Australia

Fri Aug 22

Climate change is driving a silent, sinister change in Australia’s mountain ash forests

Written by Raphael Trouve, Senior Research Fellow in Forest Dynamics and Statistical Modelling, The University of Melbourne

Something silent and sinister is happening in Australia’s mountain ash forests. As temperatures rise, these ecosystems are slowly, steadily losing their trees – and with them, their ability to store carbon.

Mountain ash (Eucalyptus regnans) forests in southeastern Australia can store from 415 ^[1] to 819 tonnes of carbon ^[2] per hectare. That’s more per hectare than the Amazon rainforest ^[3]. So the decline of these special trees, the tallest flowering plants on Earth ^[4], is a major concern – for the forests and the climate system as a whole.

Our new research ^[5] shows how mountain ash forests are changing. Half a century of forest measurements show warming is reducing the number of trees these forests can support.

For every degree of warming, a forest loses 9% of its trees. By 2080, when temperatures are expected to be 3°C warmer ^[6] than pre-industrial levels, these forests could lose a quarter of their trees – and the carbon they store. This has global significance as the trend is likely to be widespread.

A group of people take shelter inside the hollow base of a huge mountain ash tree

Giant Eucalyptus regnans provide homes for wildlife and shelter for people. Craig Nitschke

Why warming causes tree loss

Trees in forests compete with each other 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.

Growing trees need more and more resources to maintain basic functions, such as producing new leaves, flowers and branches. Some naturally grow larger as they intercept more of the available resources. Others become suppressed and eventually die, as the amount of resources available becomes insufficient to keep them alive.

This natural process is known as “self-thinning”^[7]. As a result, the number of trees decreases as the average tree size in a stand increases.

Warming accelerates this natural process by reducing the resources available to all trees. Warmer air pulls more moisture from soil and leaves, making it harder for trees to get enough water. Trees growing in the shade of larger neighbours, with shallower roots, suffer most from these changes.

Suppressed trees die sooner and in greater numbers. The loss of trees can happen slowly or, if there is a severe heatwave or drought, quite suddenly.

A path winds through _Eucalyptus regnans_ regrowth in the morning mist.

Trees in forests compete for resources. Winners grow bigger while losers become weak and eventually die. This is a natural process known as self-thinning. Tom Fairman, Author provided (no reuse)

We tested these ideas, using decades of data

We analysed data from forest management experiments collected between 1947 and 2000 in Victoria’s Central Highlands. Our research ^[8] focused on how many trees these forests can support – what ecologists call “carrying capacity”. This is the maximum number of trees of a given size that a piece of land can support.

We built on more than 1,300 measurements from more than 100 forest plots to understand how climate affects carrying capacity. We used statistical methods to quantify the effects of climate over time at specific locations and to compare warmer and cooler sites.

Changes in forest carrying capacity have important implications. As carrying capacity decreases, forests can shift from absorbing to releasing carbon. Rather than being carbon sinks, they become a carbon source.

We found forests supported fewer trees when growing in the warmest conditions where the air is drier. As temperature increased over the five decades, forests everywhere showed increased tree death rates and decreased carrying capacity.

Our research suggests these forests could lose a quarter of their trees by 2080, releasing more than 100 million tonnes of stored carbon. This equates to emissions from a million cars ^[9] driving 10,000 kilometres a year for 75 years.

Our findings echo results from other research initiatives across Australia. Citizen scientists contributing to the Dead Tree Detective project ^[10] have been documenting similar climate impacts on trees nationwide.

Drone footage shows forest dieback in Western Australia (Guardian Australia)

Global implications

Our findings have implications for global climate initiatives.

Around the world, massive tree-planting programs such as Trillion Trees ^[11] are being launched to fight climate change. But our research suggests 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. Forests in cold regions should benefit due to longer growing seasons and milder winters. As temperatures rise, they should be able to support more trees and store more carbon – provided tree growth does not become limited by moisture availability.

In contrast, forests in warmer, seasonally dry regions such as southeastern Australia will experience the opposite trend. As they become more water-limited, tree death rates will accelerate and they will store less carbon than before.

Can we help forests cope with a warmer future?

Climate change is already reshaping the towering mountain ash forests of southeastern Australia.

Understanding how forests respond to warming will help us better protect them. While we can’t stop climate change immediately, we can help forests adapt. One promising option is ecological thinning – reducing stand density by selectively removing some trees to improve the health and survival chances of others.

Decades of international research ^[12] shows thinned forests are more resilient to drought ^[13]. The trees grow faster and survive better during dry periods.

While we can’t stop climate change overnight, we can make forests more resilient to drought and less prone to carbon loss.