Understanding global tree mortality
How likely are trees to die? Do the reasons for tree death differ around the world? What impacts might climate change have on tree mortality in the future? The answers to these questions are largely unknown, yet are critical to our understanding of the exact role of forests in the global carbon cycle. Forests absorb and hold vast amounts of carbon dioxide from the atmosphere, taking up roughly 20 % of human carbon emissions from fossil fuels each year. Whether or not the size of this carbon sink will continue, shrink or grow is uncertain, in part because of a lack of understanding about the level of stress that leads to tree death. “Whilst we know the different causes of tree death, quantifying the rates of death is challenging because trees can live for a century or more,” explains Thomas Pugh, associate professor in the Department of Physical Geography and Ecosystem Science at Lund University, and TreeMort project coordinator. In the TreeMort project, which was funded by the European Research Council, Pugh led a team of scientists aiming to fill in the knowledge gaps about tree death. The project created a global database of tree mortality, and modelled the rates and causes of tree death and how they relate to environmental drivers.
A global database of tree death
The TreeMort team worked with researchers around the world to make a huge database of global tree mortality observations, which follows the lives of millions of trees over one to two decades. The researchers mapped the rates of tree death around the world, and created models that calculate the likelihood that a tree will die in any year. Then they combined these measurements with satellite observations and forest models to give a complete picture of the impacts of tree death on carbon sequestration. “Integrating these kinds of observations of tree mortality into global forest models has never been done at this scale before, but it’s really powerful,” says Pugh.
Legacies of forest carbon sinks
So far, the team has shown that the legacies of past forest disturbances – large patches of forest killed by fire, wind, and harvest, and now recovering – currently account for a quarter of the global forest carbon sink. These areas offset roughly 5 % of annual emissions from fossil fuel burning. Tree harvesting and land-use change have substantially reduced the length of time carbon remains in forest vegetation: by 32 % in northern hemisphere temperate forests over 2001-2014. “Despite their often dramatic impact on the landscape, however, our modelling showed that disturbances were responsible for a relatively modest 12 % of total tree mortality globally,” adds Pugh. The team also discovered that the impacts of humans and climate change have set a trend for the world’s forests to grow progressively younger.
Contributing to global forest science
The team’s work on the legacy of disturbances has been widely picked up, with over 370 citations since 2019. The team are now working to bring out several big analyses related to the patterns and drivers of tree mortality, and updated computational models to predict the future of the world’s forests. “We’re also working actively with other groups that model the world’s forests to use the outcomes from TreeMort to inform their work,” says Pugh.
Keywords
TreeMort, tree, death, stress, forests, carbon, sinks, legacies, forest science
