Tree Planting and Carbon Sequestration: oversimplifying the problem?

By Clemence Blanchard

Now more than ever – with COP26 fresh in our minds – we are aware of the climate change threat. Whole ecosystems and biodiversity are especially at risk, partially exacerbating the issue since certain organisms like plants act as carbon sinks. As a result, many organisations and governmental initiatives devote their energy towards tree planting as a way to mitigate climate change impacts. The idea is simple enough: plants like trees can capture atmospheric CO2, utilising and storing it during photosynthesis in plant biomass, and ultimately sequestering carbon. These manifest themselves as afforestation and reforestation projects, where the former sees trees planted in areas where there is not usually forest cover (e.g. grasslands or deserts), and the latter simply aims to plant as many trees where others have been removed by logging, for example. However, these oversimplify the problem and can even go on to have negative consequences on the environment.

Issues stem from the fact that while tree planting can in theory help tackle climate change issues1 and the consequences they threaten, like extinction, the reality is much more complex. Projects are often well-meaning but treat tree planting as a ‘fix-all’ situation, forgetting that there are numerous factors to take into consideration for projects to be successful and increase the amounts of carbon stored in plant biomass.2 For example, selecting where trees are planted and the types of species – especially whether native or not – is critical.3 In 2019 scientists responded4 to a study1 on the potential of forest cover (through tree planting) to combat climate change. They highlighted that suggestions of tree planting in historic savannas and grasslands – communities with specific and intentional species diversity – would threaten these ecosystems with lower-diversity forests. Additionally, trees planted in savannas and grasslands will be prone to fires, adding a layer of complexity to the issue. Interestingly, attempts to plant trees in such ecosystems are closely linked to fire suppression schemes. Some researchers argue that in the pursuit of carbon sequestration, costs to biodiversity are forgotten (as fire plays a role in maintaining biodiversity), and there are no resulting gains for conservation.5

Projects also need to be well planned and maintained, with clear goals and action points. All too often, tree planting efforts are supplanted by the re-use of land for crops and livestock. In the long term, this does little for carbon sequestration. A review of mangrove planting efforts (mangroves have great ecological importance) in Sri Lanka following the 2004 tsunami found that over 50% of the planting attempts failed.6 This was in part due to poor maintenance and monitoring schemes (cattle trampling was frequent) but also due to inadequate choice of planting sites. Topography, salt concentrations and inundation frequency are all vital factors in the growth of mangrove saplings that had not been adequately considered. Significant amounts of money (∼$13 million) are spent on projects that do not consider tree planting as a complex initiative.

Specifically, a lack of understanding of how carbon capture fits within general carbon fluxes undermines large-scale tree planting efforts. Total ecosystem carbon storage has to be taken into account, including carbon stored in the soil. A recent study7 found that planting native trees onto heather moorland did not increase carbon storage after either 12 or 39 years. Indeed, this led to decreased soil carbon storage, which cancelled out any sequestration effects. In the planting sites there was increased soil respiration (despite there being the same soil respiration potential in the control sites), hypothesised to be caused by changes in mycorrhizal composition (plant-fungus associations). Clearly, tree planting efforts need to consider levels of carbon stored in the soil to have any net increase in carbon storage. Heather moorlands are also found on top of peat, which is a known good carbon store, so a point is made that certain regions are best left alone to mitigate climate change impacts.

Despite the clear advantage of natural peatlands as carbon sinks, they are threatened by land-use change, draining and fires caused by climate change, which reverses their roles to carbon sources.8 Scientists thus argue9 that the focus should remain on protecting and restoring existing natural forests, which make for better carbon stores than plantation forests (in part due to established carbon flows). Action should be preventative, not reactive, and certainly must not impact current natural carbon sinks. Although there are ways of creating optimised and ‘climate-smart’ forests, there are still doubts as to just how much carbon forests can capture, and the myopic view that tree planting initiatives will help solve larger issues like where emissions are coming from in the first place needs to be addressed.

References:

1. Bastin, J-F., Finegold, Y., Garcia, C., Mollicone, D., Rezende, M., Routh, D., Zohner, C.M. & Crowther, T.W. The global tree restoration potential. Science. 2019; 365(6448), 76-79 Available from: doi: 10.1126/science.aax0848.

2. Holl, K.D. & Brancalion, P.H.S. Tree Planting is not a simple solution. Science. 2020; 368(6491), 580-581. Available from: doi: 10.1126/science.aba8232.

3. Arasumani, M., Khan, D., Das, A., Lockwood, I., Stewart, R., Kiran, R.A., Muthukumar, M., Bunyan, M. & Robin, V.V. Not seeing the grass for the trees: Timber plantations and agriculture shrink tropical montane grassland by two-thirds over four decades in the Palani Hills, a Western Ghats Sky Island. Plos

One; 2018; 13(1), e0190003. Available from: doi: 10.1371/journal.pone.0190003.

4. Veldman, J.W., Aleman, J.C., Alvarado, S.T., Anderson, T.M., Archibald, S., Bond, W.J., Boutton, T.W., Buchmann, N., Buisson, E. et al. Comment on “The global tree restoration potential”. Science. 2019; 366(6463). Available from: doi: 10.1126/science.aay7976.

5. Abreu, R.C.R., Hoffmann, W.A., Vasconcelos, H.L., Pilon, N.A., Rossatto, D.R. & Durigan, G. The biodiversity cost of carbon sequestration in tropical savanna. Science Advances. 2017; 3(8), e1701284. Available from: doi: 10.1126/sciadv.1701284.

6. Kodikara, K.A.S., Mukherjee, N., Jayatissa, L.P., Dahdouh-Guebas, F. & Koedam, N. Have mangrove restoration projects worked? An in-depth study in Sri Lanka. Restoration Ecology. 2017; 25(5), 705-716. Available from: doi: 10.1111/rec.12492.

7. Friggens, N.L., Hester, A.J., Mitchell, R.J., Parker, T.C., Subke, J-A. & Wookey, P.A. Tree planting in organic soils does not result in net carbon sequestration on decadal timescales. Global Change Biology. 2020; 26(9), 5178-5188. Available from: doi: 10/1111/gcb.15229.

8. Leifeld, J., Wust-Galley, C. & Page, S. Intact and managed peatland soils as a source and sink of GHGs from 1850 to 2100. Nature Climate Change. 2019; 9, 945-947. Available from: doi: 10.1038/s41558-019-0615-5.

9. Waring, B., Neumann, M., Prentice, I.C., Adams, M., Smith, P. & Siegert, M. Forests and Decarbonization – Roles of Natural and Planted Forests. Frontiers in Forests and Global Change. 2020; 3(58). Available from: doi: 10.3389/ffgc.2020.00058.

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