Climate change is transforming forests worldwide. Rising carbon dioxide levels and warming temperatures are transforming the world’s forests by accelerating rates of tree mortality. As a result, forest ecosystems are now dominated by younger and shorter trees.
This translates to lower levels of carbon storage and biodiversity, which both play central roles in climate change mitigation.
The study was led by Dr. Nate McDowell, who is an Earth scientist at the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL).
Based on the analysis of satellite imagery and a detailed literature review, the researchers concluded that the global average tree size has declined over the last century.
“This trend is likely to continue with climate warming. A future planet with fewer large, old forests will be very different than what we have grown accustomed to,” said Dr. McDowell. “Older forests often host much higher biodiversity than young forests and they store more carbon than young forests.”
According to study co-author Dr. Tom Pugh, there is mounting evidence that climate change is accelerating tree mortality and increasingly altering the age and stature of Earth’s forests.
“This implies a reduction in their ability to store carbon and potentially large shifts in the mix of species that compose and inhabit these forests,” said Dr. Pugh.
“This is likely to have big implications for the services those forest provide, such as mitigating climate change. Increasing rates of tree mortality driven by climate and land-use change, combined with uncertainty in the mix of species that will form the next generation, pose big challenges for conservationists and forest managers alike.”
Dramatic tree losses in recent decades can be linked to many factors including pests, clear-cutting, environmental stress, carbon dioxide fertilization, and the increasing frequency and severity of disturbances such as fires, drought, and wind damage.
When a forest experiences high tree losses, a balance is shifted among three important characteristics: recruitment of new seedlings, growth, and mortality.
“Examination of the global patterns of those three parameters over recent decades shows that mortality is rising in most areas, while recruitment and growth are variable over time, leading to a net decline in the stature of forests,” said Dr. McDowell.
“Unfortunately, mortality drivers like rising temperature and disturbances such as wildfire and insect outbreaks are on the rise and are expected to continue increasing in frequency and severity over the next century. So, reductions in average forest age and height are already happening and they’re likely to continue to happen.”
Dr. McDowell said the conditions promoting deforestation will likely accelerate, drastically altering the living conditions for plants and animals.
“Over the last hundred years we’ve lost a lot of old forests, and they’ve been replaced in part by non-forests and in part by young forests. This has consequences on biodiversity, climate mitigation, and forestry.”
The study is published in the journal Science.