Photo: Wildfire in Arizona in 2016 (photo: Dyan Bone, U.S. Forest Service, www.flickr.com).

Less wildfires, but more intense

It seems contradictory: wildfire risk is increasing while wildfires are much less frequent and widespread now than in previous centuries. However, the intensity of wildfires is much higher now than in the past. In previous centuries, wildfires were an essential part of revitalising ecosystems. They are now so intense that they cause a lot of damage, including to ecosystems. This higher intensity is a result of the heavy fuel loads and increased fuel continuity that have developed because of wildfire exclusion, thereby increasing fire severity when forests inevitably burn.

The last 400 years

A comparison of historical and present-day fire regimes for North American forests paints an interesting picture of changes in the frequency, extent and impact of wildfires in the last 400 years. Forest and woodland fire regimes were studied for the historical period of 1600–1880 and the current period of 1984–2022. The historic data were derived from records of fire scars in tree-rings. Wildfires in previous centuries often did not kill trees, but only left a scar that could be dated.

More than 1800 tree-ring fire-scar records were used to compare historical and contemporary fire activity across a broad range of forest types. The historic data cover large parts of the United States and Canada, presenting information on wildfires since 1600. The data on the contemporary wildfires refer to the same sites of these historic wildfires.

A fire deficit

The strong reduction of wildfires in recent decades resulted from the practice of preventing and suppressing nearly all wildfires. This reduced average annual area burned since the late 19th and early- to mid-20th centuries compared with historical fire regimes across many North American forests. According to the authors of this study, this resulted in a ‘widespread 20th century fire deficit relative to earlier time periods’. The authors use the word ‘deficit’ to indicate that there are not enough wildfires today to avoid fuel accumulation and increases in canopy density across many forest types in North America.

Recent increases

Area burned by wildfires has increased across much of North America over the last few decades, and many studies have linked this to climate change. Despite this increase, a significant fire deficit persists in many forests. The study shows that, even under a warming climate, wildfires are less common now than in the 1600–1880 reference period. The authors attribute this to ‘aggressive fire suppression, disruption of traditional burning, and forest loss and fragmentation from land development and other land uses’.

Good news?

The fact that there are currently relatively few wildfires seems like good news. However, this has greatly altered forest composition, structure, and continuity such that ‘the inevitable wildfires that do occur are often burning with deleterious impacts on forest ecosystems, human communities, and human health’, the authors conclude. Tree mortality as a proportion of total area burned is now much higher than in the historic reference period. Unprecedented current levels of wildfire severity have even led to the transformation of forested areas to non-forest vegetation types, where tree mortality is too high and recruitment fails. Besides, the detrimental impacts of recent fires on human lives, infrastructure, air quality, and communities have increased over the past decades.

Although the frequency and burned area of recent wildfire years are not uncommon across many North American forest types, according to the multi-century tree-ring record, these fires are likely unprecedented in terms of highly elevated fire severity and their impacts on ecosystems and humans. Current forest conditions, human infrastructure, and many communities are not well-equipped to endure these extreme fire regimes.

The authors warn that ‘without substantial investments in proactive fire management, these impacts to forests and humans are likely to intensify in future decades as fuels continue to accumulate, particularly when compounded by climate change’.

Source: Parks et al., 2025. A fire deficit persists across diverse North American forests despite recent increases in area burned. Nature Communications 16.