Mountain forests scorched by wildfires in the southwestern U.S. in recent years have failed to regenerate as forest ecosystems because of rising temperatures, decreased precipitation, and human intervention, according to a U.S. researcher. Speaking at an environmental conference this week in Colorado, Craig Allen, a research ecologist with the U.S. Geological Survey, described how since the mid-1990s the Southwest’s alpine forests have increasingly been replaced by grasslands and shrublands following fires, The New York Times reports. While southwestern fires in the distant past typically remained close to the forest floor — a natural cycle that prevented the overcrowding of trees — a combination of cattle devouring grassy surface vegetation, new government policies to prevent fires, and a drier climate have significantly altered this ecosystem. As a result, Allen said, forest fires now climb to the top of the canopy and the species that live in mountainous areas, including ponderosa pines and juniper, cannot regenerate as temperatures climb and precipitation decreases. “These forests did not evolve with this type of fire,” Allen said.