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The first time I saw the Ada Tree, I felt tiny. It was October 2015, just days after arriving in Victoria to study mountain ash forests. During a visit to the Central Highlands, we went to see the Ada Tree—one of the largest mountain ash specimens, standing 76 meters tall (equivalent to a 20-story building), with a trunk 5 meters wide.

Mountain ash (Eucalyptus regnans) is the tallest flowering plant on Earth. The , is in Tasmania, reaching almost 100 meters high.

Mountain ash can , which is more than the Amazon rainforest on a per hectare basis (a 100 meter by 100 meter square).

But these forests are slowly changing.

Forests are growing thinner as temperatures rise

Our , now published in Nature Communications, quantifies these changes and their drivers. Our research team, based at the Universities of Melbourne and New Hampshire (U.S.), analyzed 50 years of forest measurements.

We found that these forests are growing thinner as temperatures rise. For every degree of warming, the forest loses 9% of its trees.

By 2080, when temperatures are expected to be three degrees warmer, these forests could lose a quarter of their trees and their stored carbon, .

To understand why warming leads to tree loss, we need to look at how forests work.

Trees in every forest compete for essential resources—sunlight, water and nutrients, all of which are in limited supply. When one tree gets more resources, others get less. This is nature's zero-sum game.

As forests develop, trees grow larger and require more resources to survive, leading to what scientists call "self-thinning."

Larger trees gradually intercept more and more of the available resources, leaving smaller trees with insufficient supplies to survive.

Climate warming accelerates these natural processes by reducing the resources available to all trees.

Warmer air pulls more moisture from the soil and leaves, making it harder for trees to get enough water.

Suppressed trees, growing in the shadow of their larger neighbors and with shallower roots, face a higher risk of dying under these conditions.

The concern is that as trees grow, they incorporate carbon into their trunk, branches, roots, bark and leaves (mostly the stem).

So when they die, the trees rot and release carbon into the atmosphere. This can turn forests from a carbon sink (removing carbon from the atmosphere) to a carbon source (releasing carbon into the atmosphere).

Measuring forest density

To track these changes in forest density over time, we analyzed data from long-term forest monitoring plots in Victoria's Central Highlands, some dating back to 1947.

These records were originally collected by the Forestry Commission of Victoria, later maintained by the Victorian Department of Energy, Environment and Climate Action (DEECA) and its predecessors.

Foresters measured thousands of trees across more than 100 forest sites, recording each tree's size and whether it was alive or dead.

They returned to these same sites every few years for decades, creating a detailed record of forest changes over time.

By comparing these measurements with climate data over the same period, we discovered a consistent relationship between warming temperatures and decreased forest density.

This pattern held true across different sites and throughout the study period.

Impact on tree-planting programs

This discovery comes at a critical time. Around the world, massive tree-planting programs like the are being launched to fight climate change.

Our findings suggest these efforts need to account for how many trees the forests will be able to support in a warmer future, not just how many they can carry now.

The impact will vary across the globe by latitude.

Forests in cold regions will likely thrive and store more carbon as temperatures rise, while forests in warmer, seasonally dry regions like Australia's mountain ash forests will naturally thin out as they become more water-limited and store less carbon.

What we are seeing in these mountain ash forests has global significance.

By 2080, these forests could lose a quarter of their trees, releasing more than 100 million tons of stored carbon. This equals the emissions from a million cars driving 10,000 kilometers per year for 75 years.

Our findings echo results from other research initiatives across Australia.

Citizen scientists contributing to the have been documenting similar climate impacts on trees nationwide.

These studies all point to the same conclusion: Australian forests are already responding to climate change.

Helping forests adapt

So can we help these forests cope with a warmer future? While we can't stop climate change immediately, we can help forests adapt.

One promising option is ecological thinning, where some trees are selectively removed to give others access to more resources and a better chance of survival.

It is a bit like thinning lettuce in your veggie patch: if every seedling is left to grow, they crowd each other and none develop properly.

But if you thin the row early, keeping only the most robust plants, each head gets the space, light, and nutrients it needs to grow strong and healthy.

Forests work the same way. By thinning the forest, we can reduce competition.

The remaining trees have better access to sunlight, water and nutrients, helping them grow faster and become more resilient to climate extremes.

around the globe shows that thinned forests are usually more resilient to drought, with trees showing better growth and survival during dry periods.

Studies from Europe and North America consistently demonstrate these benefits across different forest types and climates.

However, we need more research on ecological thinning in Australia. We need to better understand how it affects our unique eucalyptus forests and their ecosystem services, and to triage which stands should be thinned first.

We can't thin every forest, so we should focus on the densest regrowth in drought-prone areas where the benefits could be greatest.

The Ada Tree was already ancient when Europeans arrived in Australia, having survived centuries of change. But the changes it faces now are different, happening faster and more intensely than any in its long history.

Understanding how forests respond to warming helps us protect them better. Through careful management, we can help these forests and their carbon stores survive in a warmer world.