Divergent biodiversity–carbon relationships across vertically distributed forest biomes on the Tibetan Plateau

Forests are often seen as the lungs of the planet, soaking up carbon dioxide and storing it in wood and leaves. For decades, ecologists have debated a simple but profound question: does having more different kinds of trees make a forest better at this job? The prevailing idea, known as the biodiversity-ecosystem functioning relationship, suggests that a diverse mix of species works like a well-oiled machine, where each tree fills a unique role and uses resources like sunlight and water in a way that boosts the whole group's growth. However, nature is rarely static. Forests change as they age, growing taller, denser, and more complex. A key mystery has remained: does the benefit of having many tree species stay the same as a forest matures, or does it shift, weaken, or even disappear as the trees grow old? Understanding this is critical because forests are our most powerful natural tool for slowing climate change, and we need to know if planting a mix of species is a strategy that pays off forever or only for a while.

A team of researchers set out to solve this puzzle by looking at the real, living forests of the Tibetan Plateau. Instead of studying a single small plot or a young forest, they analyzed data from 1,248 forest plots spanning a massive range of elevations, from warm lowlands to cold highlands. These plots contained nearly 120,000 trees and were monitored over eight different censuses between 1979 and 2017. This long-term view allowed them to watch how the relationship between tree diversity and carbon storage changed as the forests grew from young saplings into mature giants. They focused on four distinct types of forest found at different heights: mixed forests of evergreen and deciduous broadleaf trees, pure deciduous broadleaf forests, mixed forests of needle-leaved and broadleaf trees, and pure needle-leaved forests.

The researchers found that if you look at all the forests together, the answer seems straightforward: more tree species generally mean more stored carbon. This confirms the classic idea that diversity helps. However, when they zoomed in to see how this played out over time and across different forest types, the story became much more complicated. The positive effect of diversity was not a steady, unchanging force. Instead, it followed very different paths depending on the type of forest and how old it was. In some forests, the benefit of diversity faded away as the trees got older. In others, it grew stronger before leveling off. In a few cases, the relationship even flipped, where having more species actually started to correlate with less carbon storage in the oldest stands.

The type of forest mattered immensely. In forests dominated by deciduous broadleaf trees—those that lose their leaves in winter—the boost from diversity was strong when the forest was young but faded quickly. As these forests approached about 100 years of age, the benefit of having many species disappeared and even turned negative. The researchers suggest this happens because these fast-growing trees quickly fill up the space, crowding each other out and competing fiercely for light. Once the canopy closes, adding more species doesn't help the forest grow bigger; instead, the trees just fight over the same limited resources. In contrast, needle-leaved forests showed a different pattern. Their diversity benefits rose to a peak around 100 years of age before slowly declining, but they never turned negative. These forests, often dominated by slower-growing conifers, maintained a more stable relationship between diversity and carbon storage for a longer time.

The most stable results came from mixed forests that combined needle-leaved and broadleaf trees. In these stands, the positive effect of diversity remained consistent and strong throughout the entire life of the forest, from young saplings to old giants. The researchers believe this is because the different types of trees fit together in a way that reduces competition. The needle-leaved trees and the broadleaf trees have different shapes and growth habits, allowing them to use space and light more efficiently together than either could alone. This structural harmony allowed the forest to keep reaping the rewards of diversity even as it aged.

The study reveals that the secret to why diversity helps or hurts a forest lies in how the forest builds its structure. As trees grow, they change the environment around them. In forests where the trees grow fast and fill the space quickly without creating much variety in their shapes and sizes, the advantage of having many species vanishes. The competition between trees becomes so intense that the unique benefits of diversity are lost. But in forests where the structure develops gradually and creates a complex, layered environment, the benefits of diversity can last much longer. The researchers found that when a forest develops a complex structure—where trees of different heights and shapes create a rich, three-dimensional habitat—the positive effects of diversity are sustained. This complexity allows different species to coexist without fighting over the same spot, keeping the forest productive and carbon-rich for centuries.

This work changes how we should think about forests and climate change. It suggests that the rule "more diversity is always better" is too simple. The value of a diverse forest depends on what kind of trees are there and how the forest is growing. A forest of fast-growing broadleaf trees might get a big boost from diversity when it is young, but that boost might vanish as the forest matures. A mixed forest of different tree types, however, might keep that boost for a very long time. The findings imply that to maximize carbon storage, we cannot just plant any mix of trees; we must consider how those trees will interact as they grow and how their physical structure will evolve. The forest is not a static collection of trees but a dynamic system where the arrangement of branches and leaves determines whether diversity helps or hinders the forest's ability to fight climate change.

The researchers were careful to note that their findings come from natural forests on the Tibetan Plateau, which have specific climates and tree species. While the patterns they found offer a powerful new way to understand forest growth, they also acknowledge that other factors, such as soil conditions and human disturbance, play a role. They did not measure the rate at which carbon is added to the forest, but rather the total amount stored at any given time. Despite these limitations, the study provides a clear, evidence-based framework for understanding why forests behave differently. It moves beyond the simple question of whether diversity helps to answer the more complex question of when, where, and why it helps. By linking the number of tree species to the physical structure of the forest, the study offers a roadmap for predicting how forests will store carbon in the future, suggesting that the key to a carbon-rich forest lies in fostering a complex, layered structure that allows diverse species to thrive together over time.