Protected areas enhance long-term carbon sequestrations in China’s forests

Forests are nature's most powerful air filters. As trees grow, they pull carbon dioxide from the atmosphere and lock it away in their wood, leaves, and the soil beneath them. This process, known as carbon sequestration, is a vital tool in the fight against climate change. To protect this ability, many countries have set aside protected areas—lands where logging and development are strictly forbidden. The logic is simple: if we stop cutting down trees, the forests will grow back, store more carbon, and help cool the planet. But a crucial question has lingered in the minds of scientists and policymakers: does this protection keep working forever? As a forest matures and its trees get older, does it continue to soak up carbon at the same rapid pace, or does its appetite for carbon eventually slow down as the trees reach their full size?

A new study by researchers in China tackles this question by looking at the country's vast network of protected forests. The team combined advanced computer models that simulate how trees grow with a rigorous statistical method designed to compare protected forests with similar unprotected ones. By analyzing over 10 million hectares of forest across a 70-year timeline, they aimed to see not just how much carbon these forests currently hold, but how fast they are still adding new carbon in the future. The goal was to determine if the climate benefits of protection are a permanent gift or a temporary boost that fades as the forest ages.

The researchers found that protected areas in China have indeed been highly effective at building up carbon stocks. In 2010, the forests inside these protected zones held significantly more carbon per acre than comparable forests outside the boundaries. This advantage is expected to persist, with protected forests projected to store 134% more carbon than their unprotected counterparts by the 2060s. However, the story of how fast they are adding new carbon tells a different tale. The study reveals that the rate at which these forests absorb carbon is not a steady, endless climb. Instead, it follows a curve that rises, peaks, and then begins to fall as the trees mature.

The analysis suggests that the carbon absorption rate for China's protected forests will likely hit its highest point between 2030 and 2035. After that peak, the speed at which these forests pull carbon from the air will begin to slow down. This is a natural part of forest life: young, growing trees are like hungry eaters, gobbling up carbon rapidly to build their trunks and branches. As they age and reach their maximum size, their growth slows, and so does their ability to capture new carbon. The researchers project that by 2035, more than 90% of these protected forests will have passed their peak speed for carbon absorption. By the 2060s, the extra carbon gain provided by protection is expected to decline by an average of 0.19 tons per hectare each year.

This does not mean the forests stop helping the climate. Even as the rate of new carbon capture slows, the total amount of carbon stored remains high and continues to grow, just more gradually. The study estimates that despite this slowing rate, the protected forests will still contribute an additional 51.9 million tons of carbon storage by the 2060s compared to if they were not protected. The key finding is that the "extra" benefit of protection—the difference between a protected forest and an unprotected one—shrinks over time. This happens because unprotected forests, if left to recover naturally, also grow and store carbon, eventually catching up to the protected ones as the protected forests reach their maturity limits.

The study also highlights that this pattern is not the same everywhere. The southern regions of China, where the climate is warm and wet, show the most potential for continued rapid carbon absorption. These areas are home to younger forests that are still in their vigorous growth phase. In contrast, the northern regions, which are cooler and drier, have older forests that are closer to their maximum size, meaning their carbon absorption rates are already slowing down. The researchers used a method called propensity score matching to ensure their comparisons were fair. They paired protected forest plots with unprotected ones that had the same soil, climate, and tree age, effectively creating a controlled experiment on a massive scale. This allowed them to isolate the true effect of protection from other factors like weather or soil quality.

The implications of these findings are significant for how we manage forests in the future. The study suggests that while protected areas are essential for maintaining the massive carbon stocks we have already built up, relying on them alone to provide a rapid, accelerating carbon sink may be a mistake. As forests age, their ability to act as a fast-acting climate solution diminishes. The researchers argue that conservation strategies need to be smarter about the age of the trees. In areas where forests are already old and their carbon absorption has slowed, the primary goal should be to protect the existing carbon stock from being released back into the atmosphere through fire or logging. In younger forests, particularly in the south, protection continues to drive rapid carbon uptake.

Ultimately, the research paints a picture of a climate strategy that must evolve. Protected areas are not a one-time fix; they are a dynamic system where the benefits change as the forest grows. The study confirms that China's protected forests are a critical asset, holding far more carbon than they would otherwise. But it also warns that the window for maximum carbon absorption is closing for many of these forests. To keep the climate benefits flowing, managers may need to look beyond simple protection. They might need to consider how to maintain a mix of young and old trees or how to protect new forests as they grow, ensuring that the carbon sink remains robust even as individual trees reach the end of their rapid growth phase. The protection of these forests remains a cornerstone of climate action, but the nature of that protection must adapt to the changing life cycle of the forest itself.