The world’s forests are much shorter and younger, on average, than they were a century ago, according to the findings of a new global study published 28 May in Science (1). Changes in old and new tree growth and death have resulted in an overall loss, which is likely to have broad impacts on global ecosystems.
Forests cover almost one-third of the planet’s landmass and play host to a huge range of biodiversity. But rising temperatures and carbon dioxide have been altering the world’s forests. Indeed, human-induced climate change, wood harvesting, and a range of naturally occurring processes, including the increased prevalence of wildfires, are placing trees under enormous stress.
The team of researchers led by the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL) in collaboration with the Birmingham Institute of Forest Research (BIFoR) used satellite observations showing changes in land use and combed through over 160 published papers to assess the impact of these disturbances on the world’s forests. The authors found that human-induced climate change is at least partly to blame for a dramatic shift in forest dynamics.
One major reason: rising temperatures make it much harder for trees and plants to photosynthesise. This makes it difficult for trees to regenerate and grow. And although increased levels of carbon dioxide in the atmosphere could help some trees grow faster, the researchers say that this so-called carbon dioxide fertilisation is typically only beneficial to younger forests.
Moreover, drought, rising temperatures, forest fragmentation, and insect attacks seem to have a more profound effect on older trees. In particular, drought can place forests under enormous stress and make them more susceptible to attack from insects and disease.
Diverse and thriving forest are contingent upon the careful balance between three important conditions:
- Recruitment, or the influx of new seedlings that will one day become young trees
- Growth, resulting in a net increase in biomass
- Mortality, defined as loss of a tree’s ability to reproduce and undergo cellular metabolism.
In a statement, Dr Tom Pugh of BIFoR, who was involved in the study, 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 forests 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”.
Lead author Dr Nate McDowell of PNNL 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”.
(1) McDowell N.G. et al. Pervasive shifts in forest dynamics in a changing world. Science (2020). DOI: 10.1126/science.aaz9463