The good intentions of tree plantations to offset against carbon emissions may be counterproductive according to researchers from the Environmental Change Institute. Writing in a peer-reviewed opinion paper in the journal Trends in Ecology and Evolution they argue that these well-intended plantations might come with costs for biodiversity and other ecosystem functions. Instead, the researchers say we should prioritise conserving and restoring intact ecosystems.

Photo by Jesús Aguirre-Gutiérrez

Lead author, Dr Jesús Aguirre-Gutiérrez, a Senior Researcher and NERC Fellow at the Environmental Change Institute, (ECI), said: “Despite the broad range of ecosystem functions and services provided by tropical ecosystems, society has reduced value of these ecosystems to just one metric – carbon.”

Current and new policy should not promote ecosystem degradation via tree plantations with a narrow view on carbon capture.”

Tropical ecosystems, which include forests, grasslands, and savannahs, are attractive sites for tree plantations because their climate and physical features promote rapid tree growth (and rapid tree growth means rapid carbon capture). Although some tree plantations involve reforestation of degraded land, in many cases they involve afforestation – planting forests in undegraded and previously unforested regions such as grasslands.

It’s often assumed that tree planting for carbon capture also benefits biodiversity and enhances socioeconomic benefits, but the authors argue that this is usually not the case. Tropical ecosystems are highly biodiverse, and they provide multiple ecosystem services, such as maintaining water quality, soil health, and pollination. In comparison, carbon-capture plantations are usually monocultures and are dominated globally by just five tree species – teak, mahogany, cedar, silk oak, and black wattle – that are grown for timber, pulp, or agroforestry.

Photo by Jesús Aguirre-Gutiérrez

The authors say that there are considerable financial incentives for private companies to offset their carbon emissions by investing in carbon capture and that the boom in carbon-capture plantations is being driven by money, not ecology. Compared to parameters such as biodiversity and ecosystem services, carbon is easy to measure and monetise. But over emphasising the benefits of tree planting for carbon capture “can disincentive the protection of intact ecosystems and can lead to negative trade-offs between carbon, biodiversity, and ecosystem function,” according to the researchers.

Instead of focusing on commercial tree planting, the authors say we should prioritise conserving intact ecosystems, “An overarching view on maintaining original ecosystem functioning and maximising as many ecosystem services as possible should be prioritised above the ongoing economic focus on carbon capture projects.”

This research was supported by the Natural Environment Research Council, the John Fell fund from the University of Oxford, and the Trapnell Fund.

Although these plantations might be economically valuable, they usually support a lower level of biodiversity. For example, in the Brazilian Cerrado savannah, a 40% increase in woody cover reduced the diversity of plants and ants by approximately 30%. These plantations can also directly degrade ecosystems by reducing stream flow, depleting groundwater, and acidifying soils.

Dr Jesús Aguirre-Gutiérrez, with Dr Erika Berenguer, Senior Research Associate, ECI and Dr Nicola Stevens, Research Fellow, School of Geography and the Environment, Oxford University, believe that even ambitious commitments to carbon-capture plantations will be limited in their ability to capture carbon.

Dr Jesús Aguirre-Gutiérrez said:

“The current trend of carbon-focused tree planting is taking us along the path of large-scale biotic and functional homogenisation for little carbon gain. An area equivalent to the total summed area of USA, UK, China, and Russia would have to be forested to sequester one year of emissions.”

And tropical grasslands and savannahs are already carbon sinks. When intact, tropical grasslands and savannahs store large quantities of carbon below ground. In contrast to carbon-capture tree plantations, which predominantly store carbon above ground, these below-ground carbon sinks – which would be lost if afforested – are less susceptible to disturbances such as drought and fire.

Photo by Jesús Aguirre-Gutiérrez
Photo by Jesús Aguirre-Gutiérrez