Yes, forest trees die of old age. But the warming climate is killing them faster
- Written by: Belinda Medlyn, Distinguished Professor, Ecosystem Function and Integration, Western Sydney University
Across Australia, forests are quietly changing. Trees that once stood for decades or centuries are now dying at an accelerating rate. And this is not because of fire, storms, or logging. The chronic stress of a warming climate is killing them.
Our new research draws on 83 years of records from more than 2,700 long-term forest plots. This is the most extensive dataset ever assembled of Australia’s forests.
For the first time, we traced how background tree mortality – the slow, natural turnover of trees through time – has shifted across an entire continent. The results reveal tree mortality has been climbing steadily for more than 80 years, across all types of forests.
This is not just an Australian story. Similar increases in tree mortality have been recorded in the Amazon, Europe, and North America. Together, these independent observations point to a systemic shift. The planet’s forests, once reliable carbon sinks, are losing their capacity to buffer climate change.
Across the continent
An increase in tree death rates over time have previously been observed in tropical rainforests in Queensland and mountain ash forests in Victoria. Our new study shows such increases are the rule, rather than the exception.
To uncover this trend, we drew together a comprehensive database of forest data. This captured the diversity of Australia’s forests, from tropical rainforests and savannas in the north to cool temperate eucalypt forests in the south. Forestry researchers returned every few years to forest plots and measured the same trees. They recorded their growth or death, which created a detailed record of change over time.
This long-term record reveals a clear and sustained increase in tree deaths across the continent since the mid-20th century. We know tree deaths tend to increase with age and the number of neighbouring trees, so we took this into account. However, the increase in mortality over time remained, pointing to a fundamental shift underway.
A warming climate
The rise is tree deaths is linked to Australia’s warming and drying climate. Increased temperatures have emerged as the most powerful factor. Tree mortality has climbed fastest in hot, dry regions and dense forests, where trees compete intensely for water and light.
Increasing carbon dioxide in the atmosphere and warming temperatures can promote faster tree growth. So we investigated whether forests were simply moving through their life cycle faster, with trees dying earlier because they are growing more quickly.
However, this was not the case. We found tree growth throughout Australia’s forests has plateaued or declined, while tree death rates have continued to increase. Instead, this increasing mortality rate is linked to the physiological stress caused by hotter and drier conditions.
Australia’s forests are renowned for their toughness. Many forests are dominated by species that thrive in heat and drought and can rapidly resprout after fire or insect attack. Yet our analysis shows that ongoing climate change is pushing even these hardy species toward their limits.
Why this matters
Forests are among Earth’s most important climate regulators. Each year they absorb roughly one-third of human carbon dioxide emissions. They act as giant carbon sinks and buy us time to reduce fossil fuel use. But this buffering capacity is not guaranteed. If tree deaths continue to rise while growth stagnates, this effect will weaken. Climate feedbacks – climate processes that either intensify or minimise initial changes – will be amplified. And it will narrow the time we have left to stabilise the global climate.
What is happening across Australia offers a glimpse of what may lie ahead globally. This continent provides an ideal test case because it hosts diverse forests that are exposed to a highly variable climate noted for its “droughts and flooding rains”. Our discovery that trees deaths are happening across this entire content, regardless of forest type or location, suggests a widespread process already underway at a planetary scale.
Importantly, this story differs from that of forest dieback – the widespread decline or death of trees. Caused by factors such as extreme drought, insects, or pathogens, forest dieback is not uncommon in Australia. But it is usually noticeable when it occurs. In contrast, our study focused on a gradual transformation, which is less readily detectable but has far-reaching consequences.
Call to action
The only reason we can detect this shift is because generations of forest researchers have returned, decade after decade, to measure the same trees in the same places.
Long-term monitoring is one of our most powerful tools for understanding how forests are responding to the warming climate. There is some scope for remotely-sensed data to observe forests from afar, but scientists on the ground remain critically important.
However, support for this research has declined precipitously over the last two decades. Many of the plots in our dataset are no longer monitored. At a time when forests are changing faster than ever, this is incredibly shortsighted.
Understanding the changes currently underway is crucial for managing the many challenges facing our forests in the future. It is essential we continue to monitor our forests.
Authors: Belinda Medlyn, Distinguished Professor, Ecosystem Function and Integration, Western Sydney University
Daily Bulletin
