🕒 Last updated on August 22, 2025
Australia’s mountain ash forests, known as some of the tallest and most carbon-rich forests on Earth, are now facing alarming stress from rising temperatures. These forests were once considered powerful carbon sinks, meaning they absorbed and stored large amounts of carbon dioxide from the air. But new research shows that they are thinning rapidly and may soon become carbon sources instead, releasing huge amounts of greenhouse gases back into the atmosphere.
This transformation is being driven by heat stress and thinning caused by global warming. Over the next decades, the loss of trees in these forests may accelerate climate change and also place pressure on water resources that millions of people rely on.
Carbon-Dense Forests Now at Risk
Mountain ash forests in southeastern Australia are among the planet’s most extraordinary ecosystems. They grow in cool upland valleys and hillsides, where tall eucalyptus trees, often reaching more than 90 meters, tower over the landscape. These forests are not only visually stunning; they play one of the most important roles in nature by storing more carbon per hectare than even the Amazon rainforest.
The ability of these forests to capture and lock away carbon has long helped slow the pace of global warming. Every tree stores large amounts of carbon as it grows, keeping it out of the atmosphere. But scientists who collected long-term data from forest plots found that hotter conditions are hurting this natural balance.
As temperatures rise, trees face greater competition for water and nutrients, which are already limited. Larger, stronger trees survive more easily, but smaller trees around them weaken and die, causing gaps in the forest and a decline in overall tree density. This “natural thinning” is speeding up because of climate change, and it is reshaping the forests in ways that reduce their carbon-storing power.
Data shows that for every one degree Celsius increase in temperature, these forests may lose around nine percent of their trees. With global temperatures predicted to rise by about three degrees by 2080, this could mean almost a quarter of the trees in mountain ash forests may disappear by the end of this century.
From Carbon Sinks to Carbon Sources
The forest’s role in storing carbon has long been seen as a powerful ally against climate change, but higher tree deaths are reversing this benefit. When trees die and decompose, they release the carbon they once stored back into the atmosphere in the form of carbon dioxide. In other words, instead of capturing and locking away carbon, the forests begin to generate emissions.
The scale of this possible release is huge. Estimates suggest that future tree decline in these forests could produce emissions equivalent to the pollution from a million cars driving 10,000 kilometers every year for 75 years. That is a massive amount of carbon dioxide entering the air, accelerating climate change further.
Pakistan’s crypto pact with Trump venture sparks alarm over money laundering, terror links
What makes this situation even more serious is that the study did not take into account wildfires, which are already becoming more frequent and intense in Australia. Bushfires can burn mountain ash forests severely, causing immediate tree deaths and releasing vast amounts of carbon at once. If fire impacts are combined with growing losses from heat stress, the total carbon emissions from these forests could be much higher.
The ecosystem’s powerful cooling role is therefore under threat, leaving scientists deeply concerned about the possibility of forests flipping from being valuable carbon sinks into dangerous carbon sources in the decades ahead.
Water, Forests, and the Changing Landscape
The impacts of this thinning stretch beyond climate concerns. Mountain ash forests are closely tied to water supplies in southeastern Australia. Trees in these regions capture rainfall, and their interaction with the environment affects water flow into rivers and streams. When forests thin, the amount of water moving through the ecosystem also changes.
Research shows that thinning of trees can alter stream flows and water yields. For a city like Melbourne, which depends heavily on water catchments in forested areas, this change could directly impact future water availability. If large numbers of trees die, the balance of water that reaches reservoirs might be altered, adding another layer of challenge for communities.
Not all effects are negative—selective thinning has been highlighted as a possible management tool. By carefully removing some trees in advance, experts believe that the remaining trees may get better access to resources, making them more resilient to stress. Around the world, studies have shown that forests managed in this way often survive drought conditions better, with stronger growth in surviving trees.
Still, these actions reveal the scale of the pressures these forests face. Tree thinning, whether natural or managed, is now a visible sign of how ecosystems are struggling with climate stress. The towering mountain ash forests that once symbolized the resilience and strength of Australia’s landscapes are now displaying vulnerability.
As vast as these forests are, their survival pathway is being shaped by rising temperatures. Their fate shows how ecosystems respond directly to global climate change and why changes in the natural world are so significant for people, water systems, and the planet’s atmosphere.