Monoculture carbon plantations threaten biodiversity and have little benefit for the climate

"We risk reducing natural ecosystems to one metric - carbon"

A new paper published in the journal Trends in Ecology & Evolution finds that monoculture tree plantations threaten tropical biodiversity.

The paper is titled “Valuing the functionality of tropical ecosystems beyond carbon” and it’s written by Jesús Aguirre-Gutiérrez, Nicola Stevens, and Erika Berenguer of the Environmental Change Institute at the University of Oxford.

Aguirre-Gutiérrez explained to The Guardian that the scientists decided to write the paper after seeing the increase in industrial tree plantations in the tropics:

“We carry out a lot of fieldwork in the tropics to research what is happening with climate change and we have seen the boom in these plantations for ourselves: teaks, conifer and eucalyptus, just one or two species. These schemes are a win for the company planting these trees but not for biodiversity. This is the start of this phenomenon, hence the seriousness of the situation.”

The authors write that,

“Land-based carbon sequestration projects, such as tree planting, are a prominent strategy to offset carbon emissions. However, we risk reducing natural ecosystems to one metric – carbon.”

Vast areas of tree plantations

Carbon tree planting schemes are expanding rapidly in the Global South, because of warmer temperatures that result in rapid tree growth, as well as the availability of cheap labour and cheap land. Rapid tree growth means rapid carbon capture.

But vast areas would have to be planted in order to have an impact on global greenhouse gas emissions.

The authors write that,

If all the land area within the tropics was covered by tree plantations, we would only sequester ~1.7 years of emissions.

The authors write that an area of about 35 million square kilometres would need to planted to sequester one year of emissions.

In a statement on the Environmental Change Institute website, lead author Aguirre-Gutiérrez, says,

“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.”

The authors note that the carbon market is “poorly regulated” and “thus there is little recourse when significant mal-mitigation actions happen”.

The authors argue that, “Current and new policy should not promote ecosystem degradation via tree plantations with a narrow view on carbon capture.”

Grasslands targeted for “afforestation”

One of the serious problems with establishing industrial tree plantations as a way of sequestering carbon occurs when grasslands are targeted as suitable areas to be planted. The authors write that,

Intact savanna ecosystems have an evolutionary history shaped by interactions between grasses, droughts, fires, and herbivores. Here, increasing woody cover represents structural homogenization and a reduction in the heterogeneity of ecosystem function and local adaptations. The disbenefits are stark for grassy ecosystems where increasing above-ground woody biomass generally represents a state of degradation. Afforestation of grassy ecosystems prioritises carbon sequestration services over other multiple ecosystem services they provide (water, grazing land, nature-based tourism, and biodiversity). In this way carbon-focused tree planting exacerbates biodiversity loss, particularly of species adapted to open environments.

Aguirre-Gutiérrez told The Daily Beast that “there is a widespread effort to afforest grasslands and savannas in Africa.” He added,

“However, these ecosystems already perform crucial functions. Afforesting them could diminish their natural capacity to sequester carbon on the ground due to the suppression of natural fire regimes.”

The African Forest Landscape Restoration Initiative, for example, proposes planting trees on a total area of 100 million hectares of land. A 2020 paper about this initiative comments that,

[T]rees-for-carbon projects can be seen as a distraction from the urgent business of reducing fossil fuel emissions. Planting 100 Mha of trees, far away in Africa, might reduce the urgency of emissions reductions in industrial countries that are the major sources of greenhouse gases.

Plantations and fire suppression in the Brazilian Cerrado

The authors of the Trends in Ecology & Evolution paper give the example of the Cerrado in Brazil which is threatened by cattle ranching, soy plantations, and oil palm plantations.

Industrial tree plantations are also a threat. Giselda Durigan, a forestry engineer from the ecology and hydrology lab at the São Paulo state Environmental Research Institute, told Mongabay that,

“In the Cerrado, the biggest impact of this land use [industrial tree plantations] is reducing the amount of water that infiltrates — up to 30% is retained in the forest canopy — and increasing extraction of groundwater by the trees. As a consequence, the water table is lowered, and river flows decrease.”

Fire suppression is also a threat to the biodiversity of the Cerrado. Durigan was a co-author of a 2017 paper that found that while fire suppression resulted in increased carbon stocks, it was also “associated with acute species loss”.

Savannas have evolved to survive fires and the ecosystem will restore itself in less than six months. When fire is suppressed tree cover increases, but the authors found that,

In sites fully encroached by forest, plant species richness declined by 27%, and ant richness declined by 35%. Richness of savanna specialists, the species most at risk of local extinction due to forest encroachment, declined by 67% for plants and 86% for ants. This loss highlights the important role of fire in maintaining biodiversity in tropical savannas, a role that is not reflected in current policies of fire suppression throughout the Brazilian Cerrado.

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