A study of ancient lake sediments reveals that a recent wildfire high in the Rwenzori Mountains was the first in 12 millennia, signaling a novel threat to Africa's unique alpine ecosystems.
PROVIDENCE, R.I. [Brown University] - In 2012, a wildfire ripped through 42 square kilometers of alpine moorland in Africa's Rwenzori Mountains, a range of glaciated peaks on the border of Uganda and the Democratic Republic of Congo. The blaze, which occurred at an elevation of over 13,000 feet, was shocking to those familiar with the mountains, as the climate had been assumed to be too cold and too wet for fire to spread.
Now, using sediment cores from a pair of remote mountain lakes, a research team led by Brown University scientists has assembled a record on fire activity in the Rwenzoris dating back thousands of years. They show that the 2012 fire was the first large-scale blaze in the range's highest elevations in at least the last 12,000 years. On the range's lower slopes at altitudes closer to 9,000 feet, the researchers found no evidence of fire until about 2,000 years ago, around the time archeological evidence suggests there was an intensification of human activity in the region.
The findings, published in the journal Nature, suggest that changes in climate and human activities are having a profound effect on Africa's unique alpine ecosystems that is unprecedented in modern times.
"These findings imply that the diverse, endemic ecosystems on tropical Africa's highest mountains are vulnerable to fire and, particularly in the context of climate change, could be permanently transformed if fires become frequent," said Andrea Mason, a Ph.D. candidate in Brown's Department of Earth, Environmental and Planetary Sciences and an affiliate of the Institute at Brown for Environment and Society. "It is critically important to develop management plans that prevent ignitions to preserve these ecosystems."
For the study, the researchers trekked into the Rwenzori Mountains to collect sediment cores from small lakes nestled along the slopes. The remnants of pollen grains, leaf waxes, fossil bacteria and other biomarkers transported into the lake by runoff and wind provide researchers a well-preserved record of environmental activity dating back thousands of years. For this research, the team searched the sediments in particular for charcoal - the stable, tell-tale signal of past fire. The team collected sediment cores from two lakes: Lake Mahoma on the lower slopes at about 9,000 feet, and Lake Kopello in the alpine zone around 13,000 feet.
The researchers found that the sediments from the last 12,000 years in the high-altitude Lake Kopello contain only tiny amounts of charcoal for most of that time - the exception being the most recent sediments that were deposited around the time of the 2012 fire. Recent sediments contain more than 100 times more charcoal than older sediment, the researchers found.
"The charcoal peak associated with the 2012 fire is enormous," Mason said. "It completely overwhelms any signal from the last 12,000 years, which is how we determined that the 2012 fire is unprecedented in the Holocene epoch."
Lower on the slopes, at Lake Mahoma, the cores suggest fire was rare until around 2,000 years ago, when a sharp uptick in charcoal is recorded in the sediment. The onset of fire activity immediately preceded a profound change in vegetation surrounding the lake, as detected by pollen grains sampled from the sediment. Pollen associated with deciduous trees decline after the period when fires are detected, while pollen associated with bamboo and other grasses increase.
Those findings suggest that "rise of fire permanently transformed these tropical African mountain forests 2,000 ago," said study co-author Jim Russell, a professor of Earth, environmental and planetary sciences at Brown.
Pinpointing the cause of the increase in fire activity is difficult, the researchers say, but the timing of fire increase around 2,000 years ago coincides with archeological evidence of intensified human activity in the region. At higher elevations, it's possible that the combination of increased tourist presence and a warmer, drier climate is making it possible for fire to spread in a way that it could not over the past 12,000 years.
The implications of this new fire regime are profound. The Rwenzoris - as well as Mount Kenya and Mount Kilimanjaro, which have also experienced recent fires - are all UNESCO World Heritage Sites. These high peaks are sometimes referred to as "sky islands" that are home to plant and animal species found nowhere else on Earth. All of these ecosystems face an emerging threat from fire.
But the risks aren't confined to plants and animals. In the months following the 2012 Rwenzori fire, villages at lower elevations were subject to destructive flooding as the charred landscape above failed to hold rainwater as it normally would.
If fires like this are the new normal for Afroalpine landscapes, it will be necessary to plan for them and manage them, the researchers say. For Russell, the findings were a startling reminder of how climate change and human activity are changing the planet in profound ways.
"People are starting to experience these environmental change moments when a place that they know and love has gotten wrecked," Russell said. "That fire in 2012 was my moment. I'd been going to those mountains a lot for field work, and to see them get scorched was really eye-opening."
The research was supported in part by the National Science Foundation (DEB 2048669 and DGE 2040433).
