Western forests feeling pressure from climate change

Trees may have a harder time reproducing because of wildfires worsened by climate change

Scientists in the United States looked at the effect of climate change on forests in the American West. As reported in the journal Fire Ecology, they found that increased fire frequency may affect the forests’ ability to renew themselves. More fires are occurring in part due to warmer, drier conditions brought about by climate change.

California is experiencing wildfires on an unprecedented scale. Eight of the ten largest fires in the state have occurred in the last five years. The Dixie Fire in northern California consumed nearly a million acres in 2021. More than a million acres went up in flames the year before in what is called the August Complex Fire, the largest fire in California since records began in the 1880s. Over the last five years, fires have also grown larger in Washington State, and wildfires in Oregon have become more frequent, as well. Climate change is driving these more frequent and fierce fires, as it is doing throughout much of the rest of the world.

Western forests and their existence with fire

To understand how climate change is affecting the forests, we first need to understand forest ecology in the western United States. Through millions of years, tree species in what is now the western United States, Canada, and Mexico evolved to withstand fires that periodically burn through their forests. Historically ignited by lighting strikes, the forests burned most commonly as ground fires, consuming duff, fallen dead branches, and other organic material. Ground fires burn at lower temperatures than crown fires, which are the destructive ones that spread rapidly from the crown of one tree to the next. For the most part, ground fires clear the forest floor and leave mature trees unharmed.

Ponderosas, pinions, and other pines of the West have evolved to withstand burns to their trunks. The bark of these trees may become charred, but resins in the bark protect the inner layers from the flames. Walking through these forests, you can see burn scars that date back 10, 20, 40 years or more.

Besides fire-resistant bark, many species of western forests have learned to use fire to their reproductive advantage.Most plants, drop or disperse their seeds just after ripening. Some plants have seeds that disperse and germinate after a trigger. For example, the seeds of many chaparral plants will germinate when they sense increased moisture. For a number of tree species in western forests, they retain their seeds in cones or pods that cling to their upper branches year after year.