra 0.9 billion hectares of forest. Once mature, these forests could supposedly store 200 gigatonnes of carbon, 25% of our current atmospheric carbon pool (Bastin et al., 2019). However, this study has been slammed with criticism for the unrealistic assumptions it makes on afforestation. More realistic estimates show that restoring the same area of forest would only store around 92 gigatonnes of carbon. In the context of our current emissions, sequestering around 100 gigatonnes of carbon is only equivalent to 10 years of emissions (Lewis et al., 2019). While these estimates show that a large amount of carbon can be sequestered by tree planting, they also show that forest restoration is of lower importance than reducing fossil fuel emissions.
When it comes to afforestation, a large factor that should be considered is where these trees are being planted and how trees affect the earth in biophysical terms. In snow-covered high latitudes, trees make the overall land darker, increasing the surface albedo. The heat trapping effect could cause net warming here, offsetting carbon sequestration (Betts, 2000). The best place for afforestation would be in the tropics, where trees grow fastest and therefore store more carbon. This would have a cooling effect, while planting trees in a temperate climate, like in Europe, may have no net effect (Bala et al., 2007). With that in mind, it is very easy to misdirect efforts towards afforestation if trees are being planted in the wrong regions, especially if there are no obvious gains to be seen.
There are also some regions that could provide an opportunity for carbon sequestration, but with detrimental consequences on biodiversity. An example of these are grasslands and savannas, which should not be seen as potential land for afforestation. The formation of these habitats is a result of complex interactions between herbaceous plants, environmental change, fires, and large herbivores over millions of years. On the contrary, these regions are under threat because of fire exclusion and afforestation, replacing these regions with lower-diversity forests (Abreu et al., 2017). Having dense tree cover is completely at odds with maintaining grassy biome biodiversity, as it limits the productivity of herbaceous plants that require lots of sunlight. Areas like these should be excluded when trying to mitigate climate change through afforestation, as it would only spell ecological disaster. Furthermore, trees in grasslands may release more carbon than they store because of how prone these regions are to fire (Veldman et al., 2015).
Another important consideration is the effects of rapid climate change. At our rate of warming, the factors that affect tree planting could change in mere years. While planting trees in snowy regions seems to be a problem, it may not be for long. Polar regions are warming much faster than the rest of the planet, meaning there may not be much snow left there anyway (Betts, 2000). When we take climate change into account, planting trees there may not increase surface albedo by very much after all. Climate change could also alter the amount of land suitable for afforestation, with seedlings not being able to survive. Future land use change, such as the expansion of cattle raising, could exacerbate this problem (Bastin et al., 2019). Elevated carbon dioxide concentrations in the future could perhaps enhance the growth of trees. With that being said, increased growth rate might not actually increase carbon storage (Büntgen et al., 2019). When thinking about ecological, hydrological, and biogeochemical feedback associated with forest cover, a lot of questions on planting trees arise.
At the end of the day, climate change is an incredibly complex problem that requires multiple approaches. Alongside emission reductions and systemic change, tree planting is a powerful tool in mitigation, but only when carried out correctly. Afforestation in grasslands, tundra and boreal forests should not be considered climate mitigation tools due to consequences on albedo and biodiversity. It is very dangerous to hail tree planting as a stand-alone solution to climate change. Planting a million trees is not a license for industries to carry on polluting and emitting, and there is no quick and easy fix for climate change.
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