Uncontrollable infernos that have torched about half a million hectares and displaced more than 100,000 people have made this the worst wildfire year yet for California. From such ashes, ecosystems usually bounce back, but a new study reveals this is no longer a guarantee. Thanks to climate change, areas ravaged by wildfires may never recover, wiping out entire ecological communities forever.
Wildfires are a natural part of many environments. They are nature’s way of clearing out the dead litter on forest floors. This allows important nutrients to return to the soil, enabling a new healthy beginning for plants and animals. Fires also play an important role in the reproduction of some plants. For example, seeds in some pinecones are sealed with a resin that melts in fires, releasing them and allowing new growth.
But fires are only good if they serve their specific purpose. If they burn too long, or the ground stays dry too long, ecosystems can’t recover. Given that climate change can lead to more fires and longer droughts, researchers have wondered how forests are coping, and whether they are getting a good start on their way to becoming a new generation of trees.
To find out, ecologist Camille Stevens-Rumann of Colorado State University in Fort Collins and colleagues studied about 1500 sites in the conifer forests of the U.S. Rocky Mountains that had seen 52 wildfires between 1988 and 2011. The areas spanned elevations from about 700 to 2800 meters above sea level and comprised various types of dry and wet pine forests. The researchers collected seeds from the sites between 2010 and 2014, and, along with previously published data, analyzed the plots’ seedling presence and density. By comparing these data to the seedlings in sites that had not burned, they determined the ability of forests to regenerate. In addition, they combined these data with climate—temperature and moisture—information to see how it affected tree regrowth.
They found a dramatic difference in tree regeneration after fires late last century compared with fires earlier this century. The proportion of sites with no regrowth almost doubled after 2000, from 19% to 32%, coinciding with increasing temperatures and more droughts, the team reports this month in Ecology Letters. Although forests that burned before 2000 have had more time to grow, the presence of seedlings in the first 5 years gives a good indication of future growth.
“Essentially either you have abundant seedlings and continue to grow, or you have none, and rarely is a site filled in,” Stevens-Rumann says. Forests in the hottest and driest regions were most susceptible. So fires in these areas may cause landscapes and ecosystems to change, for example, from forests to grasslands or shrubs.
And this may become a vicious cycle. Fires pump more carbon into the atmosphere, exacerbating climate change, and fires decimate the trees that would normally suck this carbon out of the sky. “If we lose forests through increased fire and limited regeneration, this could result in more carbon in the atmosphere,” says John Abatzoglou, a climatologist at the University of Idaho in Moscow who was not associated with the study. This, in turn, may further change climate and reshape the landscape, he says.
Some forests in the moister, higher elevations may not grow the old trees back, but may change to different tree species that are better suited to the hotter, drier weather. This might be a cue for future forest management, Stevens-Rumann says. Managers may want to plant species that are adapted to the current and future climate, not the climate of the past, she says.
But, not all forests are in danger of dying out completely. As Stevens-Rumann acknowledges, the time span she and colleagues studied, 23 years, is very short compared with the life span of a forest. “Even among our sites, some of the forests we studied are regenerating very well,” she says. Although some forests may grow back, she says, at the very least we can say that it is going to take much longer that it once did.