Ongoing environmental changes are transforming forests worldwide, resulting in shorter and younger trees with broad impacts on global ecosystems, scientists say.
In a global study published in the 29 May 2020 issue of Science magazine, researchers including experts at the University of Birmingham, showed how rising temperatures and carbon dioxide have been altering the world's forests.
These alterations are caused by increased stress and carbon dioxide fertilization and through increasing the frequency and severity of disturbances such as wildfire, drought, wind damage and other natural enemies. Combined with forest harvest, the Earth has witnessed a dramatic decrease in the age and stature of forests.
The study was led by the U.S. Department of Energy's Pacific Northwest National Laboratory (PNNL), with analysis on changes in forest age carried out by the Birmingham Institute of Forest Research (BIFoR).
Dr Tom Pugh, of BIFoR, said: "This study reviews mounting evidence that climate change is accelerating tree mortality, increasingly pushing the world's forests towards being both younger and shorter. 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.
"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."
Dr Nate McDowell, a PNNL Earth scientist and the study's lead author added: "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. Older forests often host much higher biodiversity than young forests and they store more carbon than young forests."
The study, 'Pervasive shifts in forest dynamics in a changing world,' concluded that forests have already been altered by humans and will mostly likely continue to be altered in the foreseeable future, resulting in a continued reduction of the area and stature of old-growth forests globally.
The researchers used a detailed literature review along with analysis of data on land-use change to conclude that the globally averaged tree size has declined over the last century and is likely to continue declining due to continuing environmental changes.
Several factors have led to the widespread loss of trees through both human activity and natural causes - including clear-cutting, wildfire, insects and disease. High levels of tree loss leads to an imbalance in three important characteristics of a diverse and thriving forest: (1) recruitment, which is the addition of new seedlings to a community; (2) growth, the net increase in biomass or carbon; and (3) mortality, the death of forest trees.
"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."
Wide-ranging impact
The study also shows that other mechanisms of change in forests are underway too. These include:
Other factors are altering the face of the world's forests:
The US Department of Energy's Office of Science, the European Research Council (TreeMort project) and several other organisations funded the work. The study's authors include researchers from more than a dozen institutions.
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Notes to editor:
The University of Birmingham is ranked amongst the world's top 100 institutions. Its work brings people from across the world to Birmingham, including researchers, teachers and more than 6,500 international students from over 150 countries.
Pacific Northwest National Laboratory draws on signature capabilities in chemistry, Earth sciences and data analytics to advance scientific discovery and create solutions to the nation's toughest challenges in energy resiliency and national security. Founded in 1965, PNNL is operated by Battelle for the U.S. Department of Energy's Office of Science. DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit PNNL's News Center. Follow us on Facebook, Instagram, LinkedIn and Twitter.
McDowell et al (2020). 'Pervasive shifts in forest dynamics in a changing world'. Science.
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