Naturally Regrowing Forests Offer Untapped Climate Benefits

Protecting and restoring forests are essential for curbing climate change. But while efforts often focus on conserving mature forests and planting new trees (both of which are badly needed), a critical piece of the puzzle is often overlooked: managing naturally regrowing forests to increase the carbon they remove.

Until now, scientists did not have a detailed picture of the carbon removal value of naturally regrowing forests. But new research by The Nature Conservancy, WRI and partners shows that naturally regenerating “secondary forests” (which have regrown after being cleared by harvests, severe fires, agriculture or other disturbances) could be especially powerful for fighting climate change. It is the first to show where, and at what ages, they can have the biggest impact.

We found that secondary forests between 20 and 40 years old can remove carbon from the atmosphere up to 8 times faster per hectare than new natural growth — if they’re allowed to reach those older ages. The catch is that many secondary forests don’t regrow for this long, whether due to human activity (such as clearing or harvest) or climate-related disturbances (like fires or pests).

These findings highlight that countries may be underestimating the value of naturally regenerating secondary forests in their climate reporting — and that protecting them, or encouraging their regrowth for longer periods, offers untapped opportunities for climate action.

How Quickly Can Secondary Forests Remove Carbon?

The rate at which natural forests remove carbon from the atmosphere varies with location and age. Within a forest’s first 100 years of regrowth after being cleared or destroyed — the age range covered in this research — carbon removal rates generally start relatively slow, then accelerate, before slowing again. That means it may take many years before newly established forests provide their largest climate benefits.

Our new research provides the first global maps of how these carbon removal rates vary across space and time as secondary forests regenerate naturally. The maps cover any square kilometer on Earth where forests could grow. Previous estimates have not captured as much geographic or age variation or did not provide global coverage.

We found that naturally regenerating forests typically remove carbon fastest when they’re between 20 and 40 years old. This means that older secondary forests can provide more immediate and often greater carbon removals than younger regrowing forests.

However, the age at which forests reach their peak carbon removal rates varies across the globe. Tropical and subtropical rainforests (such as the Amazon and the Congo Basin) and some temperate forests (such as in in the United States) capture carbon fastest at younger ages. Meanwhile, boreal forests (like in Canada and Russia), Mediterranean forests, and forested areas in tropical and subtropical savanna regions (such as the Brazilian Cerrado), reach their maximum — and generally lower — carbon removal rates at older ages.

Maximum carbon removal rates also vary vastly by region. On average, established secondary forests at their peak removal age absorb 10% more carbon than newly growing forests. But in some areas, the difference is as large as 820%.

Carbon removal rates change most dramatically with age in tropical and subtropical wet forests, while changes were least pronounced in Mediterranean forests and woodlands. However, there is variation within each of these ecoregions, highlighting the value of knowing how carbon removal changes through time for every square kilometer of potential forest.

Many Secondary Forests Never Reach Peak Carbon Removal Age

Knowing when forests remove the most carbon matters. The world urgently needs to scale up climate action over the next 25 years (2025-2050) to achieve net-zero emissions deadlines and protect the planet from the worst effects of climate change. This new data shows that older secondary forests are some of the most effective at removing carbon within this critical window. And it pinpoints when and where regrowing forests pack the biggest climate punch.

Yet despite their importance, naturally regrowing secondary forests are frequently ignored in climate policy — and they are under threat. Across the tropics, forests regenerate for an average of 7.5 years before being cut down, with only 6% reaching 20 years of regrowth. In the Brazilian Amazon, half of secondary forests are cleared within eight years. In Costa Rica, where the clearance cycle is one of the longest in the tropics, the average age for regenerating forest is still only 20 years.

This means that many secondary forests never reach their peak carbon removal years, undercutting their climate benefits as well as the benefits they bring to people and nature. In addition to faster carbon removals, allowing forests to regrow naturally — as opposed to more active planting — can provide benefits like restoring biodiversity and protecting waterbodies at much lower costs.

What Does This Mean for Natural Climate Solutions?

Restoring and protecting forests are proven, cost-effective and scalable ways to help tackle climate change. This new information provides valuable insights into how to prioritize forest management efforts to maximize their impact:

• Secondary forests should be protected or kept growing for longer periods. 

   

  Currently, secondary forests are often not prioritized for protection. But it’s now possible to quantify the carbon removals that are foregone by cutting these forests down at a young age. While keeping intact and mature forests standing remains crucial, this research shows that protecting secondary forests — or in the case of production forests, delaying clear-cutting until after the peak age for carbon removal — warrants additional attention, as they can often provide the greatest per-hectare carbon removals. Additionally, some forests do not survive to their peak carbon removal age due to windthrow, pests, fire or drought (which are natural disturbances but are exacerbated by climate change). To effectively protect secondary forests, they must be managed for resilience to these risks.

   

• Large-scale natural forest regeneration must start now. 

   

  That way, new forests can reach the age at which their carbon removal is greatest within the window of time that carbon removal is most needed to reach climate goals. Our research shows that delaying natural forest regeneration by five or 10 years decreases potential carbon removals globally by a quarter or half through mid-century, respectively, compared to starting natural regeneration in all reforestable areas now.

   

• Policymakers and land managers can begin to make more strategic forest management decisions. 

   

  These maps can inform where and when the returns will be greatest from regrowing forests and/or letting forests continue to grow as a natural climate solution. They can be combined with forest restoration opportunity maps to estimate carbon removal in new forests over specified time horizons. They can also be integrated with maps of the age of naturally regenerating secondary forests and forest loss risk maps to estimate how much more carbon these forests could capture if they are left standing over specified time horizons.

   

• Countries can improve their carbon accounting and make more accurate climate projections. 

   

  Compared to the removal rates in Intergovernmental Panel on Climate Change (IPCC) greenhouse inventory guidelines, which are a common benchmark, the carbon removal rates we found are 26% lower for forests under 20 years old and 18% higher for those aged 20-100 years. (Differences vary by region.) This means governments and others that rely on IPCC rates are likely underestimating carbon removals by older secondary forests. Improved estimates could be used in developing Nationally Determined Contributions, place-based conservation and more.

Protecting secondary forests should go together with conserving mature forests (which have high carbon densities and biodiversity) and investing in restoring forests where they have been lost, which will bring long-term climate and nature benefits. In other words, it shouldn’t be one or the other, but all of the above.

Conservation and restoration also need to be done in ways that benefit the more than 1 billion people who live near or rely on forests. Secondary forests are often used by low-income or rural communities to support their livelihoods, and their protection must consider the needs, knowledge and wishes of local communities.

Informing More Impactful Climate Solutions

With forests under attack and under-valued, the world needs to muster every resource it can to show how much value forests provide and prioritize their protection. Maintaining existing secondary forests, protecting mature forests and enabling new forests to grow are all important for strengthening the global forest carbon sink — but these must be done while also providing food, fiber and other resources on finite land for a growing population.

Mapping carbon removal rates in forests is a rapidly advancing field. More ground data is needed from underrepresented regions, which includes most of the tropics (especially in Africa). We also need a better understanding of the human and environmental factors that affect carbon accumulation in forests. This can help inform where and how to focus forest protection efforts to maximize nature-based carbon removals within the context of social needs and other ecosystem benefits.

These maps provide another line of evidence for the value of forests. The new information can help practitioners and decision-makers focus forest restoration and protection efforts so that they are as effective as possible in averting further climate change.

This article was written in collaboration with Susan Cook-Patton and Nathaniel Robinson of The Nature Conservancy.