Younger forests outnumbering old, disturbing world’s carbon balance 

Forest age structures are transitioning, disturbing the carbon balance across different forests across the world, a new study has uncovered.

The age of forest structures influence the carbon stocks and need to be quantified, showed the study published in journal Nature Ecology and Evolution. It pointed out the primary drivers for the same are fire, deforestation and younger trees outnumbering the age old trees that store more carbon than their newer counterparts.

The researchers studied forest age across the Amazon, Congo Basin, Southeast Asia and Siberia, comprising of boreal, tropical and temperate forests, between 2010 and 2020. They observed widespread decline of forest age. It means that the forests in these regions were towards younger side. 

The researchers also noted that forests in China, Europe and North America recorded net ageing. 

It was observed that the worldwide expanse of forests aged 21 to 40 years grew by about 0.03 billion hectares, which is nearly a 17 per cent increase, between 2010 and 2020.

The researchers explained that the age of the forest matters as young, fast-growing forests have the ability to sequester carbon at higher rates compared to older forests. This is especially in the tropical regions where young regrowing forests that is less than 20 years of age can absorb carbon dioxide as much as 20 times than older forests.

However, they store much less carbon than their older counterparts. As a result, the reducing mature, ageing, carbon rich forests and increasing younger forest are reducing carbon stocks. 

“We estimate a net global loss of 140 million tonnes of carbon each year from aboveground biomass,” said first author and researcher Simon Besnard from GFZ Helmholtz Centre for Geosciences in a statement. He added that young forests provide valuable climate benefits, but they cannot fully replace the long-term carbon storage of old-growth stands.

“European forests, covering around 33 per cent of the continent, are experiencing complex age dynamics due to management and natural disturbances, such as fire, drought and insect outbreaks since 2010. Despite these disturbances, forests are generally ageing years in 2010,” read the statement. 

However, Portugal was found to be an exception where deforestation and large fires have led to younger forests.

Forests in South America and tropical Asia experienced net age decreases primarily due to increasing stand-replacing disturbances and mortality, demonstrating a shift towards younger forest stands and the replacement of old forests.

“Traditional activities such as slash-and-burn agriculture also contributed to this trend in the Amazon Basin. Whereas young forests grow faster, their regrowth does not fully compensate for the carbon loss from older forests,” the study stated.

In North America, especially in the Pacific Northwest, a mosaic of older stands and areas of stand replacement followed by regrowth is evident, probably influenced by clear-cutting, a high-burning frequency regime and other natural disturbances, such as insect outbreaks, it added.

Meanwhile, in South America, forests outside the Amazonia, namely the Atlantic forests, showed a decrease in native forest cover owing to human activities and disturbances. But recent conservation efforts showed some recovery.

In addition, they found that undisturbed, ageing forests sequester substantially higher volumes of carbon than recently disturbed ones across all age classes.

The authors observed that some regions are transitioning from carbon sinks to carbon sources. For instance in deforestation hotspots, atmospheric inversion data indicated areas such as eastern edge of Amazon as net CO2 sources.

The reasons for these were primarily fire-related emissions which can outpace the carbon uptake of remaining forests. The example emphasises the crucial role of deforestation contributing towards increasing atmospheric CO2. 

In conclusion, the the forest age transitions play a dominant role shaping the global carbon balance and that young forests can never completely replace the long-term carbon sequestration capacity and ecological functions of old forests. 

Protecting existing forests remains the most effective and immediate strategy for mitigating climate change, they said.