Gunnison. According to a recent study, the faster forests burn, the more intensely they burn. This is the central result of a study published in the journal “Science Advances” by a group led by Jonathan Coop from Western Colorado University in Gunnison (Colorado), which intensively examined forest fires in Canada and the USA.
As the speed of spread increases, the intensity of the fires, the proportion of severely burned area and the distance to surviving seed trees also increase.
The record-breaking fire season has drawn attention to how wildfire development is changing, including how they spread, the study authors write. They examined how these factors are related by evaluating satellite images of 3,499 major forest fires in Canada and the western USA, including Alaska, between 2012 and 2023. In this way, they were able to determine the daily area burned with a resolution of 30 meters. They also distinguished areas dominated by conifers from other forest areas.
The fires developed very differently: the area that burned in a single fire in one day ranged from 25 to 141,257 hectares. The straight line propagation length was 131 to 61,878 meters. The ten events with the fastest daily fire spread, each covering a distance of more than 20 miles within 24 hours, burned a combined 547,000 hectares of coniferous forest. Seven of these events occurred in Canada in 2023 and two in the western United States in 2020.
The fastest fires in forests spread through treetops when there is extreme drought and strong winds. “Because crown fires devastate tree canopies, they are generally lethal to trees in most North American coniferous forests and can leave post-fire landscapes with large, heavily burned areas,” Coop’s team writes.
As the speed of spread increases, not only does the intensity of the fire increase, but so does the proportion of severely burned area in which almost no trees survive, as the researchers found with high statistical probability. Since the seeds of the trees have also been burned in these areas, the distance to the next seed-bearing tree also increases.
Destroying the seeds can also change ecosystems in the long term. Because there are tree species that are better adapted than others to the situation after a forest fire. Fires that do not destroy entire trees, for example in the Latifolia subspecies of lodgepole pine (Pinus contorta) and in the black spruce (Picea mariana), trigger the spread of seeds. Deciduous trees such as Gambel's oak (Quercus gambelii) or American aspen (Populus tremuloides), which can disperse seeds over a large area, can also be among the winners after wildfires.
“Our results underscore the growing need to effectively reduce undesirable impacts, shape post-fire landscapes to preserve forest ecosystem functions, and promote socio-ecological adaptation,” emphasize the scientists. According to Coop, the study results can also be transferred to Europe. “In particular, it can be observed that the burning of coniferous forests under increasingly warm and dry conditions can lead to explosive forest fires – i.e. ones that both spread quickly and have a high fire intensity,” he told the dpa upon request.
Kirsten Thonicke from the Potsdam Institute for Climate Impact Research (PIK) does not rule out similar effects to those found in the study for large, contiguous forest areas in Europe. “The difference in Europe lies in the better accessibility of the locations,” says the researcher. “Measures to support reforestation – for example through planting or dropping seed bombs from the air – could be more helpful here,” says the Potsdam researcher.
