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:

Atmospheric carbon dioxide: Carbon dioxide levels in the atmosphere have increased dramatically since the Industrial Revolution and are projected to continue rising over the next century. Higher levels of carbon dioxide can increase a tree’s growth rate and seed production. However, such carbon dioxide fertilization appears to only happen in younger forests with abundant nutrients and water.  Most forests globally are exposed to limitations in nutrients and water, which drastically reduces the carbon dioxide benefits to trees.

Temperature: Rising temperatures limit life-giving photosynthesis, leading to lower growth, higher mortality, and reduced regeneration. This is one key to shorter trees, the study determined.

Droughts: They’re expected to increase in frequency, duration and severity globally. Drought can directly cause tree death or indirectly lead to mortality through associated increases in insect or pathogen attack.

Other factors are altering the face of the world’s forests:

Wildfire is increasing in many forests worldwide and future fires may be more frequent than they have been in the past 10,000 years in some regions, the study found. Plant growth following forest fires may be slow or absent due to elevated temperatures.

Biotic disturbances—by insects, fungi and choking vines—have been on the increase. The carbon storage lost to insects each year is the same as the amount of carbon emitted by 5 million vehicles, a study published last year says. This is expected to continue with warming, along with other biotic deforestation disturbances, such as fungi and bacteria. In the tropics, vines that use other plants as host structures are choking trees to death.

Wood harvests alone has had a huge impact on the shift of global forests towards younger ages or towards non-forest land, reducing the amount of forests, and old-growth forests, globally. Where forests are re-established on harvested land, the trees are smaller and biomass is reduced.

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.