Forest thinning found to improve the ‘hydrological resilience’ of forests in addition to preventing wildfires
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An environmental, economic and social nightmare, wildfires are among the most deadly natural disasters and are becoming more frequent and severe with global warming.
A range of management strategies exist to reduce the risk and severity of wildfires. One of these is forest thinning, where small trees, shrubs, and dense understory bush are shredded and mulched. Gaps are created between tree crowns, with trees that are less resistant to fire removed.
Researchers in the US have shown with observational data in Frontiers in Forests and Global Change, that tree thinning can go further than merely reducing wildfires.
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‘Forest treatments used to reduce wildfire risk also help recover snow storage that has been diminished by forest change and a warming climate, with stronger effects on north-facing slopes than on south-facing slopes,’ said corresponding author Dr Cassie Lumbrazo, a research scientist at the University of Washington and the University of Alaska Southeast.
The researchers studied the effect of forest thinning on Cle Elum Ridge in the state of Washington. The area is a regional hotspot for wildfire risk reduction actions, and is also at a high risk of drought.
‘These treatments recovered about 12.3 acre-feet of snow-stored water per 100 acres on north-facing slopes, equivalent to roughly 15 Olympic swimming pools per square kilometre, compared to about 5.1 acre-feet per 100 acres, or about six swimming pools per square kilometre, on south-facing slopes,’ said Lumbrazo.
The improvement in ‘hydrological resilience’ – the ability of a forest to deliver a sustainable supply of clean water for nature and people – is significant. Snowpack regularly yields a large amount of water for human use, meaning that forest thinning can play a valuable role on two different, yet interconnected fronts – wildfire risk and hydrological storage.
Like many features of the hydrological cycle, protective snowpack is a victim of global warming, and is projected to decrease by a further 50 per cent by the end of the century in Washington. This makes the recovery of this water as a result of forest thinning all the more welcome.
How are wildfires started?
Wildfires need three things to start: fuel (dry vegetation), oxygen and a heat source. On the climatic side, they require long periods of hot and dry weather, as well as a climate that is wet enough to ensure the growth of vegetation.
This means that wildfires are particularly common in parts of North and South America, Australia, Southeast Asia, and the Mediterranean.
1.5 million deaths a year are attributed to smoke emitted by wildfires. Vicious and unrelenting, they ruthlessly plough through forests – 3.9 million sq km of cover was destroyed in 2025 – and cause enormous economic damage. The 2025 wildfires in Los Angeles are the most costly on record at $53 billion.
The urgent need to prevent, slow down and mitigate the impacts of wildfires is paramount. Now, research suggests that forest thinning to slow the spread of wildfires simultaneously boosts the hydrological supply of forests.
