Climatic decomposition and massive logging have made the world's forests significantly shorter and younger overall, according to one analysis.
The trend is expected to continue, scientists say, with worrying consequences for the ability of forests to store carbon and mitigate the climate emergency and for the endangered wildlife that depends on ancient and rich forests.
Analysis of more than 150 previous studies found that the death rate of trees has increased, doubling in North America and increasing significantly in the Amazon, for example. The impact of forest destruction had reduced the area of ancient forest by a third since 1900, the researchers said.
But the rise in temperatures caused by global warming also slows growth and increases tree death by limiting photosynthesis and causing stress. Also, high temperatures, drought, strong storm winds, and pests and diseases affect older trees the hardest and are on the rise.
New studies on the world's forests
Tom Pugh, a scientist at the University of Birmingham, UK, said: “Our study reviews growing evidence that climate change is accelerating tree mortality, increasingly pushing the world's forests to be younger and older. short ”.
“They have gotten smaller and younger over the past century, mainly due to the effects of human land use change and disturbances such as forest fires and insect outbreaks and droughts. These are things that are increasing in frequency and severity, ”he added.
There were exceptions, Pugh said, such as forests in high latitudes: "But in a world that is generally warmer, more of the world will be covered by forests that are generally shorter."
"In the last hundred years we have lost a lot of old forests," said Nate McDowell of the US Department of Energy's Pacific Northwest National Laboratory, who led the analysis. “They have been replaced partly by non-forests and partly by young forests. This has consequences for biodiversity, climate mitigation and forestry, ”he explained.
The research, published in the journal Science including analysis of satellite data on land use change, estimated that human felling of trees had reduced total forest area by 12% since 1900. The proportion of old forests , over 140 years old, fell from 89% to 66% in that time. The lack of data meant that the researchers couldn't make an accurate estimate of how much shorter the forests had gotten.
Increased levels of carbon dioxide in the atmosphere could increase tree growth, but the researchers said this appeared to occur only in younger, nutrient- and water-rich forests. However, most forests have limited nutrients and water, dramatically reducing the benefits of carbon dioxide to trees.
Pugh said that the vast majority of forests in the UK and Europe were examples of young, unnatural forests. "They are not the size that many of those forests would have been before humans fundamentally changed them by harvesting at regular intervals and planting new species, in some cases monocultures," he said.
Professor Tom Crowther, from ETH Zurich University in Switzerland and not part of the analysis team, said the study was extremely important: “For a long time, scientists have predicted that elevated CO 2 levels and warming will increase storage of carbon in forests that help offset climate change. But this study adds to growing concern that these factors, along with human disturbance, may in fact be decreasing the amount of carbon stored in these ecosystems.
"But it also suggests that if we can protect the forests we already have and allow them to grow to maturity, there is great potential for them to capture a lot of additional carbon," he said.
Professor Simon Lewis, University College London, said: “Because ancient forest is being lost, on average, around the world, forests are getting shorter and younger. However, against this, and what the researchers do not highlight is that within many ancient forests the opposite is happening.
“The world's intact tropical and boreal forests are important globally as carbon sinks, and they are getting bigger. The world's forests are currently slowing climate change, and while future mortality trends could reverse this, the ideas in the new report do not change what the world needs to do: stabilize the climate by rapidly reducing fossil fuel emissions to zero and protect the world's forests ”.
Pugh said the analysis of changing conditions around the world also had implications for protecting forests. "When you think in terms of conservation, it can be quite moving," he said. "It could be trying to preserve a forest ecosystem that fundamentally cannot continue to exist in the way it has existed in the past."