Carbon accumulation is a key but incomplete criterion for restoring tree cover

New research elucidates patterns of in-forest carbon accumulation in single-species plantation forests. However, additional considerations must guide decisions to restore tree cover.
Carbon accumulation is a key but incomplete criterion for restoring tree cover
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Actors in the public and private sectors are increasingly looking towards reforestation—that is, restoring tree cover where it has previously been lost—as a proven and low-cost method of carbon dioxide removal. While successfully growing a forest is guaranteed to sequester carbon, forests come in all shapes and sizes and these differences greatly impact the carbon and non-carbon benefits to society.

Given their ecological simplicity and predictable yields, the forestry sector has developed abundant information on planting single-species forests (monocultures) across the globe. It is perhaps no surprise then that about half of all commitments to mitigate climate change by expanding forest cover in the tropics entail planting monocultures. While private companies in the forestry sector hold large amounts of data on how these forests are likely to grow, scientists, policymakers, and practitioners in the public domain lack this knowledge.

Our study fills this gap. Specifically, we compiled 4,756 observations of aboveground carbon stocks in monoculture plantations and analyzed how carbon accumulates in the aboveground biomass of these systems. Our publicly available database and results show how various biological (e.g., plant traits), environmental (e.g., biome), and human (e.g., management practices) factors influence how much carbon accumulates. For example, we found that carbon accumulation differed four-fold, depending on the genus of tree planted. Our science can thus help practitioners, policymakers, and investors better understand the potential carbon dioxide removal outcomes of planting monoculture forests.

However, carbon sequestration is only one factor that should be considered when undertaking large scale restoration of tree cover. Beyond carbon, forests provide other benefits to society such as conservation of biodiversity, regulation of water, recreation opportunities, and provisioning of benefits to local communities. Other forest types—for example naturally regenerated forests or mixed-species plantations—commonly provide more of these benefits than monoculture plantations. In fact, mixed-species plantations also tend to sequester greater amounts of carbon on natural landscapes than single-species plantations, particularly when niche partitioning (reduced competition for resources due to complementary growth strategies) occurs. Mixed species forests also tend to be more resilient to climate change and natural hazards, such as wildfire or biotic pests.

As scientists working in the forests and climate realm, it is critical to get the carbon accounting right, particularly for prominent reforestation strategies. Our position on monocultures is one of nuance. Although monoculture plantations can have limited biodiversity value, they provide critical goods and benefits to society, most significantly timber and fiber-based products. These plantations can reduce pressures on semi-productive or protected forests by intensifying production to meet demand. They can restore native tree cover, albeit with limited diversity. Further, naturally regenerating forests may not be possible in many instances due to the absence of seed sources, or there may need to be investments to develop the silvicultural knowledge needed to plant mixed-species forests of native species.

In addition to the carbon accumulation rates assessed here, the overall climate outcome of plantations will depend heavily on two things: 1) ensuring that reforestation takes place on cleared or heavily degraded lands and does not drive the conversion of existing forests or peatlands, and 2) the fate of the carbon held in biomass. Our study does not directly address either of these points but provides the foundation upon which to explore these topics.

While the first factor depends on the siting of plantation establishment, the fate of carbon held in plantation biomass is an open question where additional research is needed. Long-lived wood products such as timber beams can lock atmospheric carbon away for half a century or longer, potentially creating “durable” sinks of carbon storage. While naturally regenerated forests or mixed-species plantations often hold greater carbon stocks on the landscape, accounting for carbon in long-lived wood products may allow the overall climate benefits of monoculture plantations to exceed those of unharvested forests. Conversely, carbon held in short-lived products will provide only temporary carbon dioxide removal.

Monoculture plantations are a dominant but widely debated strategy for expanding forest cover to mitigate climate change. Our research provides a fundamental building block to assess the on-the-landscape climate outcomes of these investments. However, monoculture plantations are only one of many approaches for restoring tree cover on the landscape. Ultimately, a balanced portfolio of forest types on the landscape is most likely to meet a diverse array of goals: including mitigating climate change, conserving biodiversity, and meeting the needs and wants of local communities.