Science Alert: Why Our Forests Are Growing Faster but Becoming More Fragile

A global transformation invisible to the naked eye

The world’s forests are currently undergoing a radical transformation that is causing concern among the scientific community. All over the world, these ecosystems are being filled with fast-growing trees, while slow-growing, long-lived species are disappearing at an accelerated rate. While replacing century-old trees with fast-growing species benefits the timber industry and allows for rapid visual recovery after fires, this dynamic significantly weakens forests’ resilience to ongoing climate change.

To understand the scale of the phenomenon, researchers at Aarhus University conducted a global analysis of 31,001 tree species. Their work mapped the areas where characteristics associated with rapid growth are becoming dominant. The resulting maps reveal a clear trend: generalist, fast-growing species are gaining ground at the expense of specialized, slow-growing trees.

Jens-Christian Svenning, one of the study’s authors, demonstrated that this shift is particularly critical in tropical and subtropical regions. He noted that many trees with a small range are the most likely to disappear as disturbances increase. The risk is systemic: once fast-growing trees dominate a plot, storms, droughts, and pests can wipe out entire sections of that forest in a single event.

The Physical Vulnerability of the New Giants

This transition is often accelerated by human activity. Logging, road construction, and more intense wildfires leave open, sun-drenched gaps where fast-growing trees quickly take root. These species are characterized by lighter leaves and softer wood, allowing them to grow rapidly even when heat or drought makes water scarce. However, this low wood density—that is, the weight of the wood relative to its size—makes their trunks easier to break and more prone to drying out.

Over several decades, forest stands filled with light-wood trees may break or dry out more quickly, making episodes of mass mortality more likely during extreme years. In contrast, long-lived trees grow slowly, but their deep roots and sturdy trunks help maintain the forest’s cohesion when weather conditions become harsh. Their denser wood and more resilient leaves help them withstand drought and pests.

A recent report has also linked this resilience to climate protection. Jens-Christian Svenning emphasizes the vital importance of these robust species: “They form the backbone of forest ecosystems and contribute to stability, carbon storage, and resilience to change.” The loss of these stable species leaves more open space for short-lived trees, and the forest can then experience more severe fluctuations with each new disturbance.

The Silent Invasion of Introduced Species

This phenomenon is amplified by human activity. Through ports, nurseries, and plantations, humans have moved trees far beyond their native ranges, and some are now spreading on their own. Nearly 41% of naturalized tree species—that is, those capable of reproducing in the wild outside their native range—are characterized by rapid growth and small leaves. As storms, logging, and heat increasingly disrupt forests, these “outsiders” can outcompete native seedlings for light, water, and nutrients.

This pressure can push rare local trees closer to extinction, even when the newcomers appear healthy and are growing rapidly. In tropical forests, where many tree species are concentrated in small areas, the loss of just a few species can rapidly thin out entire food webs. Many threatened trees there are endemic—found only in a single region—and depend on stable temperatures and humidity. Conservation groups use the IUCN Red List to classify the extinction risk for many of these species. Once logging or a heat wave pushes conditions outside this narrow comfort zone, a local population may disappear permanently.

Further north, warmer winters and more intensive land use are creating new openings where imported trees can take root. In colder regions, many native trees are already living near their frost tolerance limits; newcomers that are more resilient to temperature fluctuations therefore spread more quickly. Trade and landscaping often move hardy, fast-growing species between continents, and repeated plantings give them multiple opportunities to escape. Over time, this spread can make distant forests more similar to one another, compounding the losses for local species.

Carbon and wildlife: the collateral victims

Globally, forests still act as carbon sinks, removing more carbon from the atmosphere than they release. Slow-growing, dense-wooded trees lock carbon away in their trunks for decades, as each centimeter of solid wood contains more material. In contrast, fast-growing trees often die younger and develop lighter trunks; as a result, storms and decomposition release this stored carbon back into the air sooner.

This dynamic has direct consequences for the environmental effectiveness of forest stands. As Jens-Christian Svenning explains: “This makes forests less stable and less effective at storing carbon over the long term.” Beyond the climate, wildlife depends on specific trees for food and shelter; the removal of a single species can therefore have a ripple effect throughout the entire forest community.

Some slow-growing trees bloom or bear fruit according to schedules that local animals follow closely, and their loss can disrupt these routines. Seed dispersers and pollinators have often synchronized their activity with the timing of these trees. If replacement trees with different life cycles take their place, the animals may find themselves starving. Without the right tree partners, forests may lose the birds and mammals that disperse seeds, thereby slowing recovery after storms or fires.

Planting for the Future: Toward a New Approach to Management

Given this reality, practices must evolve. Forest managers often prioritize fast-growing trees for rapid harvests, but long-term resilience improves when plantings also include slower-growing species. Choosing a broader mix protects rare genetics and maintains the characteristic traits of old-growth forests as climate stress increases.

Limiting repeated disturbances and early removal of invasive naturalized trees can give native seedlings enough light and water to survive. Because slow-growing trees take decades to reach maturity, the choices we make today can lock in either stability or fragility for a long time to come. Overall, forests appear to be on track to grow faster while losing the slow-growing trees that maintain stable and diverse ecosystems.

Future planning could treat these slow-growing species as essential infrastructure. It would also allow for testing which restoration mixtures hold up best under extreme conditions. This comprehensive study, which calls for urgent action, is published in the scientific journal Nature.

Source: earth.com

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Science Alert: Why Our Forests Are Growing Faster but Becoming More Fragile