rees to promote net zero?
The escalating threat of climate change has spurred global commitments to achieve net zero emissions by the middle of this century [1.]. To reach a balance between reducing emission sources and enhancing greenhouse gas sinks, land-based carbon sequestration is viewed as an important strategy to offset emissions, most prominently via nature-based solutions [2.]. This, together with the commodification of carbon and the substantial growth of the voluntary carbon market, has resulted in a boom in the number of commercial tree plantation projects across tropical ecosystems with significant financial flows from private and public sectors towards carbon offsetting projects [3.]. In practice, these offsetting projects most commonly take the form of increasing aboveground carbon, via woody biomass.

Typically, the tropics, dominated by forests and grassy ecosystems, such as grasslands and savannas, are regarded as some of the most suitable regions in which to maximise carbon sequestration (see Table S1 in the supplemental information online) due to favourable climatic and topographic conditions which promote rapid plant growth [4.]. Tropical ecosystems are also highly biodiverse and offer billions of dollars in ecosystem services provision, such as acting as carbon sinks of ~1.26PgC per year [5.], and helping sustain a large proportion of the most economically deprived parts of global society. In order to avert the climate crisis, tree planting commitments are ambitious but require a huge area of land. Assuming a single-species plantation productivity of 1.32 MgC/ha (www.bonnchallenge.org), an area of ~35 million km2 (equivalent to the total summed area of the USA, the UK, China, and Russia) would have to be forested to sequester one year of emissions. If all the land area within the tropics was covered by tree plantations, we would only sequester ~1.7 years of emissions... [6.] (Figure 1).

...Trade-offs between biodiversity and carbon capture occur across tropical ecosystems. Yet carbon capture only represents a small component of the pivotal ecological functions that tropical forests and grassy ecosystems perform such as exchanging carbon, nutrients, water, and energy [12.]. The functioning of ecosystems is mediated by the traits of the species that inhabit them. These functional traits allow species to respond to environmental changes [13.], to feedback on their environment, and to carry out ecosystem functions such as carbon fixation. Ecosystems that are compositionally and functionally diverse and heterogeneous have a higher resilience to environmental change impacts and can promote carbon sequestration [14.]. Yet, in our quest for carbon sequestration, we are not achieving this. Instead, the bulk of tree planting commitments most commonly involve the establishment of agroforestry and tree plantations (mixed species and monocultures) where five tree species, T. grandis (commonly known as teak), Swietenia macrophylla (mahogany), Cedrela odorata (cedar), Grevillea robusta (silk oak), and Acacia mangium (black wattle), dominate the majority of projects with a focus on timber, pulp, and/or agroforestry [14.]. By emphasising a particular ecosystem function at such large scales, we are effectively homogenising the functional trait composition of tropical ecosystems, for example, by selecting for species with fast carbon storage potential, we select for traits that confer rapid tree growth...

... 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. Before these projects are initiated, an in-depth understanding on the impact of carbon-focused tree planting on biodiversity, function, and people’s livelihoods is needed but clearly missing from a policy-making perspective. Despite claims that this is the case, accounting for negative impacts of carbon focused plantations, has not been widely operationalised as carbon is easy to measure at scale and is a commodity that is driving large economic benefits. In the case of savannas, this problem is exacerbated due to a persistent history of misclassification of ecosystems. The carbon market is also poorly regulated (e.g., tinyurl.com/GCJAGs), and thus there is little recourse when significant mal-mitigation actions happen. Ideally, we should be moving to a state where for any carbon project to be certified their additionality should surpass the benefits of conserving and restoring the ecosystem to their original state, so that not only carbon capture is maximised but also that no other diversity facet and ecosystem function and service is negatively impacted. Hence, 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. Current and new policy should not promote ecosystem degradation via tree plantations with a narrow view on carbon capture.

Overall, we argue that, aside from the obvious need to reduce fossil-fuel emissions, we should shift focus to conserving and restoring ecosystems...