Ponderosa pine and Douglas fir forests in the American west are struggling to regrow after they were wiped out by wildfires exacerbated by climate change.

A team of researchers analyzed the growth rings of nearly 3,000 young trees from 33 areas damaged by wildfires between 1988 and 2015 to see how the forests are recovering. They found that the conditions in wildfire-ravaged forests are increasingly becoming unsuitable for young trees to take root as the rising temperature bring hotter air and drier soil.

Experts fear that the changes in the ecosystem that otherwise would take decades or even centuries would happen abruptly due to climate change. Their findings were published in the Proceedings of the National Academy of Sciences on Monday, March 11.

No New Trees Are Growing

The researchers said that adult trees have better mechanisms in place that help them survive poor climate conditions and weather events. Ponderosa pines and Douglas firs, for example, have thick barks that allow them to live through surface-level wildfires.

However, the seedlings and juvenile trees are extremely vulnerable to the impacts of climate change. If an intense wildfire, which experts warn will become more frequent as the temperature rises, has wiped out even the adult trees in an area, there is a possibility that new trees will not grow.

"There could be a lot of areas where there is currently forest but if we have a fire we might not see regeneration," stated Kimberley Davis, a researcher from the University of Montana and the lead author of the study.

Low-Elevation Forests Disappearing

The researchers also analyzed data from 1988 to 2015 and discovered certain thresholds for humidity for the ponderosa pine, surface temperature for Douglas fir, and soil moisture for both during the summer months. They said that several sites they visited have crossed the temperature and humidity threshold in the past 20 years, resulting in a sharp decline in forest growth.

"Once a certain threshold was crossed, then the probability of tree establishment decreased rapidly," Davis explained. "The climate conditions are just a lot less suitable for regeneration."

The researchers are yet to see if forest regeneration is affected in cooler and wetter sites in the region.

The findings of the study are worrying because forests are a major part of the fight against climate change. Trees pull carbon dioxide, a greenhouse gas that traps heat, from the atmosphere.